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Biochar colloids are likely to control the environmental fate of heavy metals because of the widespread application of this carbon-based material in environmental remediation. Low-molecular-weight aromatic acids (LWMAAs), a class of common organic substances in the environment, are critical in affecting biochar colloid-mediated transport of heavy metals in aquifer media. Nevertheless, thus far, little is known about the effects of LWMAAs on their co-mobility behaviors. Herein, the influences of two aromatic acids, including cinnamic acid and caffeic acid, on the biochar colloid-assisted mobility of Cd2+ in quartz sand were examined. Generally, LMWAAs promoted biochar colloid mobility because of the enhanced electrostatic repulsion, competitive retention between LMWAAs and colloids, and steric hindrance. More importantly, biochar colloids could act as carriers for contaminants to considerably facilitate Cd2+ transport, mainly due to the strong binding affinities of colloids toward Cd2+ and the high mobility of colloids. Intriguingly, although the presence of LMWAAs inhibited Cd2+ adsorption onto biochar colloids, the two aromatic acids enhanced the contaminant-mobilizing transport capacity of colloids, owing to the increased colloid mobility and the formation of stable Cd-LMWAA complexes. Furthermore, the promotion effects of caffeic acid on colloid mobility or colloid-mediated transport of Cd2+ were greater than those of cinnamic acid. This pattern was related to the different chemical properties (e.g., molecular size and/or hydrophobicity) of the two LMWAAs. The observations of this paper provide useful information for understating the effects of coexisting LMWAAs on the co-mobility behaviors of carbonaceous materials and heavy metals in the groundwater environment. [Display omitted] • LMWAAs considerably enhance the transport of biochar colloids. • Steric repulsion and deposition site competition are critical mechanisms. • Highly mobile biochar colloids serve as carriers to enhance Cd2+ mobility markedly. • LMWAAs promote the colloid-facilitated transport of Cd2+ via multi-mechanisms. • The improved effect on colloid-assisted mobility was LMWAA-type dependent. [ABSTRACT FROM AUTHOR]

Humic acid (HA), a naturally occurring and widely distributed natural organic matter in the environment, is one of the most important factors controlling the transport and fate of mineral colloids. This study examined the influence of HA (0–10 mg/L) on the transport of goethite and hematite colloids in saturated quartz sand-packed columns under various solution chemistry conditions (including ionic strength and pH). We found that the transport-enhancement effect of HA on goethite colloids was greater than that on hematite colloids because the steric hindrance and electrostatic repulsion between HA-bound goethite colloids and sand stronger than those between HA-bound hematite colloids and sand. Besides, Ca2+ could be a bridging agent between HA and sand; and consequently, enhanced the steric hindrance between iron oxide colloids and sand, which was an important mechanism for the transport-enhancement effect of HA in the presence of Ca2+. Interestingly, with increasing HA concentration from 3 to 10 mg/L, the transport-enhancement effect of HA at higher ionic strength (i.e., 20 mM NaCl) was to a larger extent than that at lower ionic strength (i.e., 5 mM NaCl); meanwhile, the enhancement effect of HA at lower pH (i.e., 5.0) was to a larger extent than that at higher pH (i.e., 9.0). The observations were attributable to the surface charge and aggregation properties of HA-bound iron oxide colloids affected by solution chemistry coupled with HA concentration. Furthermore, DLVO theory extended by steric repulsion (XDLVO) could explain the observed trend of colloid transport under the test conditions. [ABSTRACT FROM AUTHOR]

The ionic liquids may have an impact on the interactions between fluoroquinolone antibiotics and clay minerals, which are widespread in nature. This work evaluated the adsorption properties of levofloxacin onto three clay minerals (i.e., montmorillonite, vermiculite, and kaolinite) affected by three types of soluble imidazolium-based ionic liquids with different chain lengths of cations ([C 2 mim]Cl, [C 4 mim]Cl, and [C 6 mim]Cl). The results demonstrated that the levofloxacin-binding capacity varied according to clay type (i.e., montmorillonite > vermiculite > kaolinite) regardless of ionic liquids-free or ionic liquids-addition. The observation was made concerning the physical-chemical characteristics of different clay minerals (e.g., physical structure, surface functional groups, specific surface area, and cation exchange capacity). Interestingly, ionic liquids suppressed levofloxacin adsorption on the three test clay minerals because of steric hindrance effects induced by the adsorbed ionic liquids as well as the site competition during co-adsorption of levofloxacin molecules and cations of ionic liquids. Additionally, the inhibitory impacts of ionic liquids in the sequence of [C 6 mim]Cl > [C 4 mim]Cl > [C 2 mim]Cl stemmed from steric hindrance and site competition during co-adsorption of ionic liquids and levofloxacin increased with chain lengths of ionic liquids. Another interesting finding was that the extent of inhibitory effects of [C 4 mim]Cl increased with the rise of pH values from 5.0 to 9.0, owing to the greater adsorption competition and steric effects derived from more adsorbed [C 4 mim]+ at higher pH conditions. The findings of this study are critical in assessing the effects of emerging contaminants on the fate of antibiotics in eco-environmental systems. [Display omitted] • The affinities of clays towards LEV are montmorillonite > vermiculite > kaolinite. • ILs inhibit LEV adsorption onto clays due to steric effect and site competition. • Adsorption-inhibition effects of ILs are dependent on the alkyl chain lengths. • The extent of adsorption-inhibition effects is solution pH-dependent. [ABSTRACT FROM AUTHOR]

The ionic liquids (ILs) can influence the adsorption of antibiotics on minerals but there was ambiguity about mechanisms. Herein, we demonstrated the adsorption characteristics of ciprofloxacin (CIP) onto ferrihydrite and montmorillonite by various soluble ILs. Five imidazolium-based ILs with different alkyl chain lengths ([C 2 mim]Cl, [C 4 mim]Cl, [C 6 mim]Cl) and counteranions ([C 4 mim][TOS] and [C 4 mim][PF 6 ]) were employed. The results validated that ILs could suppress CIP adsorption on both minerals due to the adsorption site competition between cations and CIP molecules, steric hindrance induced by long alkyl chains of ILs, and electrostatic interaction. Interestingly, the adsorption-inhibition effects of ILs were almost unchanged with the lengthening alkyl chain for ferrihydrite, which were ascribed to the comparable adsorption capacities of the three ILs on ferrihydrite. Whereas, the effects of ILs on CIP adsorption onto montmorillonite followed the order of [C 6 mim]Cl > [C 4 mim]Cl > [C 2 mim]Cl owing to the adsorption site competition and steric hindrance effects on the ILs increased with the lengthening alkyl chain. Furthermore, the inhibitory effects of ILs on CIP adsorption onto both studied minerals were also dependent on the counteranions (kept the same cation ([C 4 mim]+)) and in the following order of [C 4 mim][PF 6 ] > [C 4 mim][TOS] > [C 4 mim]Cl, which was mainly ascribed to the differences of anions in the ion sizes and the binding affinities to minerals. Additionally, the extent of the inhibitory impact of [C 4 mim][TOS] on CIP adsorption onto ferrihydrite declined with increasing solution pH from 5.0 to 9.0, which was attributed to the amounts of adsorbed [TOS]– declined with increasing pH. Contrarily, for montmorillonite, the extent increased with increasing pH, owing to increase in adsorption site competition and steric effect at higher pH conditions. Our findings emphasize the significance of emerging contaminants in understanding the antibiotic environmental behavior in eco-environmental systems. [Display omitted] • Adsorption-inhibition mechanisms of ILs on CIP adsorption varies with mineral types. • The adsorption-inhibition effects of ILs are less controlled by alkyl chain length. • Effects of ILs on CIP adsorption onto montmorillonite depend on alkyl chain length. • Adsorption-inhibition effects of ILs are counter anions-dependent. • The extent of adsorption-inhibition effects strongly varies with solution pH. [ABSTRACT FROM AUTHOR]

There is little understanding of how anionic surfactants affect the transport of carbon dots (CDs) nanoparticles in soil. In this study, the mobility characteristics of CDs in soil columns with sodium dodecyl sulfate (SDS, a model anionic surfactant) under various conditions were investigated. The results showed that the presence of SDS enhanced CDs mobility at pH 7.0 when Na+ was the main background electrolyte. The increased electrostatic repulsion, the competitive deposition between SDS and nanoparticles for the sites on soil particle surfaces, the decreased straining effect, and the steric hindrance were all attributed to trend. Interestingly, in the presence of divalent metal cation (i.e., Ca2+ or Cu2+), SDS suppressed the mobility of CDs. This phenomenon was most likely stemmed from the precipitation of scum (i.e., SDS–Cu2+/Ca2+ complexes) and adsorbed SDS-metal-CDs complexes. Furthermore, the extent of the enhanced effects of SDS on CDs mobility decreased with increasing solution pH from 5.0 to 9.0, which was primarily due to a decrease in the steric effect and deposition site competition. A two-site transport model was also applied to fit the observed mobility data of CDs in soil media. Overall, findings from this work provide meaningful insights into the fate of co-existing carbon-based nanoparticles and anionic surfactants in the subsurface systems. [Display omitted] • Effect of sodium dodecyl sulfate (SDS) on the mobility of CDs in soil was studied. • SDS enhances CDs transport through soil columns at pH 7.0 through multi-mechanisms. • SDS inhibits CDs mobility in the presence of Cu2+/Ca2+ due to the formation of scum. • Lower pH promotes the enhanced effect of SDS on CDs mobility. [ABSTRACT FROM AUTHOR]

