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Inappropriate treatments for the effluents from semiconductor plants might cause the releases and wide distributions of selenium (Se) into the ecosystems. In this study, Al/Si and Fe/Si coprecipitates were selected as model adsorbents as they often formed during the wastewater coagulation process, and the removal efficiency of selenite (SeO3) and selenate (SeO4) onto the coprecipitates were systematically examined. The removal efficiency of SeO3 and SeO4 was highly related to surface properties of Al/Si and Fe/Si coprecipitates. The surface-attached Al shell of Al/Si coprecipitates shielded a portion of negative charges from the core SiO2, resulting in a higher point of zero charge than that of Fe/Si coprecipitates. Thus, adsorption of SeO3/SeO4 was favorable on the Al/Si coprecipitates. Adsorptions of both SeO3 and SeO4 on Al/Si coprecipitates were exothermic reactions. On Fe/Si coprecipitates, while SeO3 adsorption also showed the exothermic behavior, SeO4 adsorption occurred as an endothermic reaction. The kinetic adsorption data of SeO3/SeO4 on Al/Si and Fe/Si coprecipitates were described well by the pseudo-second-order kinetic model. SeO4 and SeO3 adsorption on Fe/Si or Al/Si were greatly inhibited by the strong PO4 ligand, whereas the weak ligand such as SO4 only significantly affected SeO4 adsorption. The weakest complex between SeO4 and Al was implied by the essentially SeO4 desorption as SeO4/PO4 molar ratios decreased from 0.5 to 0.2. These results were further confirmed by the less SeO4 desorption (41%) from Fe/Si coprecipitates than that from Al/Si coprecipitates (78%) while PO4 was added sequentially.

Information on phosphorus (P) adsorption and its impacts on the redistribution of the P fraction in soil profiles are important for environmental management under intensive agricultural practices. To clarify the dominant factors influencing soil phosphorus adsorption in an Entisol (locally known as purple soil), P adsorption experiments were conducted in Sichuan Basin of southwestern China for cropland and woodland soils with acidic, neutral and calcareous origins throughout their profile. After various doses of P were added during incubation experiments, soil P fractions were also analyzed. The results showed that there were no significant differences in Fe-oxides and P adsorption along the vertical gradients. Agricultural practices and lower pH conditions reduced the P adsorption capacity of purple soils throughout the soil profiles. For acidic and neutral purple soil profiles, the P adsorption capability was mainly influenced by Fe-oxides and soil texture. Ca-bound P and Fe-Al-bound P represented the majority of the total inorganic P of calcareous soils. There was a saturation of adsorption capacity by sesquioxide and a high risk of dissoluble reactive P (NH4Cl-P) being released out of the soil profile in acidic and neutral purple soils after the greatest P addition, indicated by the higher proportions of NH4Cl-P (over 40%) and decreasing Fe-Al-P fraction. P fractions migrated with greater difficulty in calcareous purple soil profiles as Ca-P fraction peaked over 65% when adding a P dose at or greater than 80 g P kg-1, indicating the high potential of P adsorption. The X-Ray Diffraction analysis also verified the formation of brushite. Adaptive management practices should be designed to alleviate P losses for acidic and neutral purple soils.

Hydrotalcite (HT) is a layered double hydroxide (LDH), which is considered as a potential adsorbent to remove anion contaminants. In this study, adsorption of chromate (CrO4) and phosphate (PO4) on HT was conducted at various pH and temperatures. Related adsorption mechanisms were determined via the isotherm, kinetic, and competitive adsorption studies as well as the Cr K-edge X-ray absorption fine-structure (XAFS) spectroscopy. The maximum adsorption capacities for CrO4 and PO4 on HT were 0.16 and 0.23 mmol g−1. Regarding adsorption kinetics, CrO4 and PO4 adsorption on HT could be well described by the second order model, and the rate coefficient of CrO4 and PO4 on HT decreased significantly with the increasing pH from 5 to 9. The adsorption kinetics for CrO4 and PO4 were divided into fast and slow stages with the boundary at 15 min. This biphasic adsorption behavior might be partially attributed to multiple reactive pathways including anion exchange and surface complexation. Fitting results of Cr K-edge EXAFS analysis showed a direct bonding between CrO4 and Al on HT surfaces. Such a surface complexation appeared to be the rate-limiting step for CrO4 adsorption on HT. By contrast, the diffusion through the hydrated interlayer space of HT was the major rate-limiting step for PO4. This study determined the adsorption behaviors of CrO4 and PO4 on HT, including the initial transfer process and the subsequent adsorption mechanisms. Such information could improve the strategy to use HT as the potential adsorbent for the remediation of anionic pollutants.

Removal of arsenic (As) from water supplies is needed to reduce As exposure through drinking water and food consumption in many regions of the world. Magnetite nanoparticles (MNPs) are promising and novel adsorbents for As removal because of their great adsorption capacity for As and easy separation. This study aimed to investigate the adsorption mechanism of arsenate, As(V), and arsenite, As(III), on MNPs by macroscopic adsorption experiments in combination with thermodynamic calculation and microspectroscopic characterization using synchrotron-radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS spectra suggested predominant formation of bidentate binuclear corner-sharing complexes ((2)C) for As(V), and tridentate hexanuclear corner-sharing ((3)C) complexes for As(III) on MNP surfaces. The macroscopic and microscopic data conclusively identified the formation of inner-sphere complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the oxidation of MNPs, the oxidation of As(III) and MNPs was expected, but the observed As(V) reduction was surprising because of the role played by the reactive Fe(II).

