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There is an intimate relationship between microorganisms and the formation and stability of soil microaggregates, realized by the immobilization and occlusion of microorganisms. Little is known about the initial aggregate formation phase and the role of microorganisms in this process under the impact of environmental changes such as wetting and drying. We investigated this initial aggregate formation process of montmorillonite and goethite in combination with two bacterial strains, Pseudomonas protegens strain CHA0 and Gordonia alkanivorans strain MoAcy 2, in the presence or absence of stress conditions in form of wetting and drying cycles for up to eight days. Montmorillonite and goethite formed microaggregates instantaneously, the size of these aggregates being enhanced in the presence of microorganisms, resulting in up to twofold larger aggregates. This increase in aggregate size was strain-dependent. However, the aggregates that developed during the first 48 h broke into smaller structures later on. A microscopic analysis of the microaggregates revealed that notably the larger microaggregates harbored bacteria and that microaggregates had a sheltering effect on living cells, especially when exposed to desiccation stress. Additionally, aggregate formation was analyzed in the presence of a Pseudomonas protegens mutant strain (CHA211) with increased production capability of extracellular polymeric substances (EPS). About fivefold higher survival rates of culturable cells were observed after desiccation for this EPS overproducing mutant strain in comparison to the wild-type. Our results hint at an aggregate formation process characterized by a rapid occlusion of mineral compounds, and, after the addition of microorganisms, the bacterial colonization of small microaggregates, leading to an increase in aggregate size. The further development of the aggregate size distribution varied depending on the presence of microbial taxa and was modulated by environmental conditions like desiccation events.

The transport and retention behavior of polymer- (PVP-AgNP) and surfactant-stabilized (AgPURE) silver nanoparticles in carbonate-dominated saturated and unconsolidated porous media was studied at the laboratory scale. Initial column experiments were conducted to investigate the influence of chemical heterogeneity (CH) and nano-scale surface roughness (NR) arising from mixtures of clean, positively charged calcium carbonate sand (CCS), and negatively charged quartz sands. Additional column experiments were performed to elucidate the impact of CH and NR arising from the presence and absence of soil organic matter (SOM) on a natural carbonate-dominated aquifer material. The role of the nanoparticle capping agent was examined under all conditions tested in the column experiments. Nanoparticle transport was well described using a numerical model that facilitated blocking on one or two retention sites. Results demonstrate that an increase in CCS content in the artificially mixed porous medium leads to delayed breakthrough of the AgNPs, although AgPURE was much less affected by the CCS content than PVP-AgNPs. Interestingly, only a small portion of the solid surface area contributed to AgNP retention, even on positively charged CCS, due to the presence of NR which weakened the adhesive interaction. The presence of SOM enhanced the retention of AgPURE on the natural carbonate-dominated aquifer material, which can be a result of hydrophobic or hydrophilic interactions or due to cation bridging. Surprisingly, SOM had no significant impact on PVP-AgNP retention, which suggests that a reduction in electrostatic repulsion due to the presence of SOM outweighs the relative importance of other binding mechanisms. Our findings are important for future studies related to AgNP transport in shallow unconsolidated calcareous and siliceous sands.

The hyperarid soils of the Atacama Desert, Chile, contain the largest known nitrate deposits in the world. They also represent one of the most hostile environments for microbial life anywhere in the terrestrial biosphere. Despite known for its extreme dryness, several heavy rainfall events causing localised flash flooding have struck Atacama Desert core regions during the last five years. It remains unclear, however, whether these soils can support microbial denitrification. To answer this, we sampled soils along a hyperaridity gradient in the Atacama Desert and conducted incubation experiments using a robotized continuous flow system under a He/O2 atmosphere. The impacts of four successive extreme weather events on soil-borne N2O and N2 emissions were investigated, i) water addition, ii) NO3− addition, iii) labile carbon (C) addition, and iv) oxygen depletion. The 15N–N2O site-preference (SP) approach was further used to examine the source of N2O produced. Extremely low N2O fluxes were detected shortly after water and NO3− addition, whereas pronounced N2O and N2 emissions were recorded after labile-C (glucose) amendment in all soils. Under anoxia, N2 emissions increased drastically while N2O emissions decreased concomitantly, indicating the potential for complete denitrification at all sites. Although increasing aridity significantly reduced soil bacterial richness, microbial potential for denitrification and associated gene abundance (i.e., napA, narG, nirS, nirK, cnorB, qnorB and nosZ) was not affected. The N2O15N site preference values based on two end-member model suggested that fungal and bacterial denitrification co-contributed to N2O production in less arid sites, whereas bacterial denitrification dominated with increasing aridity. We conclude that soil denitrification functionality is preserved even with lowered microbial richness in the extreme hyperarid Atacama Desert. Future changes in land-use or extreme climate events therefore have a potential to destabilize the immense reserves of nitrate and induce significant N2O losses in the region.

The roles of graphene oxide (GO) particle geometry, GO surface orientation, surface roughness, and nanoscale chemical heterogeneity on interaction energies, aggregation, retention, and release of GO in porous media were not fully considered in previous studies. Consequently, mechanisms controlling the environmental fate of GO were incompletely or inaccurately quantified. To overcome this limitation, plate-plate interaction energies were modified to account for these factors and used in conjunction with a mathematical model to interpret the results of GO aggregation, retention, and release studies. Calculations revealed that these factors had a large influence on the predicted interaction energy parameters. Similar to previous literature, the secondary minimum was predicted to dominate on smooth, chemically homogeneous surfaces that were oriented parallel to each other, especially at higher ionic strength (IS). Conversely, shallow primary minimum interactions were sometimes predicted to occur on surfaces with nanoscale roughness and chemical heterogeneity due to adsorbed Ca2+ ions, especially when the GO particles were oriented perpendicular to the interacting surface. Experimental results were generally consistent with these predictions and indicated that the primary minimum played a major role in GO retention and the secondary minimum contributed to GO release with IS reduction. Cation exchange (Na+ replacing Ca2+) enhanced GO release with IS reduction when particles were initially deposited in the presence of Ca2+ ions. However, retained GO were always completely recovered into the excess deionized water when the sand pore structure was destroyed during excavation, and this indicates that primary minima were shallow and that the pore structure also played an important role in GO retention. Further evidence for the role of pore structure on GO retention was obtained by conducting experiments in finer textured sand and at higher input concentrations that induced greater aggregation. In both cases, greater GO retention occurred, and retention profiles became more hyperexponential in shape.

