Adsorption of nutrients and cadmium by different minerals: experimental studies and modelling

This work analyses cadmium sorption by natural vermiculite, zeolite, and pumice, which have been recently considered for their potential use in remediation of contaminated soils. Batch experiments were performed to investigate the interactions between the mineral surfaces and the main nutritive cations of Hoagland solution. In addition to their ameliorative effect of reducing metal mobility, these minerals are able to interact to different extents with various components of the nutrient solution and can change the availability of essential nutrients in solution. Therefore, the minerals were also exposed to a range of cadmium doses to compare their different affinities for this element and to analyse the cadmium–nutrient interactions. The results showed that the nutrient solution composition was significantly modified by contact with zeolite and vermiculite, and to a minor extent with pumice. The ionic equilibrium between the solid and aqueous phases was attained at several time points depending on the cation. Ammonium and potassium ions were almost entirely absorbed by zeolite, while in vermiculite endogenous magnesium ions were exchanged with the liquid phase calcium ions. The amount of cadmium adsorbed on mineral surfaces equilibrated with the nutrient solution showed the following sequence zeolite>vermiculite>pumice, and the cation concentrations of the fluid phase were sensitive to cadmium addition.     

Sorption of cadmium and zinc from aqueous solutions by zeolite 4A, zeolite 13X and bentonite

The sorption and desorption of cadmium and zinc on zeolite 4A, zeolite 13X and bentonite has been studied using batch sorption studies. Parameters such as equilibrium time, effect of pH and sorbent dose were studied. The sorbents exhibited good sorption potential for cadmium and zinc with a peak value at pH 6.0 and 6.5, respectively. The sorption followed the Freundlich sorption model. More than 70% sorption occurred within 20 min and equilibrium was attained at around 90 min for the three sorbents. The metals sorption by zeolite 4A was higher than that by zeolite 13X and bentonite. The desorption studies were carried out using NaCl solution and the effect of NaCl concentration on desorption was also studied. Maximum desorption of 76% for cadmium and 80% for zinc occurred with 10% NaCl.     

Concentration, pH, and surface charge effects on cadmium and lead sorption in three tropical soils

Reactions of heavy metals with soil are important in determining metal fates in the environment. Sorption characteristics of two heavy metals, Cd and Pb, in three tropical soils (Mollisol, Oxisol, and Ultisol) from Puerto Rico were assessed at varying metal concentrations (0 to 1.2 mM) and pH values (approximately 2 to 7). All soils sorbed more Pb than Cd. Sorption maxima were obtained for each metal for the Oxisol and Ultisol soils, but not the Mollisol. Sorption appeared to depend more on soil mineralogy than organic matter content. Sorption isotherms were linear within the sorption envelope with similar slopes for each soil-metal curve, when plotting metal sorption as a function of pH. Cadmium and Pb isotherms yielded average slopes of approximately 36+/-1 and 28+/-1 units (percent increase in metal sorption per 1-unit increase in pH), respectively. Metal sorption depended more on metal type than soil composition. Cadmium sorption displayed a greater pH dependence than Pb. Cadmium sorption was less than or equal to the amount of negative surface charge except at pH values greater than the point of zero net charge (PZNC). This suggests that Cd was probably sorbed via electrostatic surface reactions and/or possible inner-sphere complexation at pH > 3.7. However, the amount of Pb sorbed by the Oxisol was greater than the amount of negative surface charge, suggesting that Pb participates in inner-sphere surface reactions. Lead was sorbed more strongly than Cd in our soils and poses less of a threat to underlying ground water systems due to its lower mobility and availability.     

