Comparative study of chromium biosorption by red, green and brown seaweed biomass

Dried biomass of the macroalgae Fucus vesiculosus and Fucus spiralis (brown), Ulva spp. (comprising Ulva linza, Ulva compressa and Ulva intestinalis) and Ulva lactuca (green), Palmaria palmata and Polysiphonia lanosa (red) were studied in terms of their chromium biosorption performance. Metal sorption was highly pH dependent with maximum Cr(III) and Cr(VI) sorption occurring at pH 4.5 and pH 2, respectively. Extended equilibrium times were required for Cr(VI) binding over Cr(III) binding (180 and 120min, respectively) thus indicating possible disparities in binding mechanism between chromium oxidation states. The red seaweed P. palmata revealed the highest removal efficiency for both Cr(III) and Cr(VI) at low initial concentrations. However, at high initial metal concentrations F. vesiculosus had the greatest removal efficiency for Cr(III) and performed almost identically to P. lanosa in terms of Cr(VI) removal. The Langmuir Isotherm mathematically described chromium binding to the seaweeds where F. vesiculosus had the largest q(max) for Cr(III) sorption (1.21mmol g(-1)) and P. lanosa had the largest Cr(VI) uptake (0.88mmol g(-1)). P. palmata had the highest affinity for both Cr(III) and Cr(VI) binding with b values of 4.94mM(-1) and 8.64mM(-1), respectively. Fourier transform infrared analysis revealed interactions of amino, carboxyl, sulphonate and hydroxyl groups in chromium binding to Ulva spp. The remaining seaweeds showed involvement of these groups to varying degrees as well as ether group participation in the brown seaweeds and for Cr(VI) binding to the red seaweeds.     

A comparative study on metal sorption by brown seaweed

This study compared the sorption of Ag, Cd, Co, Cd, Mn, Ni, Pb and Zn by a Ca-treated Sargassum biomass at pH 5.0, under low and high ionic strength (IS) conditions. The sorption isotherms of As [As(V)] and Cr [Cr(III) and Cr(VI)] were also determined at low IS. The isotherm data for the eight cationic metals and Cr(III) were well fitted by Langmuir equations. Generally, the maximum metal uptake (Umax) followed: Cr(III) > Pb approximately Cu > Ag approximately Zn approximately Cd > Ni approximately Mn approximately Co > Cr(VI) > As(V) at low IS and Pb > Cu > Co > Mn approximately Cd > Zn approximately Ag > Ni at high IS. As(V) did not bind to the seaweed at pH 5.0. The results indicated that sorption of Pb was not affected by the increasing IS, though the percentage of free Pb ions in the water was greatly reduced as predicted by the speciation model. High IS lowered Umax by 10-36% (except Co and Pb), and lowered the affinity constant of the metal by 33-91% for all cationic metals, as compared to low IS. Moreover, the removal efficiency of the cationic metals and Cr decreased exponentially with initial metal concentrations and was lower at high IS. Ion-exchange was the mechanism responsible for the cationic metal sorption onto the seaweed, and Na ion interfered with the cationic metal binding through electrostatic interaction. In conclusion, this study showed the differential binding capacity of the Sargassm biomass for different metals and oxidation states and the differential effects of IS. According to the present results, Sargassum may be considered a good biosorbent for cationic metals (especially Pb) in both low and high-salt containing wastewater.     

Experimental and modeling studies on sorption and diffusion of radium in bentonite

The sorption and desorption behavior of radium on bentonite and purified smectite was investigated as a function of pH, ionic strength and liquid to solid ratio by batch experiments. The distribution coefficients (Kd) were in the range of 10(2) to > 10(4) ml g-1 and depended on ionic strength and pH. Most of sorbed Ra was desorbed by 1 M KCl. The results for purified smectite indicated that Ra sorption is dominated by ion exchange at layer sites of smectite, and surface complexation at edge sites may increase Ra sorption at higher pH region. Reaction parameters between Ra and smectite were determined based on an interaction model between smectite and groundwater. The reaction parameters were then used to explain the results of bentonite by considering dissolution and precipitation of minerals and soluble impurities. The dependencies of experimental Kd values on pH, ionic strength and liquid to solid ratio were qualitatively explained by the model. The modeling result for bentonite indicated that sorption of Ra on bentonite is dominated by ion exchange with smectite. The observed pH dependency was caused by changes of Ca concentration arising from dissolution and precipitation of calcite. Diffusion behavior of Ra in bentonite was also investigated as a function of dry density and ionic strength. The apparent diffusion coefficients (Da) obtained in compacted bentonite were in the range of 1.1 x 10(-11) to 2.2 x 10(-12) m2 s-1 and decreased with increasing in dry density and ionic strength. The Kd values obtained by measured effective diffusion coefficient (De) and modeled De were consistent with those by the sorption model in a deviation within one order of magnitude.     