It is considered inevitable that the increasing production and application of engineered nanoparticles will lead to their release into the environment. However, the behavior of these materials under environmentally relevant conditions is still only poorly understood. In this study the transport and deposition behavior of engineered surfactant stabilized silver nanoparticles (AgNPs) in water saturated porous media was investigated in transport experiments with glass beads as reference porous medium and in two natural soils under various hydrodynamic and hydrochemical conditions. The transport and retention processes of AgNPs in the porous media were elucidated by inverse modeling and possible particle size changes occurring during the transport through the soil matrix were analyzed with flow field-flow fractionation (FlFFF). A high mobility of AgNPs was observed in loamy sand under low ionic strength (IS) conditions and at high flow rates. The transport was inhibited at low flow rates, at higher IS, in the presence of divalent cations and in a more complex, fine-grained silty loam. The slight decrease of the mean particle size of the AgNPs in almost all experiments indicates size selective filtration processes and enables the exclusion of homoaggregation processes.

To maximize crop productivity fertilizer P is generally applied to arable soils, a significant proportion of which becomes stabilized by mineral components and in part subsequently becomes unavailable to plants. However, little is known about the relative contributions of the different organic and inorganic P bound to Fe/Al oxides in the smaller soil particles. Alkaline (NaOH–Na2EDTA) extraction with solution 31P-nuclear magnetic resonance (31P-NMR) spectroscopy is considered a reliable method for extracting and quantifying organic P and (some) inorganic P. However, any so-called residual P after the alkaline extraction has remained unidentified. Therefore, in the present study, the amorphous (a) and crystalline (c) Fe/Al oxide minerals and related P in soil aggregate-sized fractions (> 20, 2–20, 0.45–2 and < 0.45 μm) were specifically extracted by oxalate (a-Fe/Al oxides) and dithionite–citrate–bicarbonate (DCB, both a- and c-Fe/Al oxides). These soil aggregate-sized fractions with and without the oxalate and DCB pre-treatments were then sequentially extracted by alkaline extraction prior to solution 31P-NMR spectroscopy. This was done to quantify the P associated with a- and c-Fe/Al oxides in both alkaline extraction and the residual P of different soil aggregate-sized fractions. The results showed that overall P contents increased with decreasing size of the soil aggregate-sized fractions. However, the relative distribution and speciation of varying P forms were found to be independent of soil aggregate-size. The majority of alkaline-extractable P was in the a-Fe/Al oxide fraction (42–47 % of total P), most of which was ortho-phosphate (36–41 % of total P). Furthermore, still significant amounts of particularly monoester P were bound to these oxides. Intriguingly, however, Fe/Al oxides were not the main bonding sites for pyrophosphate. Residual P contained similar amounts of total P associated with both a- (11–15 % of total P) and c-Fe oxides (7–13 % of total P) in various aggregate-sized fractions, suggesting that it was likely occluded within the a- and c-Fe oxides in soil. This implies that, with the dissolution of Fe oxides, this P may be released and thus available for plants and microbial communities.

Multi-walled carbon nanotubes (MWCNTs) are increasing used in commercial applications and may be released into the environment with anionic surfactants, such as sodium dodecylbenzenesulfonate (SDBS), in sewer discharge. Little research has examined the transport, retention, and remobilization of MWCNTs in the presence or absence of SDBS in porous media with controlled chemical heterogeneity, and batch and column scale studies were therefore undertaken to address this gap in knowledge. The adsorption isotherms of SDBS on quartz sand (QS), goethite coated quartz sand (GQS), and MWCNTs were determined. Adsorption of SDBS (MWCNTs » GQS > QS) decreased zeta potentials for these materials, and produced a charge reversal for goethite. Transport of MWCNTs (5 mg L-1) dramatically decreased with an increase in the fraction of GQS from 0 to 0.1 in the absence of SDBS. Conversely, co-injection of SDBS (10 and 50 mg L-1) and MWCNTs radically increased the transport of MWCNTs when the GQS fraction was 0, 0.1, and 0.3, especially at a higher SDBS concentration, and altered the shape of retention profile. Mathematical modeling revealed that competitive blocking was not the dominant mechanism for the SDBS enhancement of MWCNT transport. Rather, SDBS sorption increased MWCNT transport by increasing electrostatic and/or steric interactions, or creating reversible interactions on rough surfaces. Sequential injection of pulses of MWCNTs and SDBS in sand (0.1 GQS fraction) indicated that SDBS could mobilize some of retained MWCNTs from the top to deeper sand layers, but only a small amount of released MWCNTs were recovered in the effluent. SDBS therefore had a much smaller influence on MWCNT transport in sequential injection than in co-injection, presumably because of a greater energy barrier to MWCNT release than retention. This research sheds novel insight on the roles of competitive blocking, chemical heterogeneity and nanoscale roughness, and injection sequence on MWCNT retention and release.

The leaching of P from the upper 20 cm of forest topsoils influences nutrient (re-)cycling and the redistribution of available phosphate and organic P forms. However, the effective leaching of colloids and associated P forms from forest topsoils was so far sparsely investigated. We demonstrated through irrigation experiments with undisturbed mesocosm soil columns, that significant proportions of P leached from acidic forest topsoils were associated with natural colloids. These colloids had a maximum size of 400 nm. By means of Field-flow fractionation the leached soil colloids could be separated into three size fractions. The size and composition was comparable to colloids present in acidic forest streams known from literature. The composition of leached colloids of the three size classes was dominated by organic carbon. Furthermore, these colloids contained large concentrations of P which amounted between 12 and 91% of the totally leached P depending on the type of the forest soil. The fraction of other elements leached with colloids ranged between 1% and 25% (Fe: 1–25%; Corg: 3–17%; Al

The surface area and pore structure of easily dispersed soil particles

Black carbon (BC), characterized by high microporosity and high specific surface area (SSA), has been demonstrated to have substantial contributions to the sorption of hydrophobic organic chemicals in soils and sediments. Other naturally occurring organic matters provide soft and penetrable sorption domains while may cling to BC and affect its original surface properties. In this work, we studied the sorption sites of a Yangtze River sediment sample with organic carbon (OC) content of 3.3 % and the preheated sediment (combusted at 375 °C) with reduced OC content (defined as BC) of 0.4 % by gas and pyrene sorption. The SSA and microporosity of the pristine and preheated sediments were characterized by N2 and CO2 adsorption. The results suggest that the adsorption of N2 was hindered by amorphous organic carbon (AOC) in the pristine sediment but CO2 was not. Instead, the uptake of CO2 was higher in the presence of AOC, likely due to the partition of CO2 molecules into the organic matter. The pyrene adsorptions to BC in pristine and preheated sediments show a similar adsorption capacity at high concentration, suggesting that AOC of ca. 2.9 % in the pristine sediment does not reduce the accessibility to the sorption sites on BC for pyrene.