Sorption of trace metals by standard and micro suction cups in the absence and presence of dissolved organic carbon

Both the bioavailability of a trace metal (TM) in a soil and the risk of leaching to the ground water are linked to the metals concentration in the soil solution. Sampling soil solution by tension lysimetry with suction cups is a simple and established technique that is increasingly used for monitoring dissolved TM in soils. Of major concern, however, is the sorption of TM by the walls of the samplers. Metal sorption by different materials used in suction cups can vary widely, depending also on the chemistry of the soil solution. We compared the sorption of Cu, Zn, Cd, and Pb by different standard-size and micro suction cups in the laboratory at two pH values (4.5 and 7.5 or 8.0) in absence and presence of dissolved organic carbon (DOC). In addition, we investigated the sorption of DOC from different origins by the cup materials. At both pH values, the weakest sorption of all four TMs was exhibited by standard-size suction cups based on nylon membranes and by hollow fibers made from polyvinyl alcohol (PVA). At alkaline pH, borosilicate glass, ceramic materials, and polytetrafluorethylene (PTFE) mixed with silicate were characterized by generally strong sorption of all investigated TMs. In addition, Cu and Pb were strongly sorbed at low pH by PTFE-silicate and a ceramic material used for the construction of standard-size suction cups. On the other hand, sorption of Cu, Zn, and Cd by ceramic capillaries produced from pure aluminum oxide was negligible at low pH. Micro suction cups made of an unknown polymerous tube sorbed Cu strongly, but were well suited to monitor Zn, Cd, and Pb at low pH, and, in the presence of DOC, also at high pH. Major cations (Na+, Mg2+, K+, Ca2+) and anions (Cl-, NO3-, SO4(2-)) were not or very weakly sorbed by all cup materials, except for Mg2+, K+, and Ca2+ by borosilicate glass at pH 7.5. Trace metal sorption by suction cups was generally greatly reduced in the presence of DOC, especially at alkaline pH. The sorption of DOC itself depended on its source. Dissolved organic carbon from leaf litter extracts with a probably large hydrophobic fraction was sorbed more strongly than mainly hydrophilic DOC from a mineral soil solution.     

Contamination time effect on lead and cadmium fractionation in a tropical coastal clay

The capability of a tropical coastal clay to immobilize lead (Pb) and cadmium (Cd) was investigated in laboratory batch sorption tests conducted under acidic, neutral, and slightly alkaline conditions. The contact time was extended to 65 d. The distribution of Pb and Cd among various sorbed phases was examined using a sequential extraction technique. The sorbed phases were fractionated into the exchangeable, carbonate, reducible, organic, and residual fractions. There were only small changes in the total Pb and Cd sorption beyond a 1-d sorption period. The metal fractionation results show that the amount of Pb and Cd in various fractions changed with sorption time, and the changes were pH-dependent. These changes could be attributed to mineral dissolution and transformation or redistribution of the sorbed phases. Transformation of the sorbed phases resulted in increasing Pb and Cd retention in the more persistent fractions with time, at the expense of reductions in the loosely bound fractions. Nevertheless, Pb and Cd fractionation in the solid phase appeared to reach equilibrium within the 65-d sorption period. These Pb and Cd fractionation results reflect the effect of contamination time on the heavy metal lability and bioavailability in the subsurface environment.     

Thermochemical esterifying citric acid onto lignocellulose for enhancing methylene blue sorption capacity of rice straw

In this paper, rice straw was esterified thermochemically with citric acid (CA) to produce potentially biodegradable cationic sorbent. The modified rice straw (MRS) and crude rice straw (CRS) were evaluated for their methylene blue (MB) removal capacity from aqueous solution. The effects of various experimental parameters (e.g., initial pH, sorbent dose, dye concentration, ion strength, and contact time) were examined. The ratio of MB sorbed on CRS increased as the initial pH was increased from pH 2 to 10. For MRS, the MB removal ratio came up to the maximum value beyond pH 3. The 1.5g/l or up of MRS could almost completely remove the dye from 250mg/l of MB solution. The ratio of MB sorbed kept above 98% over a range from 50 to 450mg/l of MB concentration when 2.0g/l of MRS was used. Increase in ion strength of solution induced decline of MB sorption. The isothermal data fitted the Langmuir model. The sorption processes followed the pseudo-first-order rate kinetics. The intraparticle diffusion rate constant (k(id)) was greatly increased due to modification.     