Tetracycline (TC) is known to be one of the most widely used antibiotics and is released into the natural environment, where amounts of iron oxides exist. Low-molecular-weight organic acids (LMWOAs), widespread water-soluble organic substances in the eco-environment systems, may affect the interactions between iron oxides and TC molecules. Nevertheless, our knowledge about the impacts of LMWOAs on the adsorption process of TC onto iron oxides still remains incomplete. In this study, we investigated the adsorption characteristics of TC onto hematite (as a model iron oxide) with various LMWOAs (i.e., acetic acid, glycolic acid, oxalic acid, malic acid, and citric acid). The results clearly showed that LMWOAs suppressed TC adsorption onto hematite particles at pH 7.0 (e.g., the maximum adsorption amounts (q max) decreased from 9.05 mg/g (without LMWOAs) to 5.90 mg/g (with 0.5 mM citric acid)), which was mainly ascribed to the competitive adsorption between TC and LMWOAs on the hematite surface, the increased electrostatic repulsion between negatively charged iron oxide particles and TC– species, and the steric hindrance. Additionally, divalent cations (Ca2+ and Cu2+) enhanced the adsorption-inhibition effects by forming cation-bridging with deprotonated LMWOAs. Interestingly, different mechanisms were involved in the adsorption process with increasing pH from 3.0 to 10.0. At pH < 5.0, electrostatic repulsion, adsorption site competition, and steric effect co-controlled the inhibitory effects. Differently, at pH > 5.0, electrostatic repulsion solely dominated the adsorption-inhibition effects. The findings of this study reveal that ubiquitous LMWOAs are critical factors controlling the fate of antibiotics in the natural environment. [Display omitted] • Effects of LMWOAs on TC adsorption onto hematite was studied. • LMWOAs inhibited TC adsorption onto hematite via multi-mechanisms. • The extent of adsorption-inhibition effects depended on organic acid species. • Cation-bridging contributed to inhibitory effects in the presence of Ca2+/Cu2+. • Different mechanisms were involved in the adsorption process with increasing pH. [ABSTRACT FROM AUTHOR]

N 2 and air-limitation environments are two common pyrolysis atmospheres to produce biochars. However, their influence on the sorption mechanism and performance of biochar toward heavy metal ions (HMs) remains unknown. Hence, biochars produced in N 2 and air-limitation environments at 300–750 °C were applied to sorb Cd(II) and Ni(II) in this study. The results show that the maximum adsorption quantities (Q m) of Cd(II) and Ni(II) on biochars of air-limitation pyrolysis were 1.08–1.84 folds and 1.66–3.11 folds, respectively, of those on biochars of N 2 pyrolysis. The Q m of both studied HMs on biochars produced in the two studied atmospheres both generally improved with increasing pyrolysis temperature due to the increased contents of inorganic elements such as P, S, and Si at higher temperatures. Besides, the Q m of both studied HMs on biochars of N 2 pyrolysis improved with the increasing graphitic C content. The results indicate the sorption mechanism of HMs on biochars of air-limitation pyrolysis was dominated by the mineral anions complexation, while the sorption mechanism of HMs on biochars of N 2 pyrolysis was controlled by both the mineral anions complexation and cation-π interaction. Additionally, the protonated oxygen-containing groups (e.g., hydroxyl groups) linked to C fraction possibly formed hydrogen-bonds with mineral anions and thus suppressed the sorption of HMs on biochars. A pyrolysis condition of the maximum adsorption efficiency for HMs is the N 2 pyrolysis at 750 °C. The results of this study are beneficial for selecting suitable pyrolysis methods to produce biochars and raising their efficiency in HMs sorption. [Display omitted] • Higher temperature generally supported greater sorption quantity of HMs on biochars. • Air-limitation (AL) made greater sorption quantity of HMs on biochar than N 2. • Mineral anions complexing controlled the sorption of HMs on biochar of AL pyrolysis. • Cation-π effect was an important sorption mechanism for biochars of N 2 pyrolysis. • The method of the N 2 pyrolysis at 750 °C presents the maximum adsorption efficiency. [ABSTRACT FROM AUTHOR]

Biochar, a cost-effective carbonaceous material, has shown great promise in many applications such as soil remediation and wastewater treatment. Citric acid in soil and water environments may affect interactions between biochar and organic contaminants (e.g., tetracycline). To evaluate the effect of citric acid on the adsorption of tetracycline onto biochars, this study investigated the adsorption behavior of tetracycline onto biochars of two different pyrolysis temperatures (300 and 450 °C, referred to as BC_300 and BC_450) in the present of citric acid, accompanying with the effects of solution pH and Cu2+. The results indicated that citric acid significantly inhibited the adsorption of tetracycline onto both biochars, that was mainly due to the multi-mechanisms including pore blocking effect, surface adsorption sites competition, and steric hindrance induced by citric acid. Moreover, citric acid more significantly suppressed the adsorption of tetracycline onto the two biochars at pH 9.0 than at pH 5.0 and 7.0. Because increasing pH could promote electrostatic repulsion between the negative tetracycline and the negatively charged surface of biochars. Cu2+ had greater enhancing effect on the sorption of tetracycline onto BC_300 than BC_450 via cation-bridging effect. Meanwhile, citric acid inhibited the adsorption of tetracycline onto BC_300 to a larger extent than the adsorption onto BC_450 in the presence of Cu2+. That is possibly due to BC_300 has more surface oxygen-containing functional groups than BC_450. Thus, the effect of coexisting citric acid needs to be taken into account when biochar is increasingly used as an amendment in soil and water systems. [ABSTRACT FROM AUTHOR]