The colloidal behavior of soil mineral colloids (quartz and illite) in Na, Ca and mixed Na–Ca systems was systematically investigated. The critical coagulation concentrations (CCCs) were determined for Na+ and Ca2+ solutions containing anions of a different nature (Cl−, SO42− and ubiquitous soil organic anions formate and oxalate) at pH 5.5 and pH 8.5. Higher CCC values in the presence of Na+ at pH 8.5 were related to a lower screening effect of monovalent cations and the ionization properties of the soil mineral colloids. Attraction energy between colloids was experimentally scaled at CCC from zeta potential and aggregation kinetics measurements. In the Na-system, a higher stability for colloidal illite and quartz particles suspensions in the presence of oxalate anions confirmed surface complexation at an acidic pH. In the Ca-system, slight CCC variations in illite suspensions in the presence of Cl−, SO42−, formate and oxalate supported attractive ion–ion correlation forces between equally highly charged clay mineral colloids. It was shown that the CCCs of mixed Na–Ca systems close to soil chemical conditions can be calculated using the CCCs for colloidal illite and quartz particles in pure Na and Ca systems at pH 5.5 and pH 8.5. Sensitive dispersion effects due to adsorbed oxalate can be analyzed as a function of the molar ratios of calcium to sodium in solution.

Nanoparticles and colloids affect the storage and hence the availability of P in forest ecosystems. We investigated the fine colloids present in forest soils and their association with inorganic and organic P. To differentiate between the different P forms, we performed liquid-state 31P-nucelar magnetic resonance (31P-NMR) measurements on forest bulk soil extracts, on colloid extracts and on the electrolyte phase of their soil suspensions. The 31P-NMR spectra indicated that soil nanoparticles and colloids were more enriched with organic than with inorganic P forms compared to the electrolyte phase. The P concentration was enriched in the colloidal fraction in comparison to the bulk soil and the phosphate diesters were more dominant in the colloidal fraction when compared to the bulk soil. The colloidal P-diester to P-monoester ratios were 2 to 3 times higher in the colloidal fraction than in the bulk soil. In contrast, relatively large percentages of inorganic P were found in the electrolyte phase. Supplementary (not shown) Data are available at the JuSER Server (juser.fz-juelich.de, reference number: FZJ-2016-01739) https://juser.fz-juelich.de/record/283057.

The perfluorooctanoic acid (PFOA) sorption behavior of two commercial multi-walled carbon nanotubes (MWCNTs) (C 150 P from Bayer MaterialScience: BA and C-MWNTs from NanoTechLabs Inc.: CP) was investigated from aqueous solution. The BA nanotubes contained Co/Mn/Mg/Al catalysts both on their outer surface and in the inner bore while CP contained Fe-based catalyst typically within the tubes. The adsorption isotherms of (14)C-radiolabeled PFOA were measured by batch experiments and fitted to the Freundlich model (r(2)>0.92). The adsorption affinity and capacity on BA were significantly higher than on CP. Increasing the pH reduced the adsorption of PFOA due to the electrostatic interaction between the pH-sensitive surface and the adsorbate. Increasing the NaCl concentration led to the aggregation of the MWCNTs reducing the available surface and thus the adsorption capacity. Removal of the catalyst from the outer surface of BA changed the electrophoretic mobility from a positive to a negative value and also decreased the adsorbed amount of PFOA. The surface charge of the surface-associated metal catalyst favors the electrostatic sorption of PFOA. Such surface modifications may be a promising way to improve the sorption capacity of MWCNTs for pollutants such as PFOA and to broaden their potential application in water purification.

Due to the limited solubility of phosphorus (P) in soil, understanding its binding in fine colloids is vital to better forecast P dynamics and losses in agricultural systems. We hypothesized that water-dispersible P is present as nanoparticles and that iron (Fe) plays a crucial role for P binding to these nanoparticles. To test this, we isolated water-dispersible fine colloids (WDFC) from an arable topsoil (Haplic Luvisol, Germany) and assessed colloidal P forms after asymmetric flow field-flow fractionation coupled with ultraviolet and an inductively coupled plasma mass spectrometer, with and without removal of amorphous and crystalline Fe oxides using oxalate and dithionite, respectively. We found that fine colloidal P was present in two dominant sizes: (i) in associations of organic matter and amorphous Fe (Al) oxides in nanoparticles

The binding of the pesticide metabolite aminobenzothiazole (ABT) to a model humic acid was studied. It is shown that, by comparing quantum chemically calculated and experimental 15 N NMR chemical shifts, proton transfer occurs in complexes of ABT with aminobenzoic acid (AB) and thiophene carboxylic acid (TC). For extended structures, i.e. the crystalline form of AB–ABT and TC–ABT, only in the latter proton transfer is observed, in agreement with previously reported analysis based on IR spectroscopy. Thus it is concluded that in the bimolecular complexes as model systems for structures found in humic substance, here specifically AB–ABT, proton transfer may be different as compared to the crystalline state. Moreover, it was shown that combining experimental NMR work and quantum chemical calculations can reveal the presence of proton transfer.

