Surface complexation modeling of proton and Cd adsorption onto an algal cell wall

This study quantifies Cd adsorption onto the cell wall of the algal species Pseudokirchneriella subcapitata by applying a surface complexation approach to model the observed adsorption behavior. We use potentiometric titrations to determine deprotonation constants and site concentrations for the functional groups on the algal cell wall. Adsorption and desorption kinetics experiments illustrate that adsorption of Cd onto the cell wall is rapid and reversible, except under low pH conditions. Adsorption experiments conducted as a function of pH and total Cd concentration yield the stoichiometry and site-specific stability constants for the important Cd-algal surface complexes. We model the acid/base properties of the algal cell wall by invoking four discrete surface functional group types, with pKa values of 3.9 +/- 0.3, 5.4 +/- 0.1, 7.6 +/- 0.3, and 9.6 +/- 0.4. The results of the Cd adsorption experiments indicate that the first, third, and fourth sites contribute to Cd adsorption under the experimental conditions, with calculated log stability constant values of 4.1 +/- 0.5, 5.4 +/- 0.5, and 6.1 +/- 0.4, respectively. Our results suggest that the stabilities of the Cd-surface complexes are high enough for algal adsorption to affect the fate and transport of Cd under some conditions and that on a per gram basis, algae and bacteria exhibit broadly similar extents of Cd adsorption.     

Adsorption of several metal ions onto a low-cost biosorbent: kinetic and equilibrium studies

Sugar beet pulp generated by sugar-refining factories has been shown to be an effective adsorbent for the removal of heavy metals from aqueous solutions. The structural components related to the metallic adsorption being determined, batch adsorption studies were performed for several metal ions, namely, Pb2+, Cu2+, Zn2+, Cd2+, and Ni2+ cations. Two simple kinetic models, that is, pseudo-first- and pseudo-second-order, were tested to investigate the adsorption mechanisms. The kinetic parameters of the models were calculated and discussed. For an 8 x 10(-4) M initial metal concentration, the initial sorption rates (v0) ranged from 0.063 mmol x g(-1) x min(-1) for Pb2+ to 0.275 mmol x g(-1) x min(-1) for Ni2+ ions, in the order Ni2+ > Cd2+ > Zn2+ > Cu2+ > Pb2+. The equilibrium data fitted well with the Langmuir and Freundlich models and showed the following affinity order of the material: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+. The metal removal was strongly dependent on pH and, to a lesser extent, ionic strength. Ion exchange with Ca2+ ions neutralizing the carboxyl groups of the polysaccharide was found to be the predominant mechanism, added with complexation for Pb2+, Cu2+, and Zn2+ metals.     

Heavy metal uptake by lignin: comparison of biotic ligand models with an ion-exchange process

Metal uptake by kraft lignin, hereafter referred to as lignin, occurs by displacement of protons or bound metals with equilibrium constants K(ex)H and K(ex), respectively. Values calculated for wide ranges of initial concentrations are reasonably constant, thereby demonstrating the validity of these displacement processes and proving that uptake in these systems is not simple adsorption. It was found that the stoichiometry for Sr and Cd uptake by Ca-loaded lignin is 1 mol of metal for 1 mol of Ca released. This observation for metals of very different binding strengths is difficult to rationalize with the biotic ligand model as generally applied but is in complete agreement with an ion-exchange process. Binding strengths to lignin, which contains only oxygen ligands, follow the order Pb > Cu > Zn > Cd > Ca (strongest to weakest). For proton displacement, only more tightly bound metals such as Pb, Cu, Zn, and Cd can compete with protons for anion-binding sites at low pH, but at high pH, uptake of Ca, Sr, and Li can occur. An observed logarithmic decrease of K(ex)H with pH can be explained by having only weaker acids available for proton displacement under more basic conditions. The advantages and disadvantages of using adsorption and biotic ligand models for an ion-exchange process are discussed.     