The marine macroalga Cystoseira baccata as biosorbent for cadmium(II) and lead(II) removal: kinetic and equilibrium studies

This work reports kinetic and equilibrium studies of cadmium(II) and lead(II) adsorption by the brown seaweed Cystoseira baccata. Kinetic experiments demonstrated rapid metal uptake. Kinetic data were satisfactorily described by a pseudo-second order chemical sorption process. Temperature change from 15 to 45 degrees C showed small variation on kinetic parameters. Langmuir-Freundlich equation was selected to describe the metal isotherms and the proton binding in acid-base titrations. The maximum metal uptake values were around 0.9 mmol g(-1) (101 and 186 mg g(-1) for cadmium(II) and lead(II), respectively) at pH 4.5 (raw biomass), while the number of weak acid groups were 2.2 mmol g(-1) and their proton binding constant, K(H), 10(3.67) (protonated biomass). FTIR analysis confirmed the participation of carboxyl groups in metal uptake. The metal sorption was found to increase with the solution pH reaching a plateau above pH 4. Calcium and sodium nitrate salts in solution were found to affect considerably the metal biosorption.     

Characterization and lead(II), cadmium(II), nickel(II) biosorption of dried marine brown macro algae Cystoseira barbata

Purpose

The objectives of this research are to identify the functional groups and determine corresponding pK _a values of the acidic sites on dried brown algae Cystoseira barbata using FTIR and potentiometric titrations, and to investigate the biosorption ability of biomass towards divalent nickel, cadmium, and lead ions. Adsorption was studied as a function of solution pH and contact time, and experimental data were evaluated by the Langmuir isotherm model.

Methods

CaCl₂ pretreatment was applied to the sorbent for enhancing the metal uptake capacity. The effect of solution pH on biosorption equilibrium was investigated in the pH range of 1.5?C5.0. Individual as well as competitive adsorption capacity of the sorbent were studied for metal cations and mixtures.

Results

The retention of the tested metal ions was mostly influenced from pH in the range of 1.5?C2.5, then stayed almost constant up to 5.0, while Ni(II) uptake showed the highest variation with pH. Potentiometric titrations were performed to find the number of strong and weak acidic groups and their acidity constants. The density of strong and weak acidic functional groups in the biomass were found to be 0.9 and 2.26?mmol/g, respectively. The FTIR spectra of the sorbent samples indicated various functionalities on the biomass surface including carboxyl, hydroxyl, and amino and sulphonate groups which are responsible for the binding of metal ions.

Conclusions

The capacity of the biomass for single metal ions (around 1?mmol/g) was increased to 1.3?mmol/g in competitive adsorption, Pb(II) showing the highest Langmuir intensity constant. Considering its extremely high abundance and low cost, C. barbata may be potentially important in metal ion removal from contaminated water and industrial effluents.     

Characterization of H+ and Cd2+ binding properties of the bacterial exopolysaccharides

The proton and Cd binding capacities of microbially produced exopolysaccharides, EPS, were quantified by the determination of stability constants and the concentration of complexing sites using H(+) or Cd(2+) selective electrodes in dynamic titrations. The influence of ionic strength, pH and the Cd to EPS ratio was evaluated over large concentration ranges. The applicability of the non-ideal competitive adsorption isotherm combined with a Donnan electrostatics approach was tested with respect to the EPS. Proton and cadmium binding data were compared with literature data examining other ubiquitous environmental ligands including humic substances, alginate, bacteria, etc. Subsequent modelling of Cd speciation in aquatic (fresh and marine waters) and soil systems suggested that the exopolysaccharides would play non-negligible role, under most conditions. The quantitative information provided in this paper thus represents an important advance in our understanding of Cd transport, bioavailability and impact in aquatic and terrestrial systems.     

Copper binding by peat fulvic and humic acids extracted from two horizons of an ombrotrophic peat bog

We studied the binding of Cu(II) to humic acids and fulvic acids extracted from two horizons of an ombrotrophic peat bog by metal titration experiments at pH 4.5, 5.0, 5.5, and 6.0 and 0.1 M KNO3 ionic strength. Free metal ion concentrations in solution were measured using an ion selective electrode. The amounts of base required to maintain constant pH conditions were recorded and used to calculate H+/Cu2+ exchange ratios. The amount of Cu(II) bound to the humic fractions was greater than the amount bound to the fulvic fractions and only at the highest concentrations of metal ion the amount of Cu(II) sorbed by both fractions became equal. The proton to metal ion exchange ratios are similar for all humic substances, with values ranging from 1.0 to 2.0, and decreasing with increased pH. The amount of Cu(II) bound is practically independent of the horizon from which the sample was extracted. The results indicate that the humic substances show similar cation binding behaviour, despite the differences in chemical composition. The copper binding data are quantitatively described with the NICA-Donnan model, which allows to characterize only the carboxylic type binding sites. The values of the binding constants are higher for the humic acids than for the fulvic acids.     

Influences of macroalga-derived dissolved organic carbon on the aquatic toxicity of copper and cadmium

In this study, the effect of dissolved organic carbon (DOC) derived from macroalga (Sargassum) on the acute toxicity of copper (Cu) and cadmium (Cd) to a freshwater cladoceran (Daphnia magna) was investigated. Potassium-loaded macroalga was incubated with ultrapure water to extract macroalgal DOC, which was then spiked with the constituents of the Elendt M7 hard water media. The 48 h median lethal concentration of Cu increased linearly with DOC levels but that of Cd was relatively independent of DOC levels (0-44 mg l(-1)). The independence of Cd toxicity on DOC level might be due to the competitive effect of high calcium concentrations in the media with Cd for the binding sites of DOC. The decreased Cu toxicity was a result of reduced Cu uptake as evidenced in a separate accumulation test. Also, the capability of the macroalgal DOC on reducing Cu toxicity was found to be comparable to DOC tested in other studies. Therefore, the present study suggested that the biosorption treatment process using macroalgae should consider the effect of DOC release from the biomass as a step of modifying the metal toxicity as well as influencing metal biosorption capacity.     

Biosorption of antimony(V) by freshwater cyanobacteria Microcystis from Lake Taihu,China: effects of pH and competitive ions

There is limited knowledge available on metalloid biosorption by freshwater algae. In this study, biosorption properties of anionic Sb(OH) ₆ ^? by naturally occurring cyanobacteria Microcystis were investigated as a function of initial pH, biosorbent dosage, contact time, and addition sequences of competitive ions, and their binding mechanisms were discussed. The biosorption process was fast and equilibrium was reached at 2 h. Sb(V) biosorption decreased with the increase of pH and the optimum pH range was 2.5–3.0, which corresponded with the changes of surface charges of the cell wall of Microcystis. The biosorption data satisfactorily followed the Freundlich model. The simultaneous addition of H₂PO₄ ^? and Ca²⁺ enhanced Sb(V) biosorption, while NO₃ ^? greatly inhibited the biosorption, compared with single Sb(V) addition. The initial addition of the competitive ions reduced Sb(V) biosorption at higher Sb(V) concentrations, compared with simultaneous addition. A fraction of biosorbed Sb(V) was replaced by the competitive ions which were added subsequently, and the exchange only occurred at higher concentrations of Sb(V). 1.0 mol/L HCl demonstrated the highest desorption efficiency. Speciation analyses indicated that no reduction of Sb(V) into Sb(III) occurred. Based on the results of zeta potential and attenuated total reflection infrared spectroscopy spectra, Sb(OH) ₆ ^? bound to the biomass through electrostatic attraction and surface complexation, and amino, carboxyl, and hydroxyl groups were involved in the biosorption process. The study suggest that Microcystis from cyanobacteria blooms could be used as a potential biosorbent to remove Sb(V) from effluents at environmentally relevant concentrations (≤10.0 mg/L).     

Removal of nickel (II) from aqueous solutions using marine macroalgae as the sorbing biomass

In the present study biosorption technique, the passive accumulation of metals by biomass, is used for the removal of nickel from aqueous medium. The brown algae, Sargassum sp., in its natural and acid treated forms are used as a low cost sorbent. The adsorption characteristics of nickel on Sargassum sp. are evaluated as a function of time, pH, adsorbent dosage and initial concentration of nickel. The equilibrium adsorption data are fitted to Freundlich and Langmuir adsorption isotherm models and the model parameters are evaluated. Both the models represent the experimental data satisfactorily. The adsorption follows Lagergren first order kinetic model. The monolayer adsorption capacities of natural and acid treated forms of algae as obtained from Langmuir adsorption model are found to be 181 and 250mg g(-1) respectively.     

Biosorption of Cr3+, Cd2+ and Cu2+ ions by blue-green algae Spirulina sp.: kinetics, equilibrium and the mechanism of the process

The process of biosorption of heavy metal ions (Cr3+, Cd2+, Cu2+) by blue-green algae Spirulina sp. is discussed in this paper. Spirulina sp. was found to be a very efficient biosorbent. The aim of the present study was to investigate quantitatively the potential binding sites present at the surface of Spirulina sp., using both potentiometric titrations and adsorption isotherms. The kinetic experiments showed that the process equilibrium was reached quickly, in less than 5-10 min. It was found that the equilibrium dependence between biosorption capacity and bulk metal ion concentration could be described with Langmuir equation. This suggests that the mechanism of biosorption is rather chemisorption than physical adsorption and was further confirmed by the low surface area associated with physical adsorption and by the presence of cations that appeared in the solution after biosorption. The maximum contribution of physical adsorption in the overall biosorption process was evaluated as 3.7%. It was proposed that functional groups on the cell surface contributed to the binding of metal ions by a biosorbent via equilibrium reaction. Three functional groups capable of cation exchange were identified on the cell surface. The biomass was described as weakly acidic ion exchanger. Since deprotonation of each functional group depends on pH, the process of biosorption is strongly pH-dependent. This was confirmed in the biosorption experiments carried out at different pH. The contribution of functional groups in the biosorption process was confirmed by chemical modification of the groups. Chemically blocked groups did not show neither biosorption nor ion-exchange capabilities. It has been shown that growth conditions can affect the metal adsorption properties of microalgae. The paper also discusses desorption characteristics of the biosorbent. The criteria for desorption were high elution efficiency and preservation of biosorptive properties. Desorbent that possessed these characteristics was nitric acid.     

Biosorption of Cr(VI) by free and immobilized Pediastrum boryanum biomass: equilibrium, kinetic, and thermodynamic studies

Background and purpose

The biosorption of Cr(VI) from aqueous solution has been studied using free and immobilized Pediastrum boryanum cells in a batch system. The algal cells were immobilized in alginate and alginate?Cgelatin beads via entrapment, and their algal cell free counterparts were used as control systems during biosorption studies of Cr(VI).

Methods

The changes in the functional groups of the biosorbents formulations were confirmed by Fourier transform infrared spectra. The effect of pH, equilibrium time, initial concentration of metal ions, and temperature on the biosorption of Cr(VI) ion was investigated.

Results

The maximum Cr(VI) biosorption capacities were found to be 17.3, 6.73, 14.0, 23.8, and 29.6?mg/g for the free algal cells, and alginate, alginate?Cgelatin, alginate?Ccells, and alginate?Cgelatin?Ccells at pH?2.0, which are corresponding to an initial Cr(VI) concentration of 400?mg/L. The biosorption of Cr(VI) on all the tested biosorbents (P. boryanum cells, alginate, alginate?Cgelatin, and alginate?Ccells, alginate?Cgelatin?Ccells) followed Langmuir adsorption isotherm model.

Conclusion

The thermodynamic studies indicated that the biosorption process was spontaneous and endothermic in nature under studied conditions. For all the tested biosorbents, biosorption kinetic was best described by the pseudo-second-order model.     

Relation between extracellular polymers' composition and its ability to complex Cd,Cu and Pb

Activated sludges originated from wastewater treatment plants (WWTPs) play an important role in heavy metal removal from effluents. Extracellular polymers (ECP) form a major part of the activated sludge and are heavily involved in biosorption of heavy metals. The complexation of three heavy metals (Cd, Cu and Pb) with ECP extracted from six activated sludges, originated from different WWTPs, was investigated at pH 7.