Sorption of pyrene and phenanthrene to model (illite and charcoal) and natural (Yangtze sediment) geosorbents were investigated by batch techniques using fluorescence spectroscopy. A higher adsorption of phenanthrene was observed with all sorbents, which is related to the better accessibility of smaller molecules to micropores in the molecular sieve sorbents. In addition, pyrene sorption in binary-solute systems with a constant initial concentration of phenanthrene (0.1 μmol L(-1) or 2 μmol L(-1)) was studied. A 0.1 μmol L(-1) concentration of phenanthrene causes no competitive effect on the pyrene sorption. A 2 μmol L(-1) concentration of phenanthrene significantly suppresses the sorption of pyrene, especially in the low concentration range; nonlinearity of the pyrene sorption isotherms thus decreases. The competitive effect of 2 μmol L(-1) phenanthrene on the pyrene sorption is overestimated by the ideal adsorbed solution theory (IAST) using the fitted single sorption results of both solutes. An adjustment of the IAST application by taking into account the molecular sieve effect is proposed, which notably improves the IAST prediction for the competitive effect.

The effect of organic carbon (OC) and mineral surface on the sorption of polycyclic aromatic hydrocarbon (PAH) pyrene molecule to four Yangtze River sediments was investigated by sorption batch techniques using fluorescence spectroscopy. Pyrene sorption to the mineral fraction was estimated with model sorbent illite, the main clay mineral in Yangtze sediment. The Freundlich model fitted sorption to illite and to sediments was normalized to the specific surface area (SSA). Comparison of the SSA-normalized sorption capacities of illite and sediments suggests a negligible contribution of the pyrene sorption to the mineral fraction. In addition, composite models, such as the linear Langmuir model (LLM) and the linear Polanyi-Dubinin-Manes model (LPDMM) were applied for fitting the sorption of pyrene to the pristine sediments. The application of composite models allows assessing the partition of pyrene into amorphous organic carbon (AOC) and the adsorption in the porous structure of black carbon (BC). The modelling results indicate that the pyrene adsorption to the minor BC components (0.2%) is more effective than the partition to AOC (0.5-1.3%). Besides the pristine sediments, sediments preheated at 375 degrees C were also studied, in which the AOC fraction was removed during the preheating treatment. The modelling results with LPDMM and Polanyi-Dubinin-Manes model (PDMM) indicate a similar adsorption capacity of BC in pristine and preheated sediments, respectively. The low AOC concentrations in sediments do not diminish the BC micropore filling with pyrene. Simulation of pyrene distribution in the investigated Yangtze River sediments support the importance of the BC fraction in the PAH immobilization under environmental conditions.

Nonylphenols (NPs) are the breakdown products of the nonionic surfactants nonylphenol ethoxylates and are toxic pollutants. Here we studied the bioaccumulation, elimination, and biotransformation of NP (12.3 mg kg(-1) soil dry weight) in a typical Chinese geophagous earthworm, Metaphire guillelmi, in a rice paddy soil, using 4-[1-ethyl-1,3-dimethylpentyl]phenol (4-NP(111)), the main constitute of technical NP, radiolabeled with (14)C. Earthworms rapidly bioaccumulated (14)C-4-NP(111) following a two-compartment first-order kinetics model. At steady state (after 20 days exposure), the normalized biota-soil accumulation factor amounted to 120, and 77% of the accumulated radioactivity were present as nonextractable bound residues. The total radioactivity was eliminated from the earthworm following an availability-adjusted decay model and controlled by the elimination rate of the bound residues (half-life = 22.6 days). The extractable residues consisted mainly of one less-polar metabolite (37%) and polar compounds (50%), including glucuronide conjugates of 4-NP(111) and the metabolite; and free 4-NP(111) accounted for only 9% of the total extractable residues. This study provides the first results of the toxicokinetics and biotransformation of 4-NP in a terrestrial organism, and underlines the significant underestimation of the bioaccumulation and risk assessment based only on free NP in earthworms.

Application of minimally invasive probes for measuring fluorescent labelled colloids inside sand columns resulted in internal breakthrough curves at given points, that provided information about mass distributions without disrupting the column. Until now, light fiber optic probes were used in combination with organic fluorophores like fluoresceine or rhodamine, which have inherent disadvantage, such as comparably small Stoke's shifts and quenching at high concentrations. These may be overcome using rare earth ion complexes to label colloidal particles like microspheres (MS). Furthermore, these dyes have large fluorescent lifetimes compared to organic fluorophores, which readily allows the elimination of background fluorescence using time-resolved spectroscopy. We demonstrate the novel application of rare earth ion-labelled microspheres as colloidal tracers in column experiments under unsaturated conditions in porous media in coupling with a detection setup based on internal light fiber optic probes. Differences in mass budgets between internal detection and detection at the outlet were confirmed afterwards by invasive refurbishment of the column matrix.

Study of bacterial transport and retention in soil is important for various environmental applications such as groundwater contamination and bioremediation of soil and water. The main objective of this research was to quantitatively assess bacterial transport and deposition under saturated conditions in calcareous soil. A series of leaching experiments was conducted on two undisturbed soil columns. Breakthrough curves of Pseudomonas fluorescens and Cl were measured. After the leaching experiment, spatial distribution of bacteria retention in the soil columns was determined. The HYDRUS-1D one- and two-site kinetic models were used to predict the transport and deposition of bacteria in soil. The results indicated that the two-site model fits the observed data better than one-site kinetic model. Bacteria interaction with the soil of kinetic site 1 revealed relatively fast attachment and slow detachment, whereas attachment to and detachment of bacteria from kinetic site 2 was fast. Fast attachment and slow detachment of site 1 can be attributed to soil calcium carbonate that has favorable attachment sites for bacteria. The detachment rate was less than 0.02 of the attachment rate, indicating irreversible attachment of bacteria. High reduction rate of bacteria was also attributed to soil calcium carbonate.

A column material containing organophilic clay particles was developed for soil remediation. 2,4-dichlorophenol adsorption on aquifer material with an effective particle diameter d

The adsorption and degradation of chlorobenzene on partially modified organoclays and by the autochthonous microorganism Rhodococcus B528 were studied by means of the batch technique. Organoclays were prepared from Na-montmorillonite (MM) by using dodecyltrimethylammonium (C12) and dioctadecyldimethylammonium (2C18) bromides. The degree of modification was 35 (2C18-35-MM) and 89% (C12-89-MM) of the cation exchange capacity of MM. The adsorption experiments were carried out using headspace GC. The intercalation of chlorobenzene into the interlayers of organo-MM was detected by X-ray diffraction. The adsorption isotherms found were of the S1 type indicating a cooperative effect. Chlorobenzene showed a higher affinity for 2C18-35-MM than C12-89-MM, which could not only be explained by the organic carbon content. The comparison with 2,4-dichlorophenol adsorption has implied that for the studied systems the different adsorption mechanisms are primarily governed by the different molecular properties and not by the type of absorbent. The presence of 2C18-35-MM caused no negative effect on the investigated microorganisms and complete biodegradation of chlorobenzene was achieved without desorption limitation for growth, demonstrating the applicability of partially modified organoclays for bioremediation.

