Studies on the recovery of uranium from nuclear industrial effluent using nanoporous silica adsorbent

In this paper, the sorption of uranium onto nanoporous silica adsorbent in the presence of nitrate, sulfate, chloride, fluoride and phosphate was studied. The effect of contact time between the nanoporous sorbent and aqueous solution, pH and initial concentration of uranium was also investigated. Uranium sorption onto nanoporous silica adsorbent is a very fast process as sorption rate increases with pH increment. Optimum pH for uranium sorption was 4?C8. Experimental sorption isotherm is successfully described by Langmuir and Freundlich models. The results obtained by batch experiments showed that the presence of high concentration of nitrate, sulfate, chloride and phosphate anions alone had no interference with uranium recovery. However, the presence of fluoride ions (>250?mg/L) decreases uranium sorption by about 55?%. The results also showed that the presence of phosphate ions (about 300?mg/L) in solution could remove fluoride interference completely. Finally, the efficiency of the nanoporous silica adsorbent for uranium recovery from wastewater of the uranium conversion facility was investigated.     

Oxytetracycline sorption onto Iraqi montmorillonite

This paper assesses the use of certified Iraqi montmorillonite clay as a potential sorbent for the removal of oxytetracycline (OTC) from aqueous solutions. The clay is characterized by a cation exchange capacity of 0.756 meq g^?1 and a zero point charge at pH 8.7. Aqueous solutions of OTC were equilibrated with montmorillonite under various experimental conditions, such as OTC concentration, pH and clay content, for 24 h at fixed ionic strength. Two forms of montmorillonite were evaluated: regular and iron-modified form. The effect of pH was minor on OTC adsorption. Kinetic study revealed that the sorption follows a pseudo-second-order model. Sorption isotherm showed a good fit with the Freundlich model. OTC sorption onto Fe-saturated montmorillonite was analyzed statistically using a response surface design to study the effects of experimental conditions. The introduction of iron improved the adsorption characteristics of the clay due to the ability of ferric ions to make stable complexes with OTC. The most favorable operating conditions for the treatment were deemed as follows: clay content, 6.85 g L^?1, oxytetracycline concentration, 1.0 mmol L^?1 and pH, 5.5 for the iron-modified form.     

Sorption of uranium(VI) at the clay mineral–water interface

Batch experiments were conducted to study the sorption of uranium on selected clay minerals (KGa-1b and KGa-2 reference kaolinite, SWy-2 and STx-1b reference montmorillonite, and IBECO natural bentonite) as a function of pH (4–9) and 0.001, 0.01, and 0.025 M NaCl in equilibrium with the CO₂ partial pressure of the atmosphere. Uranium concentrations were kept below 100 μg L⁻¹ to avoid precipitation of amorphous Uranium-hydroxides. Solely PTFE containers and materials were used, because experiments showed significant sorption at higher pH on glass ware. All batch experiments were performed over a period of 24 h, since kinetic experiments proved that the common 10 or 15 min are in many cases by far not sufficient to reach equilibrium. Kaolinite showed much greater uranium sorption than the other clay minerals due to the more aluminol sites available. Sorption on the poorly crystallized KGa-2 was higher than on the well-crystallized KGa-1b. Uranium sorption on STx-1b and IBECO exhibited parabolic behavior with a sorption maximum around pH 6.5. Sorption of uranium on montmorillonites showed a distinct dependency on sodium concentrations because of the effective competition between uranyl and sodium ions, whereas less significant differences in sorption were found for kaolinite. The presence of anatase as impurity in kaolinite enhanced the binding of uranyl-carbonate complexes with surface sites. The kinetic of uranium sorption behavior was primarily dependent on the clay minerals and pH. A multisite surface complexation model without assuming exchange is based on the binding of the most dominant uranium species to aluminol and silanol edge sites of montmorillonite, respectively to aluminol and titanol surface sites of kaolinite. For eight surface species, the log_k was determined from the experimental data using the parameter estimation code PEST together with PHREEQC.     