Soil clays (< 2,000 nm) (SC) and soil nanoclays (< 100 nm) (SNC) were used as adsorbents for removal of Cu(II) from aqueous solution. The experiments were conducted with variables including pH, interaction time, concentration of Cu(II) and temperature. Four kinetic models have been employed to investigate adsorption mechanisms, and the experimental data more closely resemble a second-order process of the kinetic model. Adsorption studies on soil nanoclays have been shown to be highly effective in removing of Cu(II) from aqueous solution. This adsorbent is widely available as a natural material, is mechanically stable and, most importantly, it is environmentally appealing. The maximum Cu(II) adsorption capacity of soil nanoclays (31.7 mg/g) is more than three times higher than natural soil clays (10.2 mg/g). Our study demonstrates that soil nanoclays can be used effectively for removal of Cu(II) from aqueous systems to achieve environmental cleaning purposes.

The products of Al hydrolysis play significant roles in terrestrial and aquatic environments, particularly through their binding of nutrients and pollutants. However, the effect of catechins on the crystallization of Al precipitates and the resultant surface alteration of Al transformation products on their kinetics and mechanisms of adsorption of Cd still remain to be uncovered. The objective of this paper was to investigate the kinetics of Cd adsorption on the Al precipitates formed under the influence of catechin. The experiments were conducted with initial catechin/Al molar ratios, interaction time, concentration of Cd ions, and temperature as the variables. Six kinetic models were employed to investigate adsorption mechanisms, and data resembled more closely a second-order process. The structural perturbation of Al precipitates by catechin, resulting in the development of their microporosity and alteration of surface and charge properties, substantially enhanced the rate constants of both fast and sl...

Wine processing waste sludge (WPWS) has been shown to be an effective sorbent for sorption of nickel, lead, and chromium, but the sorption of copper (Cu) in aqueous solution by WPWS has not been conducted. The objective of this study was to explore the sorption mechanism of WPWS for copper. Infrared analysis revealed carboxyl was the major functional group inWPWS. The WPWS sorption isotherms of copper were only well described by Langmuir sorption isotherm. The maximum adsorption capacity (Qm) was 14.26 mg/g at 50 8C. A pseudo- second-order sorption kinetic model successfully described the kinetics of copper sorption onto WPWS. The Gibb free energies (DG8) ranged from � 20.69 to � 24.29 kJ mol -1 , and the DH8 and DS8 were 5.048 kJ mol -1 and 91.05 J mol -1 K -1 , respectively. The trend of the intra-particular diffusion rate is the opposite of the adsorption constant of the pseudo- second-order equation. Water Environ. Res., 84, 733 (2012).

Dye wastewaters usually contain toxins and high chroma, making them difficult to treat with biological methods. The adsorption process plays an important role in removing dyes from wastewaters. This study aimed to explore the methylene blue (MB) adsorption mechanism by wine-processing waste sludge (WPWS). The WPWS contains a high cation-exchange capacity (64.2 cmol c kg −1 ) and organic matter (52.8%). The parameters affecting MB adsorption included pH, initial concentration of MB, reaction temperature, particle size and dosage of WPWS. The WPWS adsorption isotherms of MB were only well described by Langmuir adsorption isotherm. The maximum adsorption capacity ( Q m ) of MB was 285.7 mg g −1 at 25 °C. The activation energy determined by Arrhenius equation is 29.995 kJ mol −1 . Under steady-state reaction conditions, the Gibb free energy (Δ G °) ranged from −24.607 to −27.092 kJ mol −1 and Δ H ° was −8.926 kJ mol −1 , indicating that lower reaction temperature would favor MB adsorption. Therefore, MB adsorption by WPWS was a spontaneous, exothermic and physisorption reaction.

Humic substances are a major component of soil organic matter that influence the behavior and fate of heavy metals such as Cr(VI), a toxic and carcinogenic element. In the study, a repetitive extraction technique was used to fractionate humic acids (HAs) from a peat soil into three fractions (denoted as F1, F2, and F3), and the relative importance of O-containing aromatic and aliphatic domains in humic substances for scavenging Cr(VI) was addressed at pH 1. Spectroscopic analyses indicated that the concentrations of aromatic C and O-containing functional groups decreased with a progressive extraction as follows: F1 > F2 > F3. Cr(VI) removal by HA proceeded slowly, but it was enhanced when light was applied due to the production of efficient reductants, such as superoxide radical and H2O2, for Cr(VI). Higher aromatic- and O-containing F1 fraction exhibited a greater efficiency for Cr(VI) reduction (with a removal rate of ca. 2.89 mmol g−1 HA under illumination for 3 h). 13C NMR and FTIR spectra further demonstrated that the carboxyl groups were primarily responsible for Cr(VI) reduction. This study implied the mobility and fate of Cr(VI) would be greatly inhibited in the environments containing such organic groups.