The effect of 2,4-dichlorophenol (DCP) on the structures and phase transitions of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes was studied using FT-Raman spectroscopy. Whereas the Raman frequency shifts of the most frequently investigated bands of C-C and C-H stretching regions only indicate the main phase transition (P(beta')-L(alpha)) of the pure DPPC/water system, the Raman shift of C-H scissoring vibration at 1440 cm(-1) was found to be able to reveal the pretransition (L(beta')-P(beta')) as well. Analyzing the spectral parameters of the trans band at 1128 cm(-1), which does not overlap with DCP vibrational modes, a continuous decrease of trans conformations was found with increasing DCP concentration at 26 degrees C accompanying the phase transitions L(beta')-P(beta') and P(beta')-L(alpha). The intensity ratio of the symmetrical and asymmetrical methylene stretching bands (at 2850 cm(-1) and 2880 cm(-1)), defined as the disorder parameter by Levin [Levin, I.W., 1985. Two types of hydrocarbon chain interdigitation in sphingomielin bilayers. Biochemistry 24, 6282-6286], indicated that in the interdigitated phase (L(I)) the order is markedly high and comparable with that of L(beta). Both the phase transition P(beta')-L(alpha) in the DCP/DPPC molar ratio range of 10/100-50/100 and the phase transition L(I)-L(alpha) led to a significant increase of disordered chains and the presence of DCP molecules induced a more disordered chain region than that observed in the L(alpha) phase of DPPC. Nevertheless, it was found that the L(alpha) phase with DCP contains approximately the same amount of trans conformers than that without DCP.

The role of the type and amount of cationic surfactant in the sorption behaviour of partially modified clays was studied and the applicability of a partially modified organoclay for bioremediation was demonstrated. Partially modified surfactant/clay complexes (“organoclays”) were prepared from Na-montmorillonite by using monoalkyl-(dodecyltrimethylammonium and octadecyltrimethylammonium) and dialkylammonium (didodecyldimethylammonium and dioctadecyldimethylammonium) bromides. The degree of modification was varied between 35 and 89% of the cation exchange capacity of montmorillonite (MM). The sorption of 2,4-dichlorophenol (DCP) on organoclays was studied by using the batch technique. The intercalation of DCP into the interlayers of organo-MM was detected by X-ray diffraction. The adsorption isotherms found were non-linear indicating that the sorption is not only based on a simple partition mechanism. The adsorption coefficients Kd calculated from the initial isotherm slope turned out to be exponential with the organic carbon content of organo-MM (foc). In particular, dialkylammonium-modified MM revealed outstanding sorption characteristics for DCP. The comparison of partially modified organo-MMs with similar foc and volume fraction of surfactant indicated that the surfactant arrangement in the interlayers significantly influences the DCP adsorption. It could be assumed that both the kind and degree of exchanged surfactant play a decisive role in the adsorption of DCP on the organoclays. By performing biodegradation experiments in the presence of dioctadecyldimethylammonium montmorillonite, the complete biodegradation of DCP was achieved demonstrating the applicability of partially modified organoclays for bioremediation.

The features of the location of copper ions and the dihalogenated phenol molecules were investigated at their low concentration in the dipalmitoylphosphatidylcholine (DPPC)–water liposomes by using small-angle X-ray scattering (SAXS) and anomalous small-angle X-ray scattering (ASAXS) methods. Only the ASAXS method can provide information about the structural features of the domains rich in these ions and organic guest molecules. The copper ions are mainly located in large domains between the lamellae and only partly in the characteristic periodic water shells of the liposomes. The dihalogenated phenol molecules are embedded in the layers and do not destroy the lamellar arrangement. Their inhomogeneous distribution appears laterally and phase separation occurs. The local structures induced by the copper ions and the dihalogenated phenols were also visualised by freeze-fracture method.

The influence of sorption on the biodegradation of 2,4-dichlorophenol (DCP) by Ralstonia eutropha was investigated using organoclays. The aim was to examine the suitability of organoclays combined with biodegradation in remediation techniques. Two types of organoclays were used: a dodecyltrimethylammonium montmorillonite, where 89% of the former sodium ions were exchanged by the cationic surfactant (C12-MM), and a dioctadecyldimethylammonium montmorillonite complex (35% exchange of the sodium-ions) (2C18-MM). The organoclays showed high sorption affinity to DCP resulting in the intercalation of DCP into the interlayers of the organoclay. The sorption processes were reversible and completed within minutes. Neither organoclay was inhibitory to R. eutropha degrading fructose. This was different with DCP as sole energy and carbon source. In the presence of DCP C12-MM led to a decrease in cell numbers caused by a coupled effect of DCP and small amounts of free C12 in solution. This was not observed with 2C18-MM, as the concentration of 2C18 in solution was lower because of the lesser degree of exchange (35% compared to 89%). The addition of 2C18-MM enabled complete biodegradation of DCP in initially toxic DCP concentration ranges. Adsorption lowered the DCP concentration to a non-toxic level. Because the sorption process was reversible, DCP was desorbed when R. eutropha reduced the DCP concentration in the liquid phase by biodegradation. The whole amount of DCP—dissolved and initially adsorbed—was degraded as confirmed by oxygen consumption and cell concentration measurements. Organoclays can be very helpful tools in remediation when the exchanged amount of surfactant remains low. They even allow bioremediation under toxic circumstances, as was shown with 2C18-MM.