Surface chemistry and dissolution kinetics of divalent metal carbonates

A surface complexation model (SCM) for divalent metal carbonates (Ca, Mg, Sr, Ba, Mn, Fe, Co, Ni, Zn, Cd, and Pb) is developed based on new electrophoretic measurements and correlation between aqueous and surface reactions stability constants. This SCM postulates the formation of the following surface species: >CO3H0, >CO3-, >CO3Me+, >MeOH0, >MeO-, >MeOH2+, >MeHCO30, and MeCO3- within the framework of a constant capacitance of the electric double layer. It can be used to describe the surface-controlled dissolution kinetics of divalent metal carbonates and allows determination of the order of dissolution reactions with respect to rate-controlling protonated carbonate surface groups in acid solutions (>CO3H0) and hydrated metal groups (>MeOH2+) in neutral to alkaline solutions. The reaction order with respect to protonated carbonate groups increases from 2 for MnCO3 and ZnCO3 to 4 for NiCO3, whereas for hydrated surface metals, it augments from 2 for ZnCO3 to approximately 4 for MnCO3 and NiCO3. The dissolution rates at 5 < or = pH < or = 8 increase in the order Ni < Mg < Co < Fe < Mn < Zn < Cd < Sr < or = Ca approximately = Ba approximately = Pb and correlate nicely with water exchange rates from the aqueous solution into the hydration sphere of the corresponding dissolved cations. Such a correlation allows the generation for all carbonates of a model describing their dissolution/precipitation kinetics, including the effect of various ligands, provided that rate constants and their activation volumes for water exchange around Me(II)-ligand dissolved complexes are available.     

Electrodialytic removal of Cu, Zn, Pb, and Cd from harbor sediment: influence of changing experimental conditions

Electrodialytic remediation (EDR) was used to remove Cu, Zn, Pb, and Cd from contaminated harbor sediment. Extraction experiments were made prior to EDR, and the metal desorption was pH dependent but not liquid-to-solid ratio (L/S) dependent. The desorption order was Cd > Zn > Pb > Cu. Electrodialytic experiments were made with HCl as desorbing agent in a sediment suspension, which was stirred during EDR. Effects of different current strengths and L/S ratios on the heavy metal removal were investigated on wet and air-dried sediment. The effects of drying the sediment were negligible for the removal of Cu, Zn, and Pb, probably due to oxidation of the sediments during stirring. Contrary, Cd removal was lower in the wet sediment as compared to the air-dried. The heavy metal removal was influenced by higher current strengths and varying L/S ratios. The highest removal obtained was in an experiment with dry sediment (L/S 8) and a 70 mA applied current that lasted 14 days. These experimental conditions were thereafter used to remediate more strongly contaminated sediments. Regardless of the initial heavy metal concentrations in the sediments, 67-87% Cu, 79-98% Cd, 90-97% Zn, and 91-96% Pb were removed.     

Characterization of zinc, lead, and cadmium in mine waste: implications for transport, exposure, and bioavailability

We characterized the lability and bioaccessibility of Zn, Pb, and Cd in size-fractionated mine waste at the Tar Creek Superfund Site (Oklahoma) to assess the potential for metal transport, exposure, and subsequent bioavailability. Bulk mine waste samples contained elevated Zn (9100 +/- 2500 ppm), Pb (650 +/- 360 ppm), and Cd (42 +/- 10 ppm), while particles with the greatest potential for windborne transport and inhalation (< 10 microm) contained substantially higher concentrations, up to 220 000 ppm Zn, 16 000 ppm Pb, and 530 ppm Cd in particles < 1 microm. Although the mined ore at Tar Creek primarily consisted of refractory metal sulfides with low bioavailability, sequential extractions and physiologically based extractions indicate that physical and chemical weathering have shifted metals into relatively labile and bioaccessible mineral phases. In < 37 microm mine waste particles, 50-65% of Zn, Pb, and Cd were present in the "exchangeable" and "carbonate" sequential extraction fractions, and 60-80% of Zn, Pb, and Cd were mobilized in synthetic gastric fluid, while ZnS and PbS exhibited minimal solubility in these solutions. Our results demonstrate the importance of site-specific characterization of size-fractionated contemporary mine waste when assessing the lability and bioavailability of metals at mine-waste impacted sites.     

Heavy metal sorption at the muscovite (001)-fulvic acid interface

The role of fulvic acid (FA) in modifying the adsorption mode and sorption capacity of divalent metal cations on the muscovite (001) surface was evaluated by measuring the uptake of Cu(2+), Zn(2+), and Pb(2+) from 0.01 m solutions at pH 3.7 with FA using in situ resonant anomalous X-ray reflectivity. The molecular-scale distributions of these cations combined with those previously observed for Hg(2+), Sr(2+), and Ba(2+) indicate metal uptake patterns controlled by cation-FA binding strength and cation hydration enthalpy. For weakly hydrated cations the presence of FA increased metal uptake by approximately 60-140%. Greater uptake corresponded with increasing cation-FA affinity (Ba(2+) ≈ Sr(2+) < Pb(2+) < Hg(2+)). This trend is associated with differences in the sorption mechanism: Ba(2+) and Sr(2+) sorbed in the outer portion of the FA film whereas Pb(2+) and Hg(2+) complexed with FA effectively throughout the film. The more strongly hydrated Cu(2+) and Zn(2+) adsorbed as two distinct outer-sphere complexes on the muscovite surface, with minimal change from their distribution without FA, indicating that their strong hydration impedes additional binding to the FA film despite their relatively strong affinity for FA.     

Characterization of metal-cyanobacteria sorption reactions: a combined macroscopic and infrared spectroscopic investigation

In this study, we conducted synchrotron radiation Fourier transform infrared (IR) spectroscopy, potentiometric titration, and metal sorption experiments to characterize metal-cyanobacteria sorption reactions. Infrared spectra were collected with samples in solution for intact cyanobacterial filaments and separated exopolymeric sheath material to examine the deprotonation reactions of cell surface functional groups. The infrared spectra of intact cells sequentially titrated from pH 3.2 to 6.5 display an increase in peak intensity and area at 1400 cm(-1) corresponding to vibrational COO- frequencies from the formation of deprotonated carboxyl surface sites. Similarly, bulk acid-base titration of cyanobacterial filaments and sheath material indicates that the concentration of proton-active surface sites is higher on the cell wall compared to the overlying sheath. A three-site model provides an excellent fit to the titration curves of both intact cells and sheath material with corresponding pKa values of 4.7 +/- 0.4, 6.6 +/- 0.2, 9.2 +/- 0.3 and 4.8 +/- 0.3, 6.5 +/- 0.1, 8.7 +/- 0.2, respectively. Finally, Cu2+, Cd2+, and Pb2+ sorption experiments were conducted as a function of pH, and a site-specific surface complexation model was used to describe the metal sorption data. The modeling indicates that metal ions are partitioned between the exopolymer sheath and cell wall and that the carboxyl groups on the cyanobacterial cell wall are the dominant sink for metals at near neutral pH. These results demonstrate that the cyanobacterial surfaces are complex structures which contain distinct surface layers, each with unique molecular functional groups and metal binding properties.     

Comparison of atmospheric deposition of copper, nickel, cobalt, zinc, and cadmium recorded by Finnish peat cores with monitoring data and emission records

This study aims to determine the extent to which the accumulation rates of Cu, Ni, Co, Zn, and Cd in peat cores agree with established histories of atmospheric emission from local pointsources. Metals accumulating in three Finnish peat cores with known metal deposition histories have been measured using inductively coupled plasma-sector field mass spectrometry. Samples were age-dated using both 210Pb and 14C (bomb pulse curve). At the Outokumpu (OUT) site as well as the low-background site Hietaj?rvi (HIJ), 210Pb age dates are in excellent agreement with the 14C bomb pulse curve method results, and the precision is between 1 and 10 years for most of the samples; at the Harjavalta (HAR) site, precision is > 6 years. Mean regional "background" concentrations have been calculated from deeper peat layers of the HIJ site (microg g(-1)): Cu, 1.3 +/- 0.2 (n = 62); Co, 0.25 +/- 0.04 (n = 71); Cd, 0.08 +/- 0.01 (n = 23); and Zn, 4 +/- 2 (n = 40). For layers accumulated within the past 100 years, accumulation rates (ARs) have been calculated. At sites with < 0.06 g m(-2) cumulative Ni inventory (HIJ and OUT), ARs of Cu and Co trace the known metal deposition histories very well. At HAR, where metal inventories are much greater, Cu and Co are mobile. ARs of Zn were between 3 and 30 mg m(-2) year(-1) and those of Cd between 24 and 140 microg m(-2) year(-1) at all sites and are independent of the chronology of their inputs from the atmosphere.     

北京市售大米重金属含量监测及膳食风险评估

监测市售大米重金属含量,为北京市食品安全风险评估提供参考和数据支撑,采集了市售大米样品537件,分析其9种重金属(Cd、Cr、Cu、Fe、Mn、Ni、Pb、Sr和Zn)含量。采用内梅罗综合污染指数法评价市售大米的重金属污染水平,采用健康风险评价模型进行食用安全评估。结果如下:大米Cd、Cr、Cu、Fe、Mn、Ni、Pb、Sr和Zn含量平均值分别为0.02、0.02、2.27、2.63、9.10、0.15、0.07、0.17和14.27 mg/kg。大米重金属污染程度依次为:PbZnNiCuCdCr,其风险等级依次为:CuZnCdPbCrNi。结果表明,大米重金属的内梅罗综合污染指数较低,表明当前北京市售大米整体状况较好、处于安全水平;大米重金属对儿童的THQ贡献率高于成人,相关部门应加强有毒重金属监督与相应膳食指导。     

Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil

The complexation of heavy metals with dissolved organic matter (DOM) in the environment influences the solubility and mobility of these metals. In this paper, we measured the complexation of Cu, Cd, Zn, Ni, and Pb with DOM in the soil solution at pH 3.7-6.1 using a Donnan membrane technique. The results show that the DOM-complexed species is generally more significant for Cu and Pb than for Cd, Zn, and Ni. The ability of two advanced models for ion binding to humic substances, e.g., model VI and NICA-Donnan, in the simulation of metal binding to natural DOM was assessed by comparing the model predictions with the measurements. Using the default parameters of fulvic and humic acid, the predicted concentrations of free metal ions from the solution speciation calculation using the two models are mostly within 1 order of magnitude difference from the measured concentrations, except for Ni and Pb in a few samples. Furthermore, the solid-solution partitioning of the metals was simulated using a multisurface model, in which metal binding to soil organic matter, dissolved organic matter, clay, and iron hydroxides was accounted for using adsorption and cation exchange models (NICA-Donnan, Donnan, DDL, CD-MUSIC). The model estimation of the dissolved concentration of the metals is mostly within 1 order of magnitude difference from those measured except for Ni in some samples and Pb. The solubility of the metals depends mainly on the metal loading over soil sorbents, pH, and the concentration of inorganic ligands and DOM in the soil solution.     

Modeling of single and competitive metal adsorption onto a natural polysaccharide

Sugar beet pulp, a common agricultural waste, was studied in the removal of metal ions from aqueous solutions. Potentiometric titrations were used to characterize the surface acidity of the polysaccharide. The acid properties of the material can be described by invoking three distinct types of surface functional groups with the intrinsic acidity constants (pKa(int)) values 3.43+/-0.1, 6.05+/-0.05, and 7.89+/-0.1, respectively. The contents of each functional group (i.e., the carboxyl and phenol moieties) were also determined. Then, a simple surface complexation model with the diffuse layer model successfully described the sorption of several metal ions (Cu2+, Zn2+, Cd2+, and Ni2+) onto the polysaccharide under various experimental conditions: pH ranging from 2 to 5.5, ionic strength from 0.01 to 0.1 M, metal concentration between 10(-4) and 10(-3) M, for a constant sorbent concentration equal to 2.5 g x L(-1). It was observed experimentally that the affinity of the polysaccharide was in the sequence of Cu2+ > Zn2+ > Cd2+ > Ni2+. Predictions of sorption in binary-metal systems based on single-metal data fits represented competitive sorption data reasonably well.     

Impact of metal sorption and internalization on nitrification inhibition

The goal of this study was to explore the relationship between metal extracellular sorption, intracellular accumulation, and nitrification inhibition. Metal sorption on nitrifying biomass was rapid and could be described by linear partitioning with partition coefficients (Kp) of 20.3 +/- 0.1, 0.4 +/- 0.0, 0.1 +/- 0.0, and 0.2 +/- 0.0 L/g biomass chemical oxygen demand for Cu, Zn, Ni, and Cd, respectively. On the other hand, intracellular Zn, Ni, and Cd concentrations continued to increase with time beyond 12 h after metal addition, whereas intracellular Cu attained equilibrium after 4 h. Metal internalization kinetics could be described by an intraparticle diffusion model, with characteristic diffusion time constants (td) of 9.4, 64.6, 80.5, and 66.1 h for Cu, Zn, Ni, and Cd, respectively. Ultimate internalized percentages of the total cell-associated metal were 1.4 +/- 0.0, 4.3 +/- 0.5,7.6 +/- 1.0, and 2.7 +/- 0.2% for Cu, Zn, Ni, and Cd, respectively. Nitrification inhibition was not a function of the sorbed metal fraction but correlated well with intracellular Zn, Ni, or Cd fractions. An intraparticle diffusion model coupled with a saturation-type biological toxicity model fit the inhibition data for varying initial Cd concentrations and exposure periods. In contrast, no relationship between intracellular or sorbed Cu concentrations and nitrification inhibition was observed. In the presence of 1 mM Cu, less than 13.3 +/- 10.5% cells remained viable as compared to 72.8 +/- 7.5,104.8 +/- 1.7, and 84.7 +/- 7.0% (assumed 100% viable cells in metal-free control) in the presence of 1 mM Zn, Ni, and Cd, respectively. Hence, the observations that inhibition by metals such as Zn, Ni, and Cd is related to their intracellular fraction and the slow kinetics of metal internalization indicate that metal inhibition can easily be underpredicted from short-term batch assays. Furthermore, the inhibitory mechanism of Cu was very different from Zn, Ni, and Cd and may involve rapid loss of membrane integrity.     

Chemical speciation of Zn,Cd, Cu,and Pb in pore waters of agricultural and contaminated soils using Donnan dialysis

Knowledge of trace metal speciation in soil pore waters is important in addressing metal bioavailability and risk assessment of contaminated soils. Numerous analytical methods have been utilized for determining trace metal speciation in aqueous environmental matrixes; however, most of these methods suffer from significant interferences. The Donnan dialysis membrane technique minimizes these interferences and has been used in this study to determine free Zn2+, Cd2+, Cu2+, and Pb2+ activities in pore waters from 15 agricultural and 12 long-term contaminated soils. The soils vary widely in their origin, pH, organic carbon content, and total metal concentrations. Pore water pM2+ activities also covered a wide range and were controlled by soil pH and total metal concentrations. For the agricultural soils, most of the free metal activities were below detection limit, apart from Zn2+ for which the fraction of free Zn2+ in soluble Zn ranged from 2.3 to 87% (mean 43%). Five of the agricultural soils had detectable free Cd2+ with fractions of free metal ranging from 59 to 102% (mean 75%). For the contaminated soils with detectable free metal concentrations, the fraction of free metal as a percentage of soluble metal varied from 9.9 to 97% (mean 50%) for Zn2+, from 22 to 86% (mean 49%) for Cd2+, from 0.4 to 32.1% (mean 5%) for Cu2+, and from 2.9 to 48.8% (mean 20.1%) for Pb2+. For the contaminated soils, the equilibrium speciation programs GEOCHEM and WHAM Model VI provided reasonable estimates of free Zn2+ fractions in comparison to the measured fractions (R2 approximately 0.7), while estimates of free Cd2+ fractions were less agreeable (R2 approximately 0.5). The models generally predicted stronger binding of Cu2+ to DOC and hence lower fractions of free Cu2+ as compared with the observed fractions. The binding of Cu2+ and Pb2+ to DOC predicted by WHAM Model VI was much strongerthan that predicted by GEOCHEM.     

Characterization and preliminary assessment of a sorbent produced by accelerated mineral carbonation

This study shows that calcium silicate/aluminate-based materials can be carbonated to produce sorbents for metal removal. The material chosen for investigation, cement clinker, was accelerated carbonated, and its structural properties were investigated using X-ray diffraction (XRD), scanning electron microscopy, thermal gravimetric and differential thermal analysis, nuclear magnetic resonance spectroscopy, and nitrogen gas adsorption techniques. The principal carbonation reactions involved the transformation of dicalcium silicate, tricalcium silicate, and tricalcium aluminate into a Ca/Al-modified amorphous silica and calcium carbonate. It was found that carbonated cement had high acid buffering capacity, and maintained its structural integrity within a wide pH range. The uptake of Pb(II), Cd(II), Zn(II), Ni(II), Cr(II), Sr(II), Mo(VI), Cs(II), Co(II), and Cu(II) from concentrated (1000 mg L(-1)) single-metal solutions varied from 35 to 170 mg g(-1) of the carbonate cement. The removal of metals was hardly effected by the initial solution pH due to the buffering capability of the carbonated material. The kinetics of Pb, Cd, Cr, Sr, Cs, and Co removal followed a pseudo-second-order kinetic model, whereas the equilibrium batch data for Cu fitted the pseudo-first-order rate equation. PHREEQC simulation supported by XRD analysis suggested the formation of metal carbonates and silicates, calcium molybdate, and chromium (hydro)oxide. Cesium was likely to be adsorbed by Ca/Al-modified amorphous silica.     

Contribution of municipal effluents to metal fluxes in the St. Lawrence River

The contribution of urban effluents to the total metal fluxes carried toward the sea by the St. Lawrence, a major world river, is 60% for Ag; 8-13% for Cu, Zn, Mo, Cd, and Bi; and less than 3% for all other measured elements (Al, V, Cr, Mn, Fe Co, Ni, As, Rb, Sr, Zr, Cs, Ba, W, Re, Pb, Th, U). This is inferred from measurements at the Montreal wastewater treatment plant. Except for Ag, municipal effluents do not weigh heavily on the St. Lawrence River metal budget, likely because of the physical-chemical primary treatment applied to most effluents. Compared to direct atmospheric deposition on the surface of the river, effluents would contribute half as much Pb and one-tenth as much Zn. In contrast, effluents deliver twice as much Cd and six times as much Cu as the atmosphere. Stable Pb isotope ratios (206Pb/207Pb, 206Pb/208Pb) in suspended particulate matter from the river indicate that the total Pb content in the river water is three times higher than the pristine level. The ratios of Cr, Ni, Cu, Zn, and Cd to Al in suspended particulate matter are high as compared to pre-industrial sediments, which suggeststhattrace elementfluxes are higher today. To decrease metal levels in the St. Lawrence River further will be a challenge since the sources of metals are not well-known.     

Sorption studies of Zn(II)- and Cd(II)ions from aqueous solution on treated sawdust of sissoo wood

Sawdust, an inexpensive material, has been utilized as an adsorbent for the removal of Zn(II)- and Cd(II)ions from aqueous solution. The effects of time of equilibrium, pH, temperatures and dosage of the adsorbent on the removal of Zn(II)- and Cd(II)ions have been studied. The equilibrium nature of Zn(II)- and Cd(II)ions adsorption at different temperatures (25–60 °C) has been studied. The percent adsorption of Zn(II)- and Cd(II)ions increased with an increase in pH, temperature and dosage of treated sawdust. The applicability of the Langmuir isotherm suggests the formation of monolayer coverage of Zn(II)- and Cd(II)ions at the surface of the adsorbent. The thermodynamic parameters like free energy, enthalpy and entropy changes for the adsorption of Zn(II)- and Cd(II)ions has also been computed and discussed. The heat of adsorption [ΔH=17.706 kJmole^-1 for Zn(II) and ΔH=16.949 kJmole^-1 for Cd(II)] implied that the adsorption was an endothermic adsorption. The sawdust was found to be a metal adsorbent as effective as activated carbon.     

Deriving soil critical limits for Cu, Zn, Cd, and Pb: a method based on free ion concentrations

We present a method to calculate critical limits of cationic heavy metals accounting for variations in soil chemistry. We assume the free metal ion concentration (Mfree) to be the most appropriate indicator of toxicity, combined with a protective effect of soil cations (e.g., H+, Ca2+). Because soil metal cations tend to covary with pH, the concentration of Mfree exerting a given level of toxic effect (Mfree,toxic) can be expressed as a function of pH alone. We use linear regression equations to derive Mfree,toxic in toxicity experiments from soil pH, organic matter content, and endpoint soil metal. Chronic toxicity data from the literature, for plants, invertebrates, microbial processes, and fungi are interpreted in terms of an average log Mfree,toxic together with distributions of species sensitivity. This leads to critical limit functions to protect 95% of species, of the form log Mfree,CRIT = (pH + gamma. Appreciable effects of soil pH upon log Mfree,CRIT are found, with alpha = -1.21 (Cu), -0.34 (Zn), -0.43 (Cd), and -0.83 (Pb). Critical limit functions in terms of the geochemically active soil metal (Msoil,CRIT), that pool of metal which controls the free ion concentration, have also been derived, with soil pH and organic matter content as variables. The pH effect on Msoil,CRIT is relatively small, with slopes of 0.05 (Cu), 0.19 (Zn), 0.16 (Cd), and 0.20 (Pb), since the effect of pH on Mfree,CRIT is countered by the variation of Mfree with pH.     

Comparison of Cd(II), Cu(II), and Pb(II) biouptake by green algae in the presence of humic acid

The present study examines the role of humic acid, as a representative of dissolved organic matter, in Cd(II), Cu(II), and Pb(II) speciation and biouptake by green microalgae. Cellular and intracellular metal fractions were compared in the presence of citric and humic acids. The results demonstrated that Cd and Cu uptake in the presence of 10 mg L(-1) humic acid was consistent with that predicted from measured free metal concentrations, while Pb biouptake was higher. By comparing Cd, Cu, and Pb cellular concentrations in the absence and presence of humic acid, it was found that the influence of the increased negative algal surface charge, resulting from humic acid adsorption, on cellular metal was negligible. Moreover, the experimental results for all three metals were in good agreement with the ternary complex hypothesis. Given that metal has much higher affinity with algal sites than humic acid adsorbed to algae, the contribution of the ternary complex to metal bioavailability was negligible in the case of Cd (II) and Cu (II). In contrast, the ternary complex contributed to over 90% of total cellular metal for Pb(II), due to the comparable affinity of Pb to algal sites in comparison with humic acid adsorbed to algae. Therefore, the extension of the biotic ligand model by including the formation of the ternary complex between the metal, humic acid, and algal surface would help to avoid underestimation of Pb biouptake in the presence of humic substances by green algae Chlorella kesslerii.     

Adsorption of Me(II), HEDP, and Me(II)-HEDP onto boehmite at nonstoichiometric Me(II)-HEDP concentrations

In aqueous environments, certain heavy metals are toxic even at very low concentrations. The main pathway of metal removal in the aquatic systems is via adsorption onto surfaces. These are desired processes that help decrease the dissolved fraction of metals in natural water. The presence of organic ligands as mono- and polyphosphonates may produce drastic changes in the mobility of the heavy metals. 1-Hydroxyethane-(1,1-diphosphonic acid) (HEDP) is a very strong chelating agent widely used in industrial applications. This study examines the effect of HEDP on the adsorption of Cu(II), Zn(II), and Cd(II) onto boehmite in nonstoichiometric conditions, with the HEDP concentration higher than the corresponding Me(II) cations. At high surface loading and low pH, HEDP removes Zn(II) and Cd(II) from solution to an appreciable extent. The data are modeled assuming an anionic-ternary complex formation. In the same conditions, Cu(II) adsorption is significantly suppressed at intermediate values of pH, and this behavior is linked to Cu-HEDP complex formation in solution. At low surface covering, the effects of HEDP on metal adsorption are either negligible or slight. This behavior suggests that both ligand and metal are mainly adsorbed in separate form. All experimental data indicate that no changes are observed in the pH edges for phosphonate adsorption. The surface constants to fit the experimental data were calculated by applying the 2-K model constant capacitance (CCM).