ECP in the study were shown to be mainly composed of proteins, humic acids, uronic acids and polysaccharides along with smaller amounts of lipids and nucleic acids. IR spectra confirmed the presence of the functional groups usually found in the ECP and the content in each fraction was determined using colorimetric methods. The determination of surface charge was carried out on each ECP sample and allowed two pK_a values characteristic of two distinctive functional groups to be determined. At the pH used in the study, the value of these constants indicates that only one functional group is under protonated form.

A polarographic method was used to determine the complexation parameters (number of binding sites and complexation constant) of ECP solutions towards metals. The following orders were established for the number of binding sites: Cu > Pb  Cd and for the stability of the ECP–metal complex: Cd > Pb  Cu.

A matrix of correlation between the composition of the polymers and the complexation parameters showed that the number of binding sites and the complexation constant were strongly linked to proteins, polysaccharides and humic substances content.     

Biosorption of heavy metals from aqueous solution by UV-mutant Bacillus subtilis

To develop an efficient bio-immobilization approach for the remediation of heavy metal pollution in soil, a mutant species of Bacillus subtilis (B38) was obtained by ultraviolet irradiation and selection under high concentration of cadmium (Cd) in a previous study. In the present study, to check the applicability of this mutated species to the sorption and immobilization of other metals, the sorption of four heavy metals, Cd, chromium (Cr), mercury (Hg), and lead (Pb), on living and nonliving B38 in single- and multiple-component systems under different conditions was investigated using batch experiments. Rapid metal binding occurred on both living and nonliving B38 during the beginning of the biosorption. The sorption kinetics followed the exponential equation for living biomass and the pseudo-first-order Lagergren model for nonliving biomass, with r ² values in the range of 0.9004-0.9933. The maximum adsorptive quantity of the heavy metals on B38 changed with the solution pH, temperature, biomass dose, and ionic strength. The nonliving biomass generally showed greater or similar adsorptive capacities as compared with the living biomass and was not likely to be affected by the solution parameters. The bacterium had a stronger affinity to the cationic heavy metals than to the anionic one, and the equilibrium sorption amounts were 210.6, 332.3, and 420.9 mg/g for Cd(II), Hg(II), and Pb(II), respectively. The results of binary and ternary sorption experiments indicated that the metals with the higher sorption capacity in the single-component systems showed greater inhibitory effects on the biosorption of other metal ions in the multiple-component systems, but the sorption sites of Hg and Cd or Pb are likely to be different. The results of this study illustrated that the mutant species is a promising biosorbent for the remediation of multiple heavy metals.     

Lead and cadmium biosorption by extracellular polymeric substances (EPS) extracted from activated sludges: pH-sorption edge tests and mathematical equilibrium modelling

The sorption of Cd and Pb by extracellular polymeric substances (EPS) extracted from activated sludges originated from wastewater treatment plants (WWTPs) or Lab-scale bioreactors was investigated as a function of pH. The study was carried out using a polarographic method in the SMDE (stripping mercury dropping electrode) mode which is suited to determine labile metals in solution containing soluble ligands such as EPS. The results obtained provide evidence of the presence of a pH-sorption/desorption edge for Cd and Pb by EPS. The use of Kurbatov’s model gives information on the mechanisms involved through the determination of “relative complexation constants” (operationally defined) and the number of protons exchanged. The use of this model demonstrates that proton exchange with metals is not the only mechanism involved in metal biosorption by EPS. Other mechanisms such as cation exchange with Ca or Mg, global electric field surrounding the ligand or micro-precipitation of metals could be involved in metal sorption by EPS. The position of the pH-sorption edge curves and the “relative complexation constants” show that Pb displays a greater affinity for EPS than Cd. The studied EPS have large differences regarding binding strength of Cd and Pb. These differences are not correlated with the organic parameters measured to characterize the EPS, however the mineral fraction of the EPS could be involved to a large extent in the sorption of metal.     

满江红干体对锌离子的生物吸附

以满江红干体为生物吸附剂，研究了不同条件下对废水中Zn²⁺的净化作用。结果表明，满江红干体对Zn²⁺的吸附是一个快速的过程，前5 min的吸附量达到最大吸附量的62.9%，30 min达到吸附平衡；初始pH值对Zn²⁺的吸附有显著的影响，最适pH值为6；随着干体量的增加，吸附率逐渐提高而吸附量则降低；随着Zn²⁺初始浓度的增加，吸附率逐渐降低而吸附量则提高。满江红干体对Zn²⁺的吸附符合Langmuir吸附等温线方程，最大吸附容量达57.5 mg/g。5次吸附解吸循环实验数据表明，重复次数和再生处理对满江红干体的吸附能力没有产生显著影响。因此，满江红干体在处理含Zn²⁺废水中的重复使用是可行的。     

Biosorption of alpacide blue from aqueous solution by lignocellulosic biomass: <Emphasis Type="Italic">Luffa cylindrica</Emphasis> fibers

The aim of the present work is to develop an effective and inexpensive pollutant-removal technology using lignocellulosic fibers: Luffa cylindrica, for the biosorption of an anionic dye: alpacide blue. The influence of some experimental parameters such as pH, temperature, initial concentration of the polluted solution, and mass of the sorbent L. cylindrica on the biosorption of alpacide blue by L. cylindrica fibers has been investigated. Optimal parameters for maximum quantity of biosorption dye were achieved after 2 h of treatment in a batch system using an initial dye concentration of 20 mg/L, a mass of 1 g of L. cylindrica fibers, and pH 2. In these conditions, the quantity of dye retained is 2 mg/g and the retention rate is 78 %. Finally, a mathematical modeling of kinetics and isotherms has been used for mathematical modeling; the model of pseudo-second order is more appropriate to describe this phenomenon of biosorption. Concerning biosorption isotherms, the Freundlich model is the most appropriate for a biosorption of alpacide blue dye by L. cylindrica fibers.     

Marine macrophytes as effective lead biosorbents

Several species of seagrass and marine macrophytes were investigated for their biosorption performance in the removal of lead from aqueous solution. The effect of pH on the equilibrium of the seagrass Cymodocea nodosa as a biosorbent also was studied. It was found that increasing pH increased lead biosorption, with a maximum uptake of approximately 140 mg/g in the range pH 3.3 to 5. Equilibrium data at different pH levels were successfully fitted to competitive equilibrium models. In addition, the seaweeds belonging to different phyla (i.e., Chlorophyta, Heterokontophyta, and Rhodophyta) were studied for the effect of their structure on equilibrium at a constant pH 5. The brown algae (Heterokontophyta) showed the highest potential for lead sorption, with a maximum uptake of 220 mg/g for C. compressa and 140 mg/g for S. lomentaria. The green algae (Chlorophyta) showed lead uptake in the range 40 to 90 mg/g, and the red algae (Rhodophyta) was least effective, with uptake in the range 10 to 40 mg/g.     

Equilibrium, kinetic, and thermodynamic biosorption of Pb(II), Cr(III), and Cd(II) ions by dead anaerobic biomass from synthetic wastewater

Purpose

Heavy metals are toxic pollutants released into the environment as a result of different industrial activities. Biosorption of heavy metals from aqueous solutions is a new technology for the treatment of industrial wastewater. The aim of the present research is to highlight the basic biosorption theory to heavy metal removal.

Materials and methods

Heterogeneous cultures mostly dried anaerobic bacteria, yeast (fungi), and protozoa were used as low-cost material to remove metallic cations Pb(II), Cr(III), and Cd(II) from synthetic wastewater. Competitive biosorption of these metals was studied.

Results

The main biosorption mechanisms were complexation and physical adsorption onto natural active functional groups. It is observed that biosorption of these metals was a surface process. The main functional groups involved in these processes were hydroxyl (–OH) and carboxylic groups (C=O) with 37, 52, and 31 and 21, 14, and 34 % removal of Pb(II), Cr(III), and Cd(II), respectively. Langmuir was the best model for a single system. While extended Langmuir was the best model for binary and ternary metal systems. The maximum uptake capacities were 54.92, 34.78, and 29.99 mg/g and pore diffusion coefficients were 7.23, 3.15, and 2.76?×?10^?11 m²/s for Pb(II), Cr(III), and Cd(II), respectively. Optimum pH was found to be 4. Pseudo-second-order was the best model to predict the kinetic process. Biosorption process was exothermic and physical in nature.

Conclusions

Pb(II) offers the strongest component that is able to displace Cr(III) and Cd(II) from their sites, while Cd(II) ions are the weakest adsorbed component.