Total counts in soils are usually determined using fluorescent dyes, such as DAPI or Sybr green, due to fluorescence enhancement if they are bound to nucleic acids. Unfortunately, these commonly used dyes stain soil particles as well. Therefore, besides fluorescence enhancement, sufficient spectral differentiation is also required. We present a new procedure that overcomes the problems of visualising bacteria on surfaces in soil and avoids the separation of soil particles to a large extent. Spectral differentiation between bacteria and soil matrix is achieved by using Sytox green and a suboptimal excitation wavelength. Bacteria exhibit a bright green fluorescence, while soil particles fluoresce blue or red. Slight homogenisation and sedimentation of the sand and coarse silt that were too big for microscopic investigations were the only separation steps required. We compared the proposed Sytox green staining with Sybr green staining. The recovery of Sybr green-stained cells amounted to 38%, whereas in samples stained by Sytox green 81% of the spiked cells were counted. Sytox green can also be combined with fluorescence in situ hybridisation (FISH) using deep red dyes such as Cy5.

Aggregation and dispersion of mineral particles spontaneously take place under changing environmental conditions in natural systems. The structure of particle network in soils, the retardation or release of colloidal particles, and their mobility and transport are inherently influenced by natural organic matter bound to the mineral matrix. Since the surface properties of clay mineral and metal oxide particles, and the electrified mineral–water interfaces play a major role in formation, structure and strength of aggregates, any surface modification, especially by polyanionic organic complexants such as humic substances, has a significant affect on particle interaction in a mineral assemblage. The permanently and/or conditionally charged clay minerals (montmorillonite and kaolinite) and iron oxides (hematite and magnetite), as known major mineral components in natural waters, were selected for studying their surface charge characteristics and pH dependent interactions. We discuss how the surface charge correlates with particle aggregation through some characteristic examples for homo and heterocoagulation of similar and dissimilar mineral particles under acidic condition (at pH ∼4) in the dilute and concentrated systems studied by means of light scattering and rheology, respectively. The adsorption of a brown coal derived humic acid, and its influence on the surface charge character and particle aggregation of clay and iron oxide particles were also studied in dilute and concentrated suspensions. Humic acids can be bound to the most reactive surface sites of clay and oxide particles, i.e. to Al -OH mainly at the edges of clay lamellae, and to Fe -OH on iron oxides, in surface complexation reaction, therefore their role in particle aggregation is specific. Relations between surface complexation, surface charge modification, and particle aggregation in pure and mixed montmorillonite–iron oxide systems are explained.

The effect of 2,4-dichlorophenol (DCP) on the phosphatidylethanolamine (l-α-dipalmitoyl-phosphatidylethanolamine (DPPE))/water liposomes was studied in the temperature domains of the gel and liquid crystalline phases at the DCP/DPPE molar ratios of 10−1 and 10−3 by using differential scanning calorimetry (DSC) as well as small and wide angle X-ray scattering (SAXS and WAXS). Different character of the transitions between the gel and the liquid crystalline phases was observed in the lipid/water and by DCP-doped systems. The different DCP concentrations caused similar effects in the change of the layer arrangements of the gel phase, while the perturbation of the subcells of this phase was different. In the liquid crystalline phase, the DCP molecules did not affect the layer structure significantly. The calorimetrical behaviour of the systems were rather correlated to the changes of the subcells than to the layer arrangements.

The adsorption of anionic surfactants (sodium dodecyl sulfate (SDS) and sodium bis[2-ethylhexil]sulfosuccinate (AOT)) on hydrotalcite (HT) and the adsorption of 2,4-dichlorophenol (DCP) on HT and its modified forms from aqueous solutions was studied. The modification was carried out by calcinations (heat treatment) and hydrophobization using different anionic surfactants. The anionic surfactant isotherms were of the H-type. It was assumed that first the ion exchange mechanism takes place until approximately the anion exchange capacity of HT is reached and hereafter, the hydrophobic mechanism occurs forming surfactant bilayers. While the DCP adsorption was very poor on HT, significant uptake of DCP took place on calcined HT which was attributed to the simultaneously occurring rehydration and adsorption during the reconstruction of the layered structure. On anionic surfactant-HT complexes (“organo-hydrotalcites”) DCP was considerably adsolubilized in the anionic surfactant layers intercalated in HT. The AOT–HT complex appeared the more effective adsorbents for DCP.

The effect of 2,4-dichlorophenol (DCP) on the main transition and pretransition of fully hydrated (20 mass%) dipalmitoylphosphatidylcholine (DPPC) multilamellar liposomes has been studied by differential scanning calorimetry (DSC). It was observed that an increase in the molar ratio of DCP/DPPC (from 4·10-5 up to 2·10-2) causes progressive reductions in the temperature and enthalpy of the pretransition. The higher concentration of DCP eliminates the pretransition. The influence of DCP on the main transition in this molar ratio range is not drastic, but a decrease in temperature and in the enthalpy values was observed. In the molar ratio range (from 2·10-1 up to 4·10-1) the DSC scans show multiple main transition peaks instead of the characteristic single peak of the main transition. Above a DCP/DPPC molar ratio of 0.6 a new peak appears at 25°C having about the same transition enthalpy as the main transition of the pure system.

The adsorption behavior of 2,4-dichlorophenol and of nonionic surfactants of the Triton-X series on soil colloids, separately and in mixed systems, was studied at different pH values. Particular attention was paid to the influence of the adsorption process of the pollutant in the case of small, environmentally relevant surfactant coverages. The adsorption isotherms of 2,4-dichlorophenol on Ca montmorillonite were of the Henry type and no intercalation into the interlayer space of the clay mineral was observed using X-ray diffraction. The pollutant did not show any significant affinity to the native silica surface. The different courses of the isotherms of the nonionic surfactant and their pH dependence indicate significantly different adsorption mechanisms for the two adsorbents. This results in a greater effectiveness concerning the hydrophobization of the silica surface by the nonionic surfactant compared to the montmorillonite surface, as can be seen from a comparison of the partitioning coefficients Koc for 2,4-dichlorophenol. On both the untreated and the surfactant-covered adsorbents the increasing pH reduces 2,4-dichlorophenol adsorption drastically due to Coulomb repulsion and solubility effects.