Purpose Biochar amendments can alter phosphorus (P) availability in soils, though the influencing mechanisms are not yet fully understood. This work investigated the adsorption and desorption of P on ferrihydrite (F, a Fe-oxide widely distributed in surface environments) in order to evaluate the interactions between P and Fe-oxide in the absence or presence of biochar (F or ferrihydrite–biochar (F–B) interaction) in soils.

Wine processing waste sludge (WPWS) has shown effective removal of heavy metals in batch studies, but the adsorption in a fixed-bed WPWS column has not been examined and the mass transport in this column remains unclear. This study aimed to investigate: (1) the retardation factor ( R ) and the dispersion coefficient ( D ) of WPWS, employing one-dimensional convective–dispersive model with breakthrough curve (BTC) and using chromium, copper and zinc as the adsorbates; and (2) explore the adsorption mechanism of WPWS column for heavy metals. The retardation factor for various metal treatments is shown in the following order: Cr(18.3) > Cu(13.0) > Zn(7.2), indicating their binding affinity toward WPWS. The dispersion coefficients were calculated to be 2.36 × 10 −6 , 5.89 × 10 −7 and 6.83 × 10 −7 m 2 s −1 for Cr, Cu and Zn treatments, respectively. The high charge density for Cr 3+ ion makes it longer for the Cr column to attain exhaustion state, about 4.0 and 8.2 times the time required for the Cr and Zn columns. However, the least retardation and adsorption amount for Zn can be attributed to its full orbital in electronic configuration.

Chemical surfactants or bio-surfactants intercalated into layered double hydroxides (LDHs) are considered as potential organic pollutant sorbents. This study evaluates the effects of (1) using either calcined or uncalcined LDH, and various solid/solution ratios, (2) the concentrations of the chemical surfactant sodium dodecyl sulfate (SDS) or the bio-surfactant rhamnolipid (RL), and (3) reaction temperatures and times on the synthesis of SDS-LDH and RL-LDH. Small angle X-ray diffraction (SAXRD) and X-ray diffraction (XRD) patterns reveal that using 1000 mg L−1 RL intercalated 2:1 uncalcined LDH with 3 days reaction time, 0.5 g L−1 solid/solution ratio, and temperature of 65 °C to synthesize the RL-LDH, containing more organic carbon content, showing larger interlayer distance than SDS-LDH. RL-LDH shows subsequent higher efficiency of naphthalene sorption than SDS-LDH, depending on higher organic carbon contents and larger interlayer distance of RL-LDH. Since RL-LDH is environmentally friendly chemical, these results are of practical interesting in RL-LDH could be potentially employed as a sorbent for removal organic contaminants from aqueous solutions, and particularly in optimizing aquatic environment remediation technologies.

Interactions of toxic Cr(VI) with renewable biomaterials are considered an important pathway for Cr(VI) removal in ecosystems. Biomaterials are susceptible to dissolution, and their dissolved derivatives may provide an alternative to surface-involved pathway for scavenging of Cr(VI). In this study, dissolved organic carbon (DOC) derived from Neurospora crassa biomass was investigated. The proportion of Cr(VI) reduction by DOC to that on biomass was determined to evaluate the importance of DOC to Cr(VI) reduction. A rapid increase in DOC concentration from 145.6 to 193.7 mg L(-1) was observed when N. crassa-biomass was immersed in 0.01 M KCl solution at pH of 1-5, and polysaccharides, peptides, and glycoproteins with carboxyl, amide, and -NH functional groups, are the major compositions of DOC. On reaction of 96.2 microM Cr(VI) with N. crassa-biomass or DOC, it was estimated that DOC contributed approximately 53.8-59.5% of the total Cr(VI) reduction on biomass in the dark. Illumination enhanced Cr(VI) reduction via photo-oxidation of biomass/DOC under aeration conditions, which formed superoxide for Cr(VI) reduction. At pH 1, photoinduced Cr(VI) reduction by DOC proceeded more rapidly than reduction on the biomass surface. However, at pH >3, with a decrease in Cr(VI) reduction by DOC, photon-excited biomass may become an important electron source for Cr(VI) photoreduction.

The dynamics of Cs and Sr sorption by soils, especially in the subtropics and tropics, as influenced by soil components are not fully understood. The rates and capacities of Cs and Sr sorption by selected subtropical and tropical soils in Taiwan were investigated to facilitate our understanding of the transformation and dynamics of Cs and Sr in soils developed under highly weathering intensity. The Langmuir isotherms and kinetic rates of Cs and Sr sorption on the Ap1 and Bt1 horizons of the Long-Tan (Lt) and the A and Bt1 horizons of the Kuan-Shan (Kt), Mao-Lin (Tml) and Chi-Lo (Cl) soils were selected for this study. Air-dried soil (2mm) samples were reacted with of 7.5 x 10(-5) to 1.88 x 10(-3)M of CsCl (pH 4.0) or 1.14 x 10(-4) to 2.85 x 10(-3)M of SrCl(2) (pH 4.0) solutions at 25 degrees C. The sorption maximum capacity (q(m)) of Cs by the Ap1 and Bt1 horizons of the Lt soil (62.24 and 70.70 mmol Cs kg(-1) soil) were significantly (p0.05) higher than those by the A and Bt1 horizons of the Kt and Cl soils (26.46 and 27.49 mmol Cs kg(-1) soil in Kt soil and 34.83 and 29.96 mmol Cs kg(-1) soil in Cl soil, respectively), however, the sorption maximum capacity values of the Lt and Tml soils did not show significant differences. The amounts of pyrophosphate extractable Fe (Fe(p)) were correlated significantly with the Cs and Sr sorption capacities (for Cs sorption, r(2)=0.97, p1.0 x 10(-4); for Sr sorption, r(2)=0.82, p2.0 x 10(-3)). The partition coefficient of radiocesium sorbed on soil showed the following order: Cl soilKt soilTml soilLt soil. It was due to clay minerals. The second-order kinetic model was applied to the Cs and Sr sorption data. The rate constant of Cs or Sr sorption on the four soils was substantiality increased with increasing temperature. This is attributable to the availability of more energy for bond breaking and bond formation brought about by the higher temperatures. The rate constant of Cs sorption at 308 K was 1.39-2.09 times higher than that at 278K in the four soils. The activation energy of Cs and Sr sorbed by the four soils ranged from 7.2 to 16.7 kJ mol(-1) and from 15.2 to 22.4 kJ mol(-1), respectively. Therefore, the limiting step of the Cs(+) or Sr(2+) sorption on the soils was diffusion-controlled processes. The reactive components, which are significantly correlated with the Langmuir sorption maxima of Cs and Sr by these soils, substantially influenced their kinetic rates of Cs and Sr sorption. The data indicate that among components of the subtropical and tropical soils studied, short-range ordered sesquioxides especially Al- and Fe-oxides complexed with organics play important roles in influencing their capacity and dynamics of Cs and Sr sorption.

Lithium/aluminum layered double hydroxide intercalated by chloride (Li/Al LDH-Cl) was considered as a superior adsorbent for anionic compounds. This study aimed to establish the adsorption behavior of arsenate on Li/Al LDH-Cl by employing X-ray absorption spectroscopy (XAS) and adsorption kinetics. The XAS analysis demonstrated that inner-sphere complexes were the dominant arsenate adsorption configurations on the planar surfaces and edges of Li/Al LDH-Cl. Based on a kinetic study, arsenate adsorption on Li/Al LDH-Cl could be separated into fast and slow reactions. This biphasic arsenate adsorption behavior was partially attributable to: (i) two different adsorption sites associated with Li, exposing on planar surfaces, and Al, existing on the edges of double hydroxyl layers, and (ii) micropore adsorption sites within the Li/Al LDH-Cl surfaces. Activation energies derived by the Arrhenius equation indicated that the diffusion process was the rate-limiting step of arsenate adsorption on Li/Al LDH-Cl.

Wine-processing waste sludge (WPWS) has been shown to have powerful potential for sorption of some heavy metals (i.e., chromium, lead and nickel) in single-component aqueous solutions. But although most industrial wastewater contains two or more toxic metals, there are few sorption studies on multicomponent metals by WPWS. This study has two goals: (i) conduct competitive adsorption using Cr, Cu and Zn as sorbates and examine their interaction in binary or ternary systems; and (ii) determine the effects of temperature on the kinetic sorption reaction. The sludge tested contained a high amount of organic matter (38%) and had a high cation exchange capacity (CEC, 255 cmol(c)kg(-1)). Infrared analysis reveals that carboxyl is the main functional group in this WPWS. The (13)C NMR determination indicates alkyl-C and carboxyl-C are major organic functional groups. At steady state, there are about 40.4% (Cr), 35.0% (Cu) and 21.9% (Zn) sorbed in the initial 6.12 mM of single-component solutions. Only pseudo-second-order sorption kinetic model successfully describes the kinetics of sorption for all experimental metals. The rate constants, k(2), of Cr, Cu and Zn in single-component solutions are 0.016, 0.030 and 0.154 g mg(-1)min(-1), respectively. The sorption of metals by WPWS in this competitive system shows the trend: CrCuZn. Ions of charge, hydrated radius and electronic configuration are main factors affecting sorption capacity. The least sorption for Zn in this competitive system can be attributed to its full orbital and largest hydrated radius. Though the effect of temperature on Zn sorption is insignificant, high temperature favors the other metallic sorptions, in particular for Cr. However, the Cr sorption is lower than Cu at 10 degrees C. The Cr sorption by WPWS can be higher than that of Cu at 30 degrees and 50 degrees C.

Arsenate is one of the main contaminants threatening water supplies all over the world. Layered double hydroxides (LDHs) have large surface areas and high anion exchange capacities, so they may be used to develop rapid and cost-effective methods for scavenging As from contaminated water. In order to enhance arsenate adsorption of layered double hydroxides, it is essential to have a fundamental understanding of the interactions between arsenate and LDHs. In this study, the effects of layer charge density and interlayer nitrate orientation of Mg/Al–NO 3 LDHs on the preferential adsorption of arsenate were investigated. Mg/Al–NO 3 LDHs with a Mg:Al ratio of 2:1, 3:1 and 4:1 were synthesized, using a constant-pH co-precipitation method. The adsorption maxima of As on 2:1, 3:1 and 4:1 LDHs were 1.56, 1.08 and 0.36 mmol g − 1 , respectively, following the order of their layer charge densities. In addition, nitrate orientation in the LDHs plays a significant role in determining arsenate adsorption. The low arsenate adsorption of 4:1 LDH relative to its anion exchange capacity resulted from the low degree of access arsenate has to the interlayer space, so that arsenate adsorption largely occurred on the external surface of the material. The accessibility of the interlayer of 4:1 LDH to arsenate was restricted by the small basal spacing as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheets. Conversely, interlayer nitrate with an orientation perpendicular to the hydroxide sheets in 2:1 and 3:1 LDHs can be exchanged for arsenate more readily. Consequently, these two LDHs exhibit greater arsenate adsorption capacity, because arsenate can be adsorbed on both their external and interlayer surfaces. The arsenate adsorption of the LDHs also was highly dependent upon the type and concentration of competing anions. The negative effect of the competing anions on arsenate adsorption by various LDHs generally followed the order H 2 PO 4 − > HCO 3 − > SO 4 2− > F − > Cl − . Due to the difference in adsorption mechanism, the arsenate adsorption of 4:1 LDH is subject to the negative impact of competing anions to a greater extent than those of 3:1 and 2:1 LDH. These results demonstrated that the preferential adsorption of arsenate by LDHs is dependent upon the nitrate orientation in these materials, as a consequence of changing layer charge density. The selective retention of arsenate for wastewater treatment can be maximized by controlling the orientation of interlayer nitrate in Mg/Al–NO 3 LDHs.

This study focused on the sorption isotherms of 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane (p,p'-DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (p,p'-DDE) on different original clays (i.e., zeolite, montmorillonite and attapulgite) and organoclay complexes. Sorption of organic pollutants was determined using gas chromatographic (GC) techniques to investigate the sorption behavior, and characterize the effect of, different organic cations. The original clays only sorbed low amounts of p,p'-DDT and p,p'-DDE, and the sorptive curves can be classified as L-shaped. Organoclays exhibited higher amounts of p,p'-DDT and p,p'-DDE sorption. The p,p'-DDT and p,p'-DDE sorption increased with increasing total organic carbon (OC) content of the organoclays. For hexadecyltrimethylammonium (HDTMA)-modified organoclays, the dominant adsorptive medium showed the partitioning sorption of hydrophobic-hydrophobic interaction, indicating no competitive sorption. The sorptive curves can be classified as C-shaped of constant partition (CP). However, benzyltrimethylammonium (BTMA)-modified organoclays exhibited competitive sorption. The sorption isotherm curves can be classified as S-shaped. The sorptive capacity of the HDTMA-modified organoclays for p,p'-DDT were higher than those for p,p'-DDE, but the BTMA-modified organoclays showed a reverse trend. This can be attributed to the different structures and shapes of organic cations, giving different sorptive mechanisms. The p,p'-DDT and p,p'-DDE sorption onto HDTMA-modified organoclays were caused by chemical interaction, with the BTMA modified organoclays occuring due to physical sorption.

The aims of this study were to make use of organo-clays (i.e., Cloisite-10A, Cloisite-15A, Cloisite-30B and Cloisite-93A), to remove p -nitrophenol, phenol and aniline of organic pollutants. The organo-clays were characterized by X-ray diffraction (XRD). Sorption isotherm, kinetic and pH effect of p -nitrophenol, phenol and aniline sorbed by four organo-clays were evaluated. The d -spacings (0 0 1) of the XRD peak of Cloisite-10A, Cloisite-15A, Cloisite-30B and Cloisite-93A are 1.98, 2.76, 1.93 and 2.64 nm, respectively. The d (0 0 1)-spacings of XRD indicated that these p -nitropheno, phenol and aniline could penetrate into the interlayer of clays and expand the d (0 0 1)-spacings. The linear sorption isotherm of constant partition was employed to describe the sorption isotherms of phenols sorbed by organo-clays through hydrophobic–hydrophobic chemical reactions. The parabolic diffusion and power-function of kinetic models were employed to describe properly the kinetic experiments. The rate limiting step of the p -nitrophenol sorption reactions on organo-clays were diffusion-controlled processes (i.e., 15A, 30B, 93A) and chemical-controlled process for 10A organo-clays. The pre-exponential factor of the p -nitrophenol sorbed by four organo-clays showed the trend as follows: 10A > 30B > 93A > 15A. The efficiency of these organo-clays in removing phenol compounds in water treatments merit further study.

Goethite widely occurs in the environment and its role in photoreduction of toxic Cr(VI) to less toxic Cr(III) has been previously reported in the literature. However, natural goethite is rarely stoichiometric and usually contains certain substitutional cations, such as Mn(III) or Mn(IV), which, on the other hand, is an oxidant for Cr(III). In order to provide more insight into the fate of Cr(VI) in the environment, the influence of Mn-substitution in goethite on its Cr(VI) photoreduction capability needs to be clarified. The objectives of this study were to evaluate the adsorption and photo-transformations of Cr on Mn-substituted goethite with Mn/(Mn + Fe) molar ratios ranging from 0 to 0.1. The results showed that Cr adsorption and photo-catalytic transformation on goethite were strongly influenced by the Mn content of goethite. When the system was under illumination, Cr(VI) reduction was decreased with increasing Mn-substitution in goethite. The re-oxidation of photo-reduced product, i.e., Cr(III), at the Mn sites on the surface of goethite is responsible for the lower Cr(VI) photo-reduction capacity of Mn-substituted goethite. The role of goethite in the reduction of Cr(VI) may be overestimated unless its Mn-substituted counterparts are considered. The effectiveness of Mn sites in goethite to increase toxic Cr(VI) has important implications in determining the fate and threat of Cr(VI) contamination.

The present study describes the photocatalytic degradation of toluene in gas phase on different porous manganese oxide doped titanium dioxide. As synthesized birnessite and cryptomelane type porous manganese oxide were doped with titania and tested for photocatalytic decomposition of toluene in gas phase. The effects of the inlet concentration of toluene, flow rate (retention time) were examined and the relative humidity was maintained constantly. Thermal and textural characterization of manganese oxide doped titania materials were characterized by X-ray diffraction (XRD), thermogravemetry (TG), BET and TEM-EDAX studies. The aim of the present study is to synthesize the porous manganese oxide doped titania and to study its photocatalytic activity for toluene degradation in gas phase. Cryptomelane doped titania catalyst prepared in water medium [K-OMS-2 (W)] is shown the good toluene degradation with lower catalysts loading compared to commercial bulk titania in annular type photo reactor. The higher photocatalytic activity due to various factors such as catalyst preparation method, experimental conditions, catalyst loading, surface area, etc. In the present study manganese oxide OMS doped titania materials prepared by both aqueous and non-aqueous medium, aqueous medium prepared catalyst shows the good efficiency due to the presence of OH bonded groups on the surface of catalyst. The linear forms of different kinetic equations were applied to the adsorption data and their goodness of fit was evaluated based on the R2 and standard error. The goodness to the linear fit was observed for Elovich model with high R2 (>or=0.9477) value.

Aluminum and iron oxides have been often used in the coagulation processes during water purification due to their unique surface properties toward anions. In the presence of silica, the coprecipitation of Al/Si or Fe/Si might decrease the efficiency of wastewater purification and reuse. In this study, surface properties and molecular structures of Al/Si and Fe/Si coprecipitates were characterized using spectroscopic techniques. Also, the selenite removal efficiency of Al/Si and Fe/Si coprecipitates in relation to their surface and structural properties was investigated. While dissolved silicate increased with increasing pH from Fe/Si coprecipitates, less than 7% of silicate was discernible from Al/Si samples over the range from acidic to alkaline conditions. Our spectroscopic results showed that the associations between Al and Si were relatively stronger than that between Fe and Si in coprecipitates. In Al/Si coprecipitates, core-shell structures were developed with AlO6/AlO4 domains as the shells and Si frameworks polymerized from the SiO2 as the cores. However, Si framework remained relatively unchanged upon coprecipitation with Fe hydroxides in Fe/Si samples. The Si core with Al shell structure of Al/Si coprecipitates shielded the negative charges from SiO2 and thereby resulted in a higher adsorption capacity of selenite than Fe/Si coprecipitates.

The organoclays Wyoming montmorillonite (SWy-1, Mon), W. R. Grace vermiculite (Ver), and Tung-Wei (Tw) soil montmorillonite were prepared to sorb or remove chlorophenoxy propionic acids (CPA) pollutants such as 2,4-dichlorophenoxy acid (2,4-DP), 2,4,6-tri chlorophenoxy acid (2,4,6-TCP), and pentachlorophenoxy propionic acid (PCP). The objective of this study was to evaluate the behavior of organoclays that have sorbed three types of CPA and to assess the applicability to basaltic-developed iron-rich calcareous soil. This type of soil contains high amounts of montmorillonite. The mean-layer charge per formula unit of Mon, Ver, and Tw clays, characterized by the alkylammonium method, was 0.27, 0.70, and 0.52 mol(c)/O10(OH)2, respectively. The sorption isotherms of 2,4-DP, 2,4,6-TCP, and PCP sorbed by hexadecyltrimethyl-ammonium (HDTMA organoclay, 150% cation-exchange capacity) were evaluated. The data on sorption of CPAs fit well with the Freundlich isotherm equation. In general, the sorption of three types of CPA by the organoclays showed a linear relationship, with a high correlation coefficient (r > 0.935). The Kf value of PCP was the same as that of 2,4,6-TCP sorbed by the organoclays and was higher than that of 2,4-DP. However, the 1/nf values of the three CPAs sorbed by the organoclays did not show significant differences. In general, maximum CPA sorption occurred at a pH of approximately 3, but very significant differences were observed for the 2,4,6-TCP and PCP sorbed by the organoclays. However, there was no significant difference in pH for 2,4-DP sorption. This should be related to the solubility and pKa of each CPA. The experimental results showed that solubility of 2,4-DP, 2,4,6-TCP, and PCP is a function of pH, ionic strength, and temperature. © 2006 Wiley Periodicals, Inc. Environ Toxicol 21: 71–79, 2006.

Reactions of Al(III) at the interface between SiO2(s) and aqueous solution were characteristically and quantitatively studied using electrophoretic methods and applying a surface complexation/precipitation model (SCM/SPM). The surface and bulk properties of Al(III)/SiO2 suspensions were determined as functions of pH and initial Al(III) concentration. Simulated modeling results indicate that the SCM, accounting for the adsorption mechanism, predicts sorption data for low surface coverage only reasonably well. Al(III) hydrolysis and surface hydroxide precipitation must be invoked as the Al(III) concentration and/or pH progressively increase. Accordingly, the three processes in the Al(III) sorption continuum, from adsorption through hydrolysis to surface precipitation, could be identified by the divergence between the SCM/SPM predictions and the experimental data. SiO2(s) suspensions with low Al(III) concentrations (1 x 10(-4) and 1 x 10(-5) M) exhibit electrophoretic behavior similar to that of a pure SiO2(s) system. In Al(III)/SiO2 systems with high Al concentrations of 1 x 10(-3), 5 x 10(-3) and 1 x 10(-2) M, three charge reversals (CR) are observed, separately representing, in order of increasing pH, the point of zero charge (PZC) on the SiO2 substrate (CR1), the onset of the surface precipitation of Al hydroxide (CR2), and at a high pH, the PZC of the Al(OH)3 coating (CR3). Furthermore, in the 1 x 10(-3) M Al(III)/SiO2(s) system, CR2 is consistent with the modeling results of SCM/SPM and provides evidence that Al(III) forms a surface precipitate on SiO2(s) at pH above 4. SiO2(s) dissolution was slightly inhibited when Al(III) was adsorbed onto the surface of SiO2(s), as compared to the dissolution that occurs in a pure SiO2(s) suspension system. Al hydroxide surface precipitation dramatically reduced the dissolution of SiO2(s) because the Al hydroxide passive film inhibited the corrosion of the SiO2(s) surface by OH- ions.

Many factors affect adsorption phenomena in solid–liquid systems. One of the most important factors is the sorbent/water (S/W) ratio in the system. However, the effect of varying S/W ratios on the adsorption is still unclear. In this study, batch experiments were examined to observe the adsorption of four contaminants (copper, cadmium, Butachlor, and Deltamethrin) in six soils with texture ranging from silty clay to loamy sand and with different S/W ratios. Dimensional analysis was used to assess the relationship between adsorption phenomena and S/W ratio. We have assumed that the total amount of sorbate sorbed in soil is a function of the equilibrium concentration, the volume of sorbate solution, and the sorbent amount in the system. A power function (Freundlich-like) model was obtained from the dimensional analysis. It can describe precisely the adsorption phenomena of different sorbents and sorbates in the moisture regime of paddy soils. Therefore, proper adsorption parameters can be obtained by this power function model regardless of the solids effect, which can then be utilized to describe the fate of solute in soil using solute transport models.

Wyoming montmorillonite (SWy-1, MON), Charng-Bin (SCB) soil montmorillonite, and W.R. Grace vermiculite (VER) have been used as adsorbents to prepare organo-clays. Sorption of 2,4-dichlorophenoxy propionic acid (2,4-DP) by organo-clays is greater after adding of hexadecyltrimethyl-ammonium (HDTMA) to 200% CEC of the clay than after adding the same amount of HDTMA to 100% CEC of clay (HDTMA–clay). The Freundlich adsorption isotherm describes 2,4-DP sorption by HDTMA–clays (Kf=1.828–11.474 l kg−1, nf=l.130–1.454, R=0.919–0.944) quite well. Low surface charge montmorillonite can allow a high concentration of 2,4-DP to penetrate into the interlayer with low residual charge on the wedge. Thus, with increasing 2,4-DP concentration, the basal d-spacings of HDTMA-montmorillonite complexes increased from 2.15 to 2.71 nm. However, increasing the concentration of 2,4-DP did not result in further swelling of vermiculite owing to the high surface charge. In general, HDTMA–montmorillonite (O-MON) sorbed more 2,4-DP than HDTMA–soil montmorillonite (O-SCB) and HDTMA–vermiculite (O-VER). Thus, the interlayer sorption of 2,4-DP exhibited a critical level, beyond which further expansion of the clays does not occur. The equilibrated pH in the suspension affected the adsorption by HDTMA–clay complexes. The maximum 2,4-DP adsorption occurs at a pH level of approximately 3 corresponding to pKa=3.0 of 2,4-DP in the adsorption–pH curve.

Surface fire could induce heat transferring into the soil, creating a carbonized environment, which may alter the chemical compositions of soil organic matters (SOM). In the study, a surface soil was carbonized at up to 600 °C with limited air to simulate soils experiencing a surface fire, and Cr(VI) removal on the carbonized soils was investigated. NMR and FTIR analyses demonstrated a remarkable change of SOM structures at 300-400 °C. TGA-MS spectra indicated that (e.g. C(2)H(4), CH(3)OH and C(3)H(8)) were the major components in the evolved gases from the pyrolyzed soil. A maximum amount of Cr(VI) removal (ca. 4 mg g(-1) soil) occurred for the 200 °C-carbonized soils, attributed mainly to a significant increase of Cr(VI) reduction by 0.1 M KCl extractable organic carbon (EOC) with abundant carboxylic groups. Nonetheless, the formation of aromatic C upon carbonization of the soil at400 °C may be responsible for Cr(VI) reduction.

Biosorption, as an effective and low-cost technology treating industrial wastewaters containing Cr(VI), has become a significant concern worldwide. In this work, acid-modified byproducts of beer production (BBP) were used to remove Cr(VI) from aqueous solutions. Removal of Cr(VI) increases as the pH is decreased from 4.0 to 1.5, but the maximum of total Cr removal is obtained in a pH range from 2.0 to 2.5. Nearly 60% of the initial Cr(VI) (100 mg L −1 ) was adsorbed or reduced to Cr(III) within the first 10 min at pH 2.0. The Cr(VI) removal capability of acid-modified BBP materials was almost completely retained after regenerating with acid. FT-IR and XPS spectra revealed that carboxylate and carboxyl groups on the surface of modified BBP materials play a major role in Cr(VI) binding and reduction, whereas amide and other groups play a minor role in the Cr(VI) removal process.

The role of short-range order (SRO) metal oxides, which are common in acid soils and associated environments, in influencing the abiotic transformations of catechin, which is common in the soil of tea plantations, still remains poorly understood. The aim of this study was to investigate the catalytic power of SRO Mn(IV)-, Fe(III)- and Al-oxides in influencing the abiotic transformations of catechin. At the end of a 90-h reaction period, the release of CO 2 in all the oxide-catechin systems is higher than that for the system with only catechin. Polymerization of catechin is catalyzed and enhanced by SRO-oxides, as is indicated by the absorbance values of the supernatants, which were obtained via visible adsorption spectroscopy, and the yields of humic polymers. The sequence of the oxides that increased the yield of total humic polymers in these systems under ambient atmosphere is: Fe(III)-oxide > Mn(IV)-oxide > Al-oxide ≫ no catalyst (catechin). The electron spin resonance (ESR) and Fourier transformation infrared absorption spectrometry (FT-IR) of humic polymers formed in the oxide-catechin systems were similar to the spectra obtained from the humic polymers extracted from the soil. The catalytic power of SRO-oxides in promoting the oxidative polymerization of catechin, the resultant formation of humic substances, and C turnover in acid soils thus merit attention.

Radio pollutant removal is one of several priority restoration strategies for the environment. This study assessed the effect of low molecular weight organic acid on the lability and mechanisms for release of (137)Cs from contaminated soils. The amount of (137)Cs radioactivity released from contaminated soils reacting with 0.02 M low molecular weight organic acids (LMWOAs) specifically acetic, succinic, oxalic, tartaric, and citric acid over 48 h were 265, 370, 760, 850, and 1002 Bq kg(-1), respectively. The kinetic results indicate that (137)Cs exhibits a two-step parabolic diffusion equation and a good linear relationship, indicating that the parabolic diffusion equation describes the data quite well, as shown by low p and high r(2) values. The fast stage, which was found to occur within a short period of time (0.083-3 h), corresponds to the interaction of LMWOAs with the surface of clay minerals; meanwhile, during the slow stage, which occurs over a much longer time period (3-24 h), desorption primarily is attributed to inter-particle or intra-particle diffusion. After a fifth renewal of the LMWOAs, the total levels of (137)Cs radioactivity released by acetic, succinic, oxalic, tartaric, and citric acid were equivalent to 390, 520, 3949, 2061, and 4422 Bq kg(-1) soil, respectively. H(+) can protonate the hydroxyl groups and oxygen atoms at the broken edges or surfaces of the minerals, thereby weakening Fe-O and Al-O bonds. After protonation of H(+), organic ligands can attack the OH and OH(2) groups in the minerals easily, to form complexes with surface structure cations, such as Al and Fe. The amounts of (137)Cs released from contaminated soil treated with LMWOAs were substantially increased, indicating that the LMWOAs excreted by the roots of plants play a critical role in (137)Cs release.

Removal of selenium oxyanions by the binary oxide systems, Al- or Fe-oxides mixed with X-ray noncrystalline SiO(2), was previously not well understood. This study evaluates the adsorption capacity and kinetics of selenium oxyanions by different metal hydroxides onto SiO(2), and uses X-ray absorption spectroscopy (XAS) to assess the interaction between selenium oxyanions and the sorbents at pH 5.0. The binary oxide systems of Al(III)- or Fe(III)-oxides mixed with SiO(2) were prepared, and were characterized for their surface area, point of zero charge (PZC), pH envelopes, X-ray diffraction analysis (XRD), and then macro-scale adsorption isotherm and kinetics of selenite and selenate, micro-scale adsorption XAS. The adsorption capacity of selenite and selenate on Al(III)/SiO(2) is greater than on Fe(III)/SiO(2). Adsorption isothermal and kinetic data of selenium can be well fitted to the Langmuir isotherm and pseudo-second-order kinetic models. Based on simple geometrical constraints, selenite on both the binary oxide systems forms bidentate inner-sphere surface complexes, and selenate on Fe(III)/SiO(2) forms stronger complexes than on Al(III)/SiO(2).