Influence of quaternary ammonium on sorption of selected metal cations onto clinoptilolite zeolite

Clay minerals and zeolites have large cation exchange capacities, which enable them to be modified by cationic surfactant to enhance their sorption of organic and anionic contaminants. In this study, the influence of quaternary ammonium surfactants on sorption of five metal cations (Cs+, Sr+, La3+, Pb2+, and Zn2+) onto a clinoptilolite zeolite was investigated. Generally, the metal cation sorption capacity and affinity for the zeolite decreased, indicating that presorbed cationic surfactants blocked sorption sites for metal cations, as the surfactant loading on the zeolite increased. Cesium and Pb2+ sorption was affected to a small extent, indicating that selective sorption for Cs+ and specific sorption for Pb2+ play an important role in addition to cation exchange. Sorption of cationic surfactants on zeolite preloaded with different metal cations showed a strong correlation with the chain length of the surfactant tail group, while the roles of the charges and types of the metal cations were minimal. As the chain length increases, the critical micelle concentration decreases and the surfactant molecules become more hydrophobic, resulting in progressive bilayer coverage. Desorption of presorbed metal cations by cationic surfactants was strongly affected by the surfactant chain length and metal type. More metal cations, particularly Sr2+ and Zn2+, desorbed with an increase in surfactant chain length. The results, in combination with those from organic and oxyanion sorption on surfactant-modified zeolite, may be used for future surfactant modification to target sorption and desorption of a specific type of contaminant or a mixture of different types of contaminants.     

Cation-pi bonding: a new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces

Recent molecular modeling and spectroscopic studies have suggested that relatively strong interactions can occur between aromatic pi donors and metal cations in aqueous solutions. The objective of this study was to characterize potential cation-pi interactions between pi donors and exchangeable cations accumulated at mineral surfaces via both spectroscopic and batch sorption methods. Quadrupolar splitting in deuterium nuclear magnetic resonance ((2)H NMR) spectroscopy for d(2)-dichloromethane, d(6)-benzene, and d(8)-toluene (C(6)D(5)- moiety) in aqueous suspensions of a Na-saturated reference montmorillonite unambiguously indicated the ordering of solute molecules with respect to the clay surface. The half line broadening (Delta nu(1/2)) of (2)H NMR of d(6)-benzene in montmorillonite suspensions showed that soft exchangeable cations generally resulted in more benzene sorption compared with harder cations (e.g., Ag(+) > Cs(+) > Na(+) > Mg(2+), Ba(2+)). In batch sorption experiments, saturating minerals (e.g., porous silica gels, kaolinite, vermiculite, montmorillonite) with a soft transition metal or softer base cations generally increased the polycyclic aromatic hydrocarbon (PAH) sorption relative to harder cations (e.g., Ag(+) > Cs(+) > K(+) > Na(+); Ba(2+) > Mg(2+)). Sorption of phenanthrene to Ag(+)-saturated montmorillonite was much stronger compared with 1,2,4,5-tetrachlorobenzene, a coplanar non-pi donor having slightly higher hydrophobicity. In addition, a strong positive correlation was found between the cation-dependent sorption and surface charge density of the minerals (e.g., vermiculite, montmorillonite > silica gels, kaolinite). These results, coupled with the observations in (2)H NMR experiments with montmorillonite, strongly suggest that cation-pi bonding forms between PAHs and exchangeable cations at mineral surfaces and affects PAH sorption to hydrated mineral surfaces.     

Sorption of iron-cyanide complexes on goethite in the presence of sulfate and desorption with phosphate and chloride

Soils are contaminated with potentially toxic iron-cyanide complexes by some industrial activities. The influence of sulfate on the sorption of the iron-cyanide complexes ferricyanide, [Fe(CN)6]3-, and ferrocyanide, [Fe(CN)6]4-, on goethite was investigated in batch experiments. The experiments were conducted as influenced by pH and varying sulfate/iron-cyanide complex concentration ratios. Furthermore, the desorption of iron-cyanide complexes sorbed on goethite was studied using phosphate and chloride solutions as influenced by pH and anion concentration. Over the whole pH range (pH 3.5 to 8), ferricyanide and sulfate showed similar affinities for the goethite surface. The extent of ferricyanide sorption strongly depended on sulfate concentrations and vice versa. In contrast, ferrocyanide sorption was only decreased (approximately 12%) by sulfate additions at pH 3.5. Ferricyanide was completely desorbed by 1 M chloride, ferrocyanide not at all. Unbuffered phosphate solutions (pH 8.3) desorbed both iron-cyanide complexes completely. Even in 70-fold excess, pH-adjusted phosphate solutions could not desorb ferrocyanide completely at pH 3.5. For ferricyanide we propose a sorption mechanism that is similar to the sulfate sorption mechanism, including outer-sphere and weak inner-sphere surface complexes on goethite. Ferrocyanide appears to form inner-sphere surface complexes. Additionally, we assume that ferrocyanide precipitates probably as a Berlin Blue-like phase at pH 3.5. Hence, ferrocyanide should be less mobile in the soil environment than ferricyanide or sulfate.     

Metal immobilization in soils using synthetic zeolites

In situ immobilization of heavy metals in contaminated soils is a technique to improve soil quality. Synthetic zeolites are potentially useful additives to bind heavy metals. This study selected the most effective zeolite in cadmium and zinc binding out of six synthetic zeolites (mordenite-type, faujasite-type, zeolite X, zeolite P, and two zeolites A) and one natural zeolite (clinoptilolite). Zeolite A appeared to have the highest binding capacity between pH 5 and 6.5 and was stable above pH 5.5. The second objective of this study was to investigate the effects of zeolite addition on the dissolved organic matter (DOM) concentration. Since zeolites increase soil pH and bind Ca, their application might lead to dispersion of organic matter. In a batch experiment, the DOM concentration increased by a factor of 5 when the pH increased from 6 to 8 as a result of zeolite A addition. A strong increase in DOM was also found in the leachate of soil columns, particularly in the beginning of the experiment. This resulted in higher metal leaching caused by metal-DOM complexes. In contrast, the free ionic concentration of Cd and Zn strongly decreased after the addition of zeolites, which might explain the reduction in metal uptake observed in plant growth experiments. Pretreatment of zeolites with acid (to prevent a pH increase) or Ca (to coagulate organic matter) suppressed the dispersion of organic matter, but also decreased the metal binding capacity of the zeolites due to competition of protons or Ca.     

Bioaccessibility of lead sequestered to corundum and ferrihydrite in a simulated gastrointestinal system

Lead (Pb) sorption onto oxide surfaces in soils may strongly influence the risk posed from incidental ingestion of Pb-contaminated soil. Lead was sorbed to model oxide minerals of corundum (alpha-Al(2)O(3)) and ferrihydrite (Fe(5)HO(8).4H(2)O). The Pb-sorbed minerals were placed in a simulated gastrointestinal tract (in vitro) to simulate ingestion of Pb-contaminated soil. The changes in Pb speciation were determined using extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES). Both corundum (sorption maximum of 2.13 g kg(-1)) and ferrihydrite (sorption maximum of 38.6 g kg(-1)) have been shown to sorb Pb, with ferrihydrite having a very high affinity for Pb. The gastric bioaccessible Pb for corundum was >85% for corundum when the concentration of Pb was >200 mg kg(-1). Bioaccessible Pb was not detectable at 4. However, much of the sorbed Pb will become bioaccessible under gastric conditions (pH 1.5-2.5) if this soil is ingested. Caution should be used before using these materials to remediate a soil where soil ingestion is an important exposure pathway.     

Adsorption of trace metals on glass fiber filters

Filters, containing glass-fiber (GF) filter material, are commonly used as the primary filter or as the prefilter in sampling natural waters and laboratory experiments with high concentrations of suspended solids. We observed that GF filter material removed substantial quantities of trace metals from solutions of low ionic strength at near neutral and slightly acidic pH. The GF material sorbed essentially all Pb and Ag from 5-mL aliquots of solutions containing 0.054 and 0.093 mM, respectively. Somewhat less Ni was sorbed from a 0.099 mM solution. This material retained about 43 micromol of Ag per gram of GF material (4600 microg/g). The Ag and Ni sorption was highest at low KNO(3) concentrations (as background electrolyte) and decreased to a constant concentration of sorbed metal at approximately 10 mM KNO(3). Glass-fiber filter material should only be used with careful testing for the elements of concern under conditions that closely match expected environmental or experimental conditions.     

Removal of perchlorate from contaminated waters using surfactant-modified zeolite

We investigated the potential of using surfactant (hexadecyltrimethylammonium)-modified zeolite (SMZ) as an inexpensive sorbent for removing perchlorate (ClO(4)(-)) from contaminated waters in the presence of competing anions. In batch systems, the presence of 10 mM OH(-) (i.e., pH 12), CO(3)(2-), Cl(-), or SO(4)(2-) had little effect on the sorption of ClO(4)(-) by SMZ, indicating that the sorption of ClO(4)(-) by SMZ was very selective. The presence of 10 mM NO(3)(-), however, reduced the sorption of ClO(4)(-) at low initial concentrations. The maximum sorption capacity for ClO(4)(-) by the SMZ remained relatively constant (40-47 mmol kg(-1)), in the absence or presence of the competing ions. In flow-through systems, ClO(4)(-) broke through the SMZ columns much later than other anions present in an artificial ground water. The affinity of the anions for SMZ followed the sequence of ClO(4)(-) > > NO(3)(-) > SO(4)(2-) > Cl(-). Perchlorate loading under dynamic flow-through conditions was 34 mmol kg(-1), somewhat less than the maximum loading of 40 to 47 mmol kg(-1) determined by the batch method. Less than 1% of previously sorbed ClO(4)(-) was leached out by ultra-pure water, by extraction fluid #1 of the standard toxicity characteristic leaching procedure (TCLP), or by a solution of 0.28 M Na(2)CO(3)/0.5 M NaOH. About 40% of the previously sorbed ClO(4)(-) was leached out from SMZ by a 0.5 M NO(3)(-) solution. The exchange of ClO(4)(-) with NO(3)(-) corroborated results of the batch tests where NO(3)(-) was shown to compete with ClO(4)(-) sorption.     

A comparative study for the sorption of Cd(II) by K-feldspar and sepiolite as soil components, and the recovery of Cd(II) using rhamnolipid biosurfactant

This study investigated the sorption characteristics and recovery of selected heavy metal Cd(II) from K-feldspar and sepiolite, representative soil components, using rhamnolipid biosurfactant. Although the proposed technique was classified as a soil bioremediation process, it can also be applied to treatment of waste waters containing Cd(II) ions with minor modifications. The effect of initial Cd(II) concentration on sorption capacity was characterized by determining the sorption isotherms. Of the four models examined, the Freundlich model showed the best fit for the sorption of Cd(II) on K-feldspar, whereas the Langmuir-model was used successfully to characterize the sorption of Cd(II) on sepiolite. Although a high Cd(II) uptake of 7.49 mmol/kg by K-feldspar was obtained, sepiolite was a superior Cd(II) accumulater, with a maximum Cd(II) uptake of 24.66 mmol Cd(II)/kg. The presence of Cd(II) in the sepiolite or K-feldspar prior to addition of the rhamnolipid generally resulted in less rhamnolipid sorption to sepiolite or K-feldspar. The maximum Cd(II) desorption efficiency by rhamnolipid from K-feldspar was substantially higher than that of sepiolite and determined to be 96% of the sorbed Cd(II), whereas only 10.1% of the sorbed Cd(II) from sepiolite was recovered by rhamnolipid solution.     

Sorption of the antibiotic tetracycline to humic-mineral complexes

Humic substances, by altering the surface properties and/or competing for available reaction sites, can either suppress or promote sorption of organic compounds to mineral surfaces. Limited literature evidence points to the reduction in sorption of the antibiotic tetracycline to clay minerals in the presence of humic substances. We investigated the surface interaction of Elliott soil humic acid (ESHA) with hydrous Al oxide (HAO) and the effect of this association on tetracycline sorption. Strong interaction between ESHA and HAO led to ESHA-promoted dissolution of HAO and surface charge reversal. The ESHA-HAO sorption-desorption isotherms were successfully described using a modified Langmuir model that accounted for the heterogeneity of HAO surface and ESHA. Ligand exchange was proposed as the major interaction mechanism, and the edge Al atoms on HAO surface were considered as the sorption sites for ESHA macromolecules. ESHA was coated onto HAO to achieve two different organic content (foc) levels of 0.81 and 1.52%. Sorption results were compared for the binary ESHA-tetracycline and HAO-tetracycline systems, and the ternary ESHA-HAO-tetracycline system. The coating of ESHA on HAO significantly suppressed tetracycline sorption levels, attributable to altered HAO surface charge characteristics and/or direct competition between ESHA and tetracycline for potential sorption sites. Higher foc level, besides increasing the extent of sorption suppression, also resulted in greater ionic strength dependence and increased nonlinearity of sorption behavior. It, therefore, appears that the presence of humic substances, in both dissolved and mineral-bound forms, is likely to increase the environmental mobility of tetracycline compounds.     

Removal of uranium(VI) from contaminated sediments by surfactants

Uranium(VI) sorption onto a soil collected at the Melton Branch Watershed (Oak Ridge National Laboratory, TN) is strongly influenced by the pH of the soil solution and, to a lesser extent, by the presence of calcium, suggesting specific chemical interactions between U(VI) and the soil matrix. Batch experiments designed to evaluate factors controlling desorption indicate that two anionic surfactants, AOK and T77, at concentrations ranging from 60 to 200 mM, are most suitable for U(VI) removal from acidic soils such as the Oak Ridge sediment. These surfactants are very efficient solubilizing agents at low uranium concentrations: ca. 100% U(VI) removal for [U(VI)]o,sorbed = 10(-6) mol kg-1. At greater uranium concentrations (e.g., [U(VI)]o,sorbed = ca. 10(-5) mol kg-1), the desorption efficiency of the surfactant solutions increases with an increase in surfactant concentration and reaches a plateau of 75 to 80% of the U(VI) initially sorbed. The most probable mechanisms responsible for U(VI) desorption include cation exchange in the electric double layer surrounding the micelles and, to a lesser extent, dissolution of the soil matrix. Limitations associated with the surfactant treatment include loss of surfactants onto the soil (sorption) and greater affinity between U(VI) and the soil matrix at large soil to liquid ratios. Parallel experiments with H2SO4 and carbonate-bicarbonate (CB) solutions indicate that these more conventional methods suffer from strong matrix dissolution with the acid and reduced desorption efficiency with CB due to the buffering capacity of the acidic soil.     

Sorption and stability of the polycyclic nitramine explosive CL-20 in soil

The polycyclic nitramine CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) is being considered for use as a munition, but its environmental fate and impact are unknown. The present study consisted of two main elements. First, sorption-desorption data were measured with soils and minerals to evaluate the respective contributions of organic matter and minerals to CL-20 immobilization. Second, since CL-20 hydrolyzes at a pH of >7, the effect of sorption on CL-20 degradation was examined in alkaline soils. Sorption-desorption isotherms measured using five slightly acidic soils (5.1 < pH < 6.9) containing various amounts of total organic carbon (TOC) revealed a nonlinear sorption that increased with TOC [K(d) (0.33% TOC) = 2.4 L kg(-1); K(d) (20% TOC) = 311 L kg(-1)]. Sorption to minerals (Fe(2)O(3), silica, kaolinite, montmorillonite, illite) was very low (0 < K(d) < 0.6 L kg(-1)), suggesting that mineral phases do not contribute significantly to CL-20 sorption. Degradation of CL-20 in sterile soils having different pH values increased as follows: sandy agricultural topsoil from Varennes, QC, Canada (VT) (pH = 5.6; K(d) = 15 L kg(-1); 8% loss) < clay soil from St. Sulpice, QC, Canada (CSS) (pH = 8.1; K(d) = 1 L kg(-1); 82% loss) < sandy soil provided by Agriculture Canada (SAC) (pH = 8.1, K(d) = approximately 0 L kg(-1); 100% loss). The faster degradation in SAC soil compared with CSS soil was attributed to the absence of sorption in the former. In summary, CL-20 is highly immobilized by soils rich in organic matter. Although sorption retards abiotic degradation, CL-20 still decomposes in soils where pH is >7.5, suggesting that it will not persist in even slightly alkaline soils.     

Oxytetracycline sorption to organic matter by metal-bridging

The sorption of oxytetracycline to metal-loaded ion exchange resin and to natural organic matter by the formation of ternary complexes between polyvalent metal cations and sorbent- and sorbate ligand groups was investigated. Oxytetracycline (OTC) sorption to Ca- and Cu-loaded Chelex-100 resin increased with increasing metal/sorbate ratio at pH 7.6 (OTC speciation: 55% zwitterion, 45% anion). Greater sorption to Cu- than Ca-loaded resin was observed, consistent with the greater stability constants of Cu with both the resin sites and with OTC. Oxytetracycline sorption to organic matter was measured at pH 5.5 (OTC speciation: 1% cation, 98% zwitterion, 1% anion). No detectable sorption was measured for cellulose or lignin sorbents that contain few metal-complexing ligand groups. Sorption to Aldrich humic acid increased from "clean" < "dirty" (no cation exchange pretreatment) < Al-amended < Fe(III)-amended clean humic acid with K(d) values of 5500, 32000, 48000, and 250000 L kg(-1) C, respectively. Calcium amendments of clean humic acid suggested that a portion of the sorbed OTC was interacting by cation exchange. Oxytetracycline sorption coefficients for all humic acid sorbents were well-correlated with the total sorbed Al-plus-Fe(III) concentrations (r(2) = 0.87, log-log plot), suggesting that sorption by ternary complex formation with humic acid is important. Results of this research indicate that organic matter may be an important sorbent phase in soils and sediments for pharmaceutical compounds that can complex metals by the formation of ternary complexes between organic matter ligand groups and pharmaceutical ligand groups.     

Sorption of arsenic from soil-washing leachate by surfactant-modified zeolite

Post-treatment of leachate from soil-washing remedial actions may be necessary depending on the amounts of dissolved contaminants present. Uptake of arsenic species by surfactant-modified zeolite (SMZ) from a synthetic soil leachate (pH of approximately 12 [NaOH]) was measured as a test of SMZ as a post-treatment sorbent. Batch sorption isotherms were prepared using leachate to SMZ ratios from 40:1 to 4:1, and temperatures of 25 and 15 degrees C. Equilibrium levels of dissolved and total solution arsenic were similar. At each temperature, sorption appeared to reach a plateau or maximum, then decreased at the highest solution concentration, corresponding to the lowest amount of zeolite added (2.5 g). A maximum sorption value of 72.0 mmol of arsenic per kg of SMZ (5400 mg/kg) was observed at 25 degrees C, and 42.1 mmol/kg (3150 mg/kg) at 15 degrees C. Total arsenic recoveries varied from 74 to 125%. Surfactant-modified zeolite removed up to 97% of dissolved organic carbon and decolorized the leachate solutions. Excluding the points for the highest arsenic to SMZ ratio, the sorption isotherms were well described by the linearized form of the Langmuir equation, with coefficients of determination greater than 0.90 at both temperatures. Sorption of arsenic by SMZ is attributed to anion exchange with counterions on the surfactant head groups, and/or partitioning of organic carbon-complexed arsenic into the surfactant bilayer.