The aim of this work was to study the sorption behaviour of atrazine on clay minerals at low environmentally relevant concentrations. Adsorption and desorption isotherms of atrazine were determined on different clay minerals using the 14C tracer technique. The adsorption isotherms are linear at pH 5.8 in the low concentration range studied. The adsorption constant Kd is proportional to the external surface in Na+ layer silicates, such as kaolinite, illite and montmorillonite. This implies that atrazine molecules do not intercalate even in swelling Na+ clay minerals. The experiments with homoionic montmorillonites (Na+, Ca2+, Ni2+, Cu2+ and Fe3+) indicate a correlation between the adsorption constant and the hydrolysis constant of the exchangeable cation. This suggests a participation of the protonated atrazine molecules in sorption due to electrostatic interactions. It is assumed that adsorption shifts the chemical equilibrium to the side of the protonated form for Men+ montmorillonites with a low hydrolysis constant of Men+. In contrast, protonation clearly dominates in Fe3+ montmorillonite because of the high hydrolysis constant of the Fe(III) ion and the adsorption isotherm obtained is not linear. The desorption isotherms show a hysteresis on all the Men+ montmorillonites examined for the time interval of 3.5 weeks. It is suggested that only that fraction of the bound atrazine, which is adsorbed due to the relatively weak physical forces, can be desorbed. The larger the fraction of protonated atrazine molecules on the surface, the less is remobilized.

The adsorption of octa(oxyethylene) monododecyl ether (C12E8) from water onto silica gela system of technical as well as environmental interestis investigated at various temperatures using liquid chromatographic techniques. The isotherms obtained are of the two-step type. The first step at low adsorbed amounts has scarcely been observed in the past. Therefore, this study focuses on this area. The sorption isotherms and the influence of temperature on these isotherms indicate different adsorption mechanisms for the two isotherm steps. Whereas the first step seems to be related to hydrogen bonds between hydrophilic surfactant groups and surface silanol groups, the second step can be well explained by the formation of hemimicelles on the surface. For the determination of the sorption isotherms, a liquid chromatographic technique, an elution analysis, is introduced. The so-called “elution on a plateau” technique measures the derivative of an isotherm and therefore can provide additional details about the cour...

The adsorption of the cationic surfactant dodecyl trimethyl ammonium bromide (DTAB) and of the nonionic surfactant dodecyl octaethylene glycol ether (C12E8) on four different layer silicates and their influence on the sorption processes of the fungizide biphenyl were studied. Unexpectedly, no great differences were found in comparing the adsorption of the two surfactants on the basis of physicochemical investigations, although the adsorption mechanism up to monolayer formation is fundamentally different (ion exchange and physisorption). Thus, the plateau values of the adsorption isotherms and the molar enthalpies of displacement Δ 21h are of the same order of magnitude for both surfactants and the same basal spacing by intercalation is observed in the case of swelling clays. The isotherms of the hydrophobic contaminant biphenyl are of the linear Cl-type at all layer silicates and very low adsorption takes place approximately proportionally to the BET (N2) surface area. If the surface is weakly hy...

The aggregation of multi-walled carbon nanotubes (MWCNTs) in the aqueous phase not only inhibits their extensive utilization in various aspects but also dominates their environmental fate and transport. The role of surfactants at low concentration in the aggregation of MWCNTs has been studied, however the effect of perfluorinated surfactants at low concentration is uncertain. To understand this interfacial phenomenon, the influences of perfluorooctanoic acid (PFOA), and sodium dodecyl sulfate (SDS) as a control, on MWCNT aggregation in the aqueous phase were examined by the UV absorbency method. Influences of pH and cationic species on the critical coagulation concentration (CCC) value were evaluated. The CCC values were dependent on the concentration of PFOA, however a pronounced effect of SDS concentration on the CCC values was not observed. The CCC values of the MWCNTs were 51.6 mmol/L in NaCl and 0.28 mmol/L in CaCl2 solutions, which suggested pronounced differences in the effects of Na+ and Ca2+ ions on the aggregation of the MWCNTs. The presence of both PFOA and SDS significantly decreased the CCC values of the MWCNTs in NaCl solution. The aggregation of the MWCNTs took place under acidic conditions and was not notably altered under neutral and alkaline conditions due to the electrostatic repulsion of deprotonated functional groups on the surface of the MWCNTs.

The sorption of nitrobenzene andn-pentanol from dilute aqueous solution on swelling clay minerals and their organophilized derivatives (organo clays) was studied. Adsorption excess isotherms were obtained by the immersion method. The basal spacings of the clay minerals were determined by X-ray diffraction measurements. By combining these two independent methods, composition and structure of the interlamellar space could be calculated. On the hydrophilic surface of montmorillonite negative adsorption of the organic component was observed at low molar fractions of nitrobenzene or pentanol, i.e., water was preferentially adsorbed. On organophilized montmorillonite and vermiculite adsorption of nitrobenzene and pentanol was positive over the whole range of liquid composition. The amount of interlamellar alkyl chains which is determined by the surface charge of clay mineral inversely affected the adsorption of both solutes.

Risk assessment of xenobiotics requires a comprehensive understanding of their transformation in the environment. As most of the transformation processes usually involve a redox reaction or a hydrolysis as the first steps of the transformation, we applied an approach that uses an electrochemical cell to investigate model "redox" reactions in aqueous solutions for environmental processes. We investigated the degradation of a variety of xenobiotics from polar to nonpolar and analyzed their degradation products by on-line coupling of electrochemistry with mass spectrometry (EC-MS). Furthermore, we evaluated possible binding reactions with regard to the generation of non-extractable residues with some model substances (catechol, phthalic acid, γ-L-Glutamyl-L-cysteinyl-glycine (GSH) and L-histidine) deduced from a natural organic matter (NOM) structure model and identified possible binding-sites. Whereas typically investigations in soil/water-systems have been applied, we used to our knowledge for the first time a bottom-up approach, starting from the chemicals of interest and different model substances for natural organic matter to evaluate chemical binding mechanisms (or processes) in the EC-MS under redox conditions. Under oxidative conditions, bindings of the xenobiotics with catechol, GSH and histidine were found, but no reactions with the model compound phthalic acid were observed. In general, no chemical binding has yet been found under reductive conditions. In some cases (i.e. benzo[a]anthracene) the oxidation product only underwent a binding reaction, whereas the xenobiotic itself did not undergo any reactions. EC-MS is a promising fast and simple screening method to investigate the environmental behavior of xenobiotics and to evaluate the potential risks of newly synthesized substances.