Sorption of Cesium on Clay Colloids: Kinetic and Thermodynamic Studies

Sorption of radionuclides onto stable colloids can significantly enhance their transport in groundwater. Batch adsorption studies were performed to evaluate the influence of various experimental parameters like initial pH, contact time, temperature and concentration of Na⁺ and Ca²⁺ ions on the sorption of Cs on clay. The sorption process is dependent on pH of the solution with distribution coefficient (K _d) found to increase with increase in pH. The kinetic experiments were carried out at different temperatures, and the results have shown that the sorption process fits well into a pseudo-second-order mechanism with apparent activation energy of 45.7?kJ/mol. The rate constant was found to decrease with increase in temperature. The thermodynamic parameters such as ?G ⁰, ?H ⁰ and ?S ⁰ were calculated. The negative value of ?H ⁰ indicates that the reaction is exothermic. The negative values obtained for ?G ⁰ indicated that the sorption of cesium on clay was spontaneous at all studied concentrations. The distribution coefficient was found to decrease with increasing concentration of Na⁺ and Ca²⁺ ions. The cesium sorption data were fitted to Freundlich, Langmuir, Temkin and Dubinin–Radushkevich (D–R) isotherms. The values of Langmuir separation factor (R _L) indicate a favorable Cs adsorption. The values of mean free energy of sorption (E) at various temperatures ranged from 10.5 to 11.1?kJ/mol, which indicates that the sorption process follows chemisorption.     

Geochemical and thermodynamic aspects of sorption of strontium on kaolinite dominated clay samples at Kalpakkam

The mobility of strontium in subsurface is largely influenced by sorption on to clay minerals. In the present study, kaolinite clay samples collected from the Kalpakkam nuclear plant site were employed to understand the sorption characteristics of strontium by batch method. The effect of several parameters such as time, strontium ion concentration, pH, temperature and ionic strength was investigated. The kinetic studies suggested pseudo-second-order mechanism. The experimental sorption data was fitted to Langmuir adsorption model for obtaining the sorption capacity of the sorbent. The maximum sorption capacity was 5.77 mg/g at 298 K and was found to increase with an increase in temperature. It was observed that the distribution coefficient (K _d) of strontium on clay increased as the pH of the solution increased. The distribution coefficient was found to decrease with an increase in concentration of Na⁺ and Ca²⁺ ions. This variation of K _d suggests that cation exchange is the predominant sorption process. It was also observed that sorption process is endothermic. The thermodynamic parameters such as ∆G ⁰, ∆H ⁰ and ∆S ⁰ were calculated. The negative values obtained for ∆G ⁰ indicated that the sorption of strontium on clay was spontaneous at all studied concentrations. ∆G ⁰ becomes more negative with an increase in temperature, suggests that the sorption process is more favorable at higher temperatures.     

Removal of phosphorus from water using scallop shell synthesized ceramic biomaterials

This study describes the development of scallop shell synthesized ceramic biomaterial for phosphorus removal from water. The synthesized biomaterial was characterized by scanning electron microscope, Brunauer–Emmett–Teller and X-ray diffractometer methods. The influences of contact time, initial phosphate concentration, initial solution pH, co-existing ions and temperature for phosphorus removal were investigated by batch experiments. The results indicated that the equilibrium data can be fitted by the Langmuir isotherm model at temperatures ranging from 15 to 55 °C, with the maximum sorption capacity of 13.6 mg/g. Sorption kinetics followed a pseudo-second-order kinetic equation model. The sorption process was optimal at a wide range of solution pH (above 2.4), with a relatively high sorption capacity level. Phosphorus sorption was slightly impeded by the presence of F^?, HCO₃ ^? and NH₄ ⁺ ions, and significantly inhibited by Cl^?, SO₄ ^2? and NO₃ ^? ions. Sorption process appeared to be controlled by a chemical precipitation processes. The mechanism may be attributed to ion complexation during subsequent sorption of phosphorus on scallop shell synthesized ceramic biomaterial.     

溶液介质条件对重金属离子与石英表面反应的影响

实验研究表明,随着溶液ＰＨ值的升高石芟夺Ｃｕ＾２＋、Ｐｂ＾２＿、Ｃｄ＾２＋等重金属离子的吸附量和表面吸附覆盖率逐渐增大,而表面反应产物的结合开矿相应地出现由单核化合物、多核化合物〖ＳＯＣｕ４（ＯＨ）３＾４＋〗,直至表面沉淀（ＳＯＨ…Ｃｕ（ＯＨ）２（ｓ）〗。随着温度升高,石英对Ｃｕ＾２＋、Ｐｂ＾２＋、Ｃｄ＾２＋等重金属离子的吸附量逐渐减是随着溶液离子强度的增大,石英对Ｃｕ＾２＋离子的吸附量和表面离了     

Sorption of silica by clay minerals

Clay minerals were reacted with silica-spiked solutions of unbuffered distilled water; water buffered at pH 5.5, 8 and 10; alkali chloride solutions; natural and artificial sea water to assess the influence of pH, silica and cation activities. The data are plotted as silica produced by dissolution or sorption of silica by clay surface as a function of initial silica concentration at a given pH and solution composition. This allows the determination of the dissolved silica value at which the clay mineral surface neither dissolves nor sorbs silica. The values of the various activities in different solutions are used to infer the phase equilibria between solution, clay mineral and the surface phase produced either by dissolution or sorption. Most intensively investigated were sorption reactions of kaolinite in sea water and other ionic solutions to form silica-rich, cation-rich surface phases in cationic solutions and silica-rich phases in cation-free solutions.Inferred equilibrium constants imply that silicate reconstitution is doubtful as a mechanism for partial control of silica and cation composition of sea water but is reasonable in silica-rich interstitial waters.     

镉及其络合物在黏土上的吸附行为

淋洗剂EDTA 二钠和柠檬酸三钠与镉形成的络合物及镉本身在黏土上的吸附行为研究表明: 上述两种淋洗剂能有效提取表层土中的Cd,形成的络合物在迁移过程中又能较好地被黏土层吸附。黏土对Cd2 +、Cd--EDTA 和Cd--柠檬酸的吸附容量顺序为: Cd--柠檬酸＞ Cd2 + ＞ Cd － EDTA; 3 种形态的吸附过程均以离子交换吸附为主,并在15 min 内完成。黏土对Cd2 + 和Cd － 柠檬酸的吸附容量随溶液初始pH 的升高而增大,对Cd--EDTA 的吸附容量随pH 升高略有降低。     

Phosphate sorption desorption characteristics of some ferruginous soils of tropical region in Eastern India

Phosphate sorption and desorption experiments were conducted with four ferruginous soils (alfisols) of Eastern India, in view of the low native phosphate concentrations in tropical Indian soils. From the P-isotherm curve, standard P requirement (SPR) of the soils was determined. Phosphate sorption data were fitted to both Langmuir and Freundlich equations and mean sorption maximum values obtained for the different soil series were in the decreasing order as Matimahal > Anandapur > Mrigindih > Kashipur. The fraction of added P sorbed followed the same trend as SPR, P sorption maximum (P_max), phosphate affinity constant (K), maximum phosphate buffering capacity (MPBC), Freundlich constant K′ and phosphate desorption values. Phosphate sorption maximum was significantly correlated with MPBC, Freundlich 1/n, SPR, clay and different forms of Fe and Al. The value of K (bonding energy) was significantly correlated with MPBC, Freundlich K′ and pyrophosphate extractable Fe and Al. The MPBC was significantly correlated with Freundlich K′, Freundlich constant 1/n, clay, oxalate and dithionite extractable, amorphous and crystalline form of Fe and Al. Freundlich K′ was significantly correlated with Freundlich 1/n, pH_water, clay, dithionite extractable and crystalline form of Fe and Al. The results suggested that the soils having higher amount of extractable and reactive Fe and Al shared higher P sorbtion capacity and such soils may need higher levels of P application     

Immobilization of Ni by Al-modified montmorillonite: A novel uptake mechanism

The sorption capacity of montmorillonite clay minerals for small cations, such as Ni²⁺, can be greatly enhanced by modifying the clay mineral with Al(III). In this study, the mechanisms of Ni uptake by Al-modified montmorillonite were studied using extended x-ray absorption fine structure (EXAFS) spectroscopy of powders and polarized EXAFS spectroscopy of self-supporting clay films to delineate the binding structure of Ni formed as a function of the reaction conditions. Analysis of powder EXAFS spectra of wet pastes, collected from Ni-treated Al-modified montmorillonites reacted at pH 5-8, 25°C or 80°C (to accelerate the reaction process), and reaction times ranging from 1 month to 9 yrs, showed that Ni was surrounded on average by 6 O atoms at a distance of 2.05 Å and 6 Al atoms at 3.01 Å, suggesting the incorporation of Ni into a gibbsite-like structure. Only at pH 8, Ni-containing precipitates were congruently formed. Polarized EXAFS spectroscopy of self-supporting Ni-reacted Al-modified montmorillonite clay films showed a pronounced angular dependency of the spectra of the Ni-doped gibbsite, indicating that the orientation of this Ni-doped gibbsite coincided with the layering of the montmorillonite. Data analysis suggested that Ni is included slightly above and below the vacant octahedral sites of the postulated interstitial gibbsite monolayer. This newly identified mechanism of metal uptake by Al-modified montmorillonite provides a large metal sorption capacity and, because the metal is included in a monolayer gibbsite or gibbsite “islands” formed in the interstitial space of the clay mineral, it potentially leads to a permanent sequestration of the metal from the environment.     

Citrate impairs the micropore diffusion of phosphate into pure and C-coated goethite

Anions of polycarboxylic low-molecular-weight organic acids (LMWOA) compete with phosphate for sorption sites of hydrous Fe and Al oxides. To test whether the sorption of LMWOA anions decreases the accessibility of micropores (<2 nm) of goethite (α-FeOOH) for phosphate, we studied the kinetics of citrate-induced changes in microporosity and the phosphate sorption kinetics of synthetic goethite in the presence and absence of citrate in batch systems for 3 weeks (500 μM of each ion, pH 5). We also used C-coated goethite obtained after sorption of dissolved organic matter in order to simulate organic coatings in the soil. We analyzed our samples with N₂ adsorption and electrophoretic mobility measurements. Citrate clogged the micropores of both adsorbents by up to 13% within 1 h of contact. The micropore volume decreased with increasing concentration and residence time of citrate. In the absence of citrate, phosphate diffused into micropores of the pure and C-coated goethite. The C coating (5.6 μmol C m⁻²) did not impair the intraparticle diffusion of phosphate. In the presence of citrate, the diffusion of phosphate into the micropores of both adsorbents was strongly impaired. We attribute this to the micropore clogging and the ligand-induced dissolution of goethite by citrate. While the diffusion limitation of phosphate by citrate was stronger when citrate was added before phosphate to pure goethite, the order of addition of both ions to C-coated goethite had only a minor effect on the intraparticle diffusion of phosphate. Micropore clogging and dissolution of microporous hydrous Fe and Al oxides may be regarded as potential strategies of plants to cope with phosphate deficiency in addition to ligand-exchange.     

Removal of Cadmium Ions from Aqueous Solution by Silicate-incorporated Hydroxyapatite

This article reports a preliminary research on silicate-incorporated hydroxyapatite as a new environmental mineral used to remove cadmium ions from aqueous solutions. The silicate-incorporated hydroxyapatite was prepared by coprecipitation and calcining, and silicate was incorporated into the crystal lattice of hydroxyapatite by partial substitution of phosphate. The amount of cadmium ions removed by silicate-incorporated hydroxyapatite was significantly elevated, which was 76% higher than that of pure hydroxyapatite. But the sorption behavior of cadmium ions on silicate-incorporated hydroxyapatite was similar to that of pure hydroxyapatite. Morphological study revealed that silicate incorporation confined the crystal growth and increased the specific surface area of hydroxyapatite, which were in favor of enhancing the cadmium ion sorption capacity of the samples. Incorporation of silicate into hydroxyapatite seems to be an effective approach to improve the environmental property of hydroxyapatite on removal of aqueous cadmium ions.     

X-ray absorption spectroscopy investigation of aqueous Co(II) and Sr(II) sorption at clay–water interfaces

Sorption processes typically control trace metal concentrations in aquatic systems. To illustrate the impact of various types of surface sites on metal ion sorption behavior, Co(II) and Sr(II) sorption by several clay minerals under a range pH and background electrolyte conditions was studied. X-ray absorption spectroscopy (XAS) was used to characterize the surface complexes formed to explain the basis for the sorption trends. At low pH, Co(II) could be displaced from the surface by increasing the Na ion concentration. XAS analysis of these samples showed that sorbed Co(II) retained the coordination structure of aqueous phase Co(II), suggesting the formation of weakly associated, outer-sphere, mononuclear Co complexes at permanent charge sites. At high pH, sorbed Co could not be displaced by increasing the Na ion concentration. The XAS analyses of these samples indicated the formation of Co coprecipitates. The results of the Sr(II) sorption experiments suggested weaker bonding between sorbed Sr and the solid surfaces, regardless of solution conditions and adsorbent. XAS analysis of Sr sorption samples revealed the formation of mononuclear, outer-sphere complexes of Sr at clay–water interfaces, similar to the outer-sphere Co sorption samples observed only at low pH.     

黄铁矿净化水中低浓度磷

以产自安徽铜陵新桥矿的黄铁矿为典型样品,研究黄铁矿对磷的吸附作用。静态实验考察黄铁矿粒度、固液比、pH值、离子强度、温度、吸附时间等因素对黄铁矿去除磷效果的影响,XPS和FE-SEM研究吸附磷后黄铁矿颗粒表面形貌和成分特征。结果表明：黄铁矿粒径越小（50～180目）,去除磷效率越高（9.3%～90.7%）;提高固液比（0.2～2 g/L）,磷的去除率增加（6.5%～97.1%）;在pH值3～9.65范围内黄铁矿对磷都有很好的去除效率（95%以上）;NO3-对磷的去除效果表现为微弱的促进作用,Cl-有微弱的抑制作用,溶液中SO42-、HCO3-对黄铁矿吸附磷表现出较强的抑制作用。温度对黄铁矿吸附效率基本没有影响。黄铁矿对磷吸附动态实验表明除磷效率在16 h后接近最大值。除磷作用机理是黄铁矿表面缓慢氧化产生的三价铁对磷的化学吸附。成果表明黄铁矿用于净化污水中低浓度磷具有很大的潜力。     

Sorption of lanthanides on smectite and kaolinite

Experiments were carried out to investigate the sorption of the complete lanthanide series (Ln or rare earth elements, REE) on a kaolinite and an a Na-montmorillonite at 22°C over a wide range of pH (3-9). Experiments were conducted at two ionic strengths, 0.025 and 0.5 M, using two different background electrolytes (NaNO₃ or NaClO₄) under atmospheric conditions or N₂ flow (glove box). The REE sorption does not depend on the background electrolyte or the presence of dissolved CO₂, but is controlled by the nature of the clay minerals, the pH and the ionic strength. At 0.5 M, both clay minerals exhibit the same pH dependence for the Ln sorption edge, with a large increase in the sorption coefficient (K_D) above pH 5.5. At 0.025 M, the measured K_D is influenced by the Cation Exchange Capacity (CEC) of the minerals. Two different behaviours are observed for smectite: between pH 3 and 6, the K_D is weakly pH-dependent, while above pH 6, there is a slight decrease in log K_D. This can be explained by a particular arrangement of the particles. For kaolinite, the sorption coefficient exhibits a linear increase with increasing pH over the studied pH range. A fractionation is observed that due to the selective sorption between the HREEs and the LREEs at high ionic strength, the heavy REE is being more sorbed than the light REE. These results can be interpreted in terms of the surface chemistry of clay minerals, where two types of surface charge are able to coexist: the permanent structural charge and the variable pH-dependent charge. The fractionation due to sorption observed at high ionic strength can be interpreted either because of a competition with sodium or because of the formation of inner-sphere complexes. Both processes could favour the sorption of HREEs according to the lanthanide contraction.     

Restructuring of polygalacturonate on alumina upon hydration—Effect on phosphate sorption kinetics

Hydration of organic coatings in soils is expected to affect the sorption of oxyanions onto hydrous Fe and Al oxides. We hypothesized that the hydration of polygalacturonate (PGA) coatings on alumina (Al₂O₃) increases their permeability for phosphate. Pure and PGA-coated alumina were equilibrated in deionized water for 2 and 170 h at pH 5 and 20 °C before studying (i) their porosity with N₂ gas adsorption and ¹H NMR relaxometry, (ii) structural changes of PGA-coatings with differential scanning calorimetry (DSC), and (iii) the kinetics of phosphate sorption and PGA desorption in batch experiments. Scanning electron micrographs revealed that PGA molecules formed three-dimensional networks with pores ranging in size from <10 to several hundred nanometers. Our NMR results showed that the water content of intraparticle alumina pores decreased upon PGA sorption, indicating a displacement of pore water by PGA. The amount of water in interparticle alumina pores increased strongly after PGA addition, however, and was attributed to water in pores of PGA and/or in pores at the PGA-alumina interface. The flexibility of PGA molecules and the fraction of a PGA gel phase increased within one week of hydration, implying restructuring of PGA. Hydration of PGA coatings increased the amount of phosphate defined as instantaneously sorbed by 84%, showing that restructuring of PGA enhanced the accessibility of phosphate to external alumina surfaces. Despite the fact that the efficacy of phosphate to displace PGA was higher after 170 h than after 2 h, a higher phosphate surface loading was required after 170 h to set off PGA desorption. Our findings imply that the number of PGA chain segments directly attached to the alumina surface decreased with time. We conclude that hydration/dehydration of polymeric surface coatings affects the sorption kinetics of oxyanions, and may thus control the sorption and transport of solutes in soils.     

Sorption of Zr~(4+) and Hf~(4+) onto Hydrous Ferric Oxide and Their Fractionation Behaviors:An experimental Study

1 Introduction Coherent element pairs (Y3+-Ho3+, Zr4+-Hf4+, Nb5+-Ta5+ and Sr2+-Eu2+) have the same valences and very similar ionic radii. They are seldom fractionated during silicate mineral/melt interaction systems because the partitioning of these element pairs between melts and minerals is tightly controlled by charge valence and crystal structure (lattice site size) or ionic radius. The concentration ratios of these pairs in igneous rocks are close to the chondritic values (Y/Ho=28, Zr…     

Adsorption Behavior of Hexavalent Chromium in Vadose Zone

Adsorption behavior of Cr (VI) in vadose zone, which is silty clay and clayey soil, was studied through kinetics experiments, isothermal adsorption experiments under various conditions, including different ph, temperature and organic contents. The results from kinetics experiments showed that the sorption progress of Cr (VI) has clear features in different stages, and adsorption equilibrium showed at 30 min, the adsorption rate of silty clay and clayey soil were 60%. The isothermal adsorption curve of Cr (VI) fitted closely with Freundlich equation model. When pH is 3-5 a plateau were seen, thereafter with increase in pH the adsorption rate of Cr (VI) dropped sharply and the minimum achieved at pH 10, the adsorption rate were only 35%. Adsorption rate of Cr (VI) increased gradually with the increase of temperature, the temperature of vadose zone is 14.7 ℃, according to the experimental results, the adsorption rate of Cr (VI) is about 40%. The use of organics represents an important contribution to the sorption of Cr (VI), sorption rate up to 100% when 30% of organic content. These studies will provide basis for manager to minimize the impacts, and provide basic data for pollution prevention and remediation of vadose zone.     

Sorption of Cu and Pb to kaolinite-fulvic acid colloids: Assessment of sorbent interactions

The sorption of Cu(II) and Pb(II) to kaolinite-fulvic acid colloids was investigated by potentiometric titrations. To assess the possible interactions between kaolinite and fulvic acid during metal sorption, experimental sorption isotherms were compared with predictions based on a linear additivity model (LAM). Suspensions of 5 g L⁻¹ kaolinite and 0.03 g L⁻¹ fulvic acid in 0.01 M NaNO₃ were titrated with Cu and Pb solutions, respectively. The suspension pH was kept constant at pH 4, 6, or 8. The free ion activities of Cu²⁺ and Pb²⁺ were monitored in the titration vessel using ion selective electrodes. Total dissolved concentrations of metals (by ICP-MS) and fulvic acid (by UV-absorption) were determined in samples taken after each titration step. The amounts of metals sorbed to the solid phase, comprised of kaolinite plus surface-bound fulvic acid, were calculated by difference. Compared to pure kaolinite, addition of fulvic acid to the clay strongly increased metal sorption to the solid phase. This effect was more pronounced at pH 4 and 6 than at pH 8, because more fulvic acid was sorbed to the kaolinite surface under acidic conditions. Addition of Pb enhanced the sorption of fulvic acid onto kaolinite at pH 6 and 8, but not at pH 4. Addition of Cu had no effect on the sorption of fulvic acid onto kaolinite. In the LAM, metal sorption to the kaolinite surface was predicted by a two-site, 1-pK basic Stern model and metal sorption to the fulvic acid was calculated with the NICA-Donnan model, respectively. The LAM provided good predictions of Cu sorption to the kaolinite-fulvic acid colloids over the entire range in pH and free Cu²⁺ ion activity (10⁻¹² to 10⁻⁵). The sorption of Pb was slightly underestimated by the LAM under most conditions. A fractionation of the fulvic acid during sorption to kaolinite was observed, but this could not explain the observed deviations of the LAM predictions from the experimental Pb sorption isotherms.