International audience; We report the adsorption of phenol and dopamine probe molecules, from aqueous solution with NaCl, on commercial multiwall carbon nanotubes (MWCNT) and on their carboxylated derivative. The nanotubes were fully characterized by high resolution transmission electron microscopy (HRTEM), small angle X-ray scattering (SAXS), potentiometric titration, electrophoretic mobility and nitrogen adsorption (77 K) measurements. The experimental pollutant isotherms, evaluated using the Langmuir model, showed that only 8-12 % and 21-32 % of the BET surface area was available for phenol and dopamine, respectively, which is far below the performance of activated carbons. Influence of the pH was more pronounced for the oxidized MWCNT, particularly with dopamine. The strongest interaction and the highest adsorption capacity occurred at pH 3 with both model pollutants on both types of nanotube. Although the surface area available for adsorption is far lower in MWCNTs than in activated carbons, it is nonetheless substantial. In particular delayed release of toxic molecules that are either adsorbed on the surface or trapped in the inner bore of such systems could constitute an environmental hazard. The need for further adsorption studies with regard to their environmental aspects is therefore pressing, particularly for MWCNTs in their functionalized state.

Using (14)C- and (13)C-ring-labeling, degradation of five p-nonylphenol (4-NP) isomers including four branched (4-NP(38), 4-NP(65), 4-NP(111), and 4-NP(112)) and one linear (4-NP(1)) isomers in a rice paddy soil was studied under oxic conditions. Degradation followed an availability-adjusted first-order kinetics with the decreasing order of half-life 4-NP(111) (10.3 days) > 4-NP(112) (8.4 days) > 4-NP(65) (5.8 days) > 4-NP(38) (2.1 days) > 4-NP(1) (1.4 days), which is in agreement with the order of their reported estrogenicities. One metabolite of 4-NP(111) with less polarity than the parent compound occurred rapidly and remained stable in the soil. At the end of incubation (58 days), bound residues of 4-NP(111) amounted to 54% of the initially applied radioactivity and resided almost exclusively in the humin fraction of soil organic matter, in which chemically humin-bound residues increased over incubation. Our results indicate an increase of specific estrogenicity of the remaining 4-NPs in soil as a result of the isomer-specific degradation and therefore underline the importance of understanding the individual fate (including degradation, metabolism, and bound-residue formation) of isomers for risk assessment of 4-NPs in soil. 4-NP(1) should not be used as a representative of 4-NPs for studies on their environmental behavior.

By using dialysis equilibrium experiments, the sorption of a branched nonylphenol isomer [4-(1-ethyl-1,3-dimethylpentyl)-phenol] (NP111) on various humic acids (HAs) isolated from river sediments and two reference HAs was studied. The HAs were characterized by solid-state 13 C direct polarization/magic angle spinning nuclear magnetic resonance ( 13 C DP/MAS NMR) spectroscopy. Sorption isotherms of NP111 on HAs were described by a linear model. The organic carbon-normalized sorption coefficient ( K OC ) ranged from 2.3 × 10 3 to 1.5 × 10 4 L kg −1 . Interestingly, a clear correlation between K OC value and alkyl C content was observed, indicating that the aliphaticity of HAs markedly dominates the sorption of NP111. These new mechanistic insights about the NP111 sorption indicate that the fate of nonylphenols in soil or sediment depends not only on the content of HA, but also on its structural composition.

Electrochemistry (EC) coupled to mass spectrometry (MS) has already been successfully applied to metabolism research for pharmaceutical applications, especially for the oxidation behaviour of drug substances. Xenobiotics (chemicals in the environment) also undergo various conversions; some of which are oxidative reactions. Therefore, EC-MS might be a suitable tool for the investigation of oxidative behaviour of xenobiotics. A further evaluation of this approach to environmental research is presented in the present paper using sulfonamide antibiotics. The results with sulfadiazine showed that EC-MS is a powerful tool for the elucidation of the oxidative degradation mechanism within a short time period. In addition, it was demonstrated that EC-MS can be used as a fast and easy method to model the chemical binding of xenobiotics to soil. The reaction of sulfadiazine with catechol, as a model substance for organic matter in soil, led to the expected chemical structure. Finally, by using EC-MS a first indication was obtained of the persistence of a component under chemical oxidation conditions for the comparison of the oxidative stability of different classes of xenobiotics. Overall, using just a few examples, the study demonstrates that EC-MS can be applied as a versatile tool for mechanistic studies of oxidative degradation pathways of xenobiotics and their possible interaction with soil organic matter as well as their oxidative stability in the environment. Further studies are needed to evaluate the full range of possibilities of the application of EC-MS in environmental research.

Toxic 2,4-dichlorophenol (DCP) molecules, originating from the environment, drastically affect the main functions of organic cell membranes. To yield some information on the destroyed membrane structures, 1,2-dipalmitoyl-snglycero-3-phosphatidylcholine (DPPC)/water liposome systems containing different amounts of DCP were studied by differential scanning calorimetry and small-angle X-ray scattering (SAXS) methods. The transition points of the main transition and its enthalpy are changed only in a relatively narrow range of about 0–2.5%, while the enthalpy of the pretransition is affected by DCP more strongly. The disappearance of the pretransition occurs in a range of about 3 orders of magnitude of the DCP/DPPC ratio (from 10−5 to 10−2). The changes in the first Bragg profiles of the SAXS curves depend not only on the temperature but also very strongly on the DCP/DPPC molar ratios. Especially, at higher concentrations DCP causes a strong broadening in the Bragg profiles and a difference from the profiles of the nonrippled phase. The general destruction in the layer arrangements and the reductions in the transition enthalpies between them indicate defect structures consisting of domains with nonlayer structure which are formed simultaneously by DCP molecules.

The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated. The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base titration. The intrinsic equilibrium constants for surface charge formation were logK a,1 int=6.7±0.2 and logK a,2 int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK 1 = 3.7±0.1 and pK 2 = 6.6±0.1 and the total acidity was 4.56 mmol g−1. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the aqueous phase is negligible (less than 10−5 mol dm−3) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5, where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm−3 NaNO3. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm−3 salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5 and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed.