Novel chelating resin with cyanoguanidine group: useful recyclable materials for Hg(II) removal in aqueous environment

A novel chelating resin containing cyanoguanidine moiety has been successfully prepared by the functionalizing reaction of a macroporous bead based on chloromethylated copolymer of styrene-divinylbenzene (CMPS) with dicyandiamide (DCDA) in the presence of phase transfer catalyst. The Fourier transform-infrared spectra (FT-IR) and scanning electron microscopy (SEM) were employed in the characterization of the resulting chelating resin, meanwhile, the adsorption properties of the resin for Hg(II) were investigated by batch and column methods. The results indicated that the resin displayed a marked advantage in Hg(II) binding capacity, and the saturated adsorption capacity estimated from the Langmuir model was dramatically up to 1077 mg g(-1) at 45 °C. Furthermore, it was found that the resin was able to selectively separate Hg(II) from multicomponent solutions with Zn(II), Cu(II), Pb(II) and Mg(II). The desorption process of Hg(II) was tested with different eluents and the ratio of the highest recovery reached to 96% under eluting condition of 1M HCl+10% thiourea. Consequently, the resulting chelating resin would provide a potential application for treatment process of Hg(II) containing wastewater.     

Use of rice straw as biosorbent for removal of Cu(II), Zn(II), Cd(II) and Hg(II) ions in industrial effluents

Adsorption experiments were carried out using waste rice straw of several kinds as a biosorbent to adsorb Cu(II), Zn(II), Cd(II) and Hg(II) ions from aqueous solutions at room temperature. To achieve the best adsorption conditions the influence of pH and contact time were investigated. The isotherms of adsorption were fitted to the Freundlich equation. Based on the experimental data and Freundlich model, the adsorption order was Cd(II) > Cu(II) > Zn(II) > Hg(II) on the rice straw. This quick adsorption process reached the equilibrium before 1.5 h, with maximum adsorptions at pH 5.0. Thermodynamic aspects of the adsorption process were investigated. The biosorbent material was used in columns for the removal of ions Cu, Zn, Cd and Hg of real samples of industrial effluent and its efficiency was studied.     

Removal of mercury(II) from aqueous solutions and chlor-alkali industry effluent by steam activated and sulphurised activated carbons prepared from bagasse pith: kinetics and equilibrium studies

The adsorption of mercury from aqueous solutions and chlor-alkali industry effluent on steam activated and sulphurised steam activated carbons prepared from bagasse pith have been studied comparatively. The uptake of mercury(II) (Hg(II)) was maximum by steam activated carbon in presence of SO(2) and H(2)S (SA-SO(2)-H(2)S-C) followed by steam activated carbon in presence of SO(2) (SA-SO(2)-C), steam activated carbon in presence of H(2)S (SA-H(2)S-C) and steam activated carbon (SA-C) at the same concentration, pH and temperature of the solution. Adsorption experiments demonstrate that the adsorption process corresponds to the pseudo-second-order kinetic model and equilibrium results correspond to the Langmuir adsorption isotherm. Kinetic parameters as a function of initial concentration, for all adsorbents were calculated. Batch studies indicated that the optimum pH range for the adsorption of Hg(II) on sulphurised carbons was between 4 and 9 and for sulphur free carbon was between 6 and 9 at 30 degrees C. The adsorptive behaviour of the activated carbons is explained on the basis of their chemical nature and porous texture. Decrease in ionic strength and increase in temperature of the solution has been found to improve the uptake of Hg(II). Synthetic and chlor-alkali industrial wastewaters were also treated by sulphurised activated carbons to demonstrate their efficiencies in removing Hg(II) from wastewaters. Some feasibility experiments have been carried out with a view to recover the adsorbed Hg(II) and regenerate the spent activated carbons using 0.2M HCl solution. The data obtained point towards viable adsorbents, which are both effective as well as economically attractive for Hg(II) removal from wastewaters.     

Study of the removal of mercury(II) and chromium(VI) from aqueous solutions by Moroccan stevensite

The objective of the present study was to investigate the adsorption of the heavy metals mercury(II) and chromium(VI), from aqueous solutions, onto Moroccan stevensite. A mineralogical and physicochemical characterization of natural stevensite was carried out. In order to improve the adsorption capacity of stevensite for Cr(VI), a preparation of stevensite was carried out. It consists in saturating the stevensite by ferrous iron Fe(II) and reducing the total Fe by Na(2)S(2)O(4). Then, the adsorption experiments were studied in batch reactors at 25+/-3 degrees C. The influence of the pH solution on the Cr(VI) and Hg(II) adsorption was studied in the pH range of 1.5-7.0. The optimum pH for the Cr(VI) adsorption is in the pH range of 2.0-5.0 while that of Hg(II) is at the pH values above 4.0. The adsorption kinetics were tested by a pseudo-second-order model. The adsorption rate of Hg(II) is 54.35 mmol kg(-1)min(-1) and that of Cr(VI) is 7.21 mmol kg(-1)min(-1). The adsorption equilibrium time for Hg(II) and Cr(VI) was reached within 2 and 12 h, respectively. The adsorption isotherms were described by the Dubinin-Radushkevich model. The maximal adsorption capacity for Cr(VI) increases from 13.7 (raw stevensite) to 48.86 mmol kg(-1) (modified stevensite) while that of Hg(II) decreases from 205.8 to 166.9 mmol kg(-1). The mechanism of Hg(II) and Cr(VI) adsorption was discussed.     

Preparation and use of chemically modified MCM-41 and silica gel as selective adsorbents for Hg(II) ions

Adsorbents for Hg(II) ion extraction were prepared using amorphous silica gel and ordered MCM-41. Grafting with 2-(3-(2-aminoethylthio)propylthio)ethanamine was used to functionalize the silica. The functionalized adsorbents were characterized by nitrogen adsorption, X-ray diffraction, 13C MAS NMR spectroscopy and thermogravimetric analysis. The adsorption properties of the modified silica gel and MCM-41 were compared using batch method. The effect of pH, stirring time, ionic strength and foreign ions were studied. The extraction of Hg(II) ions occurred rapidly with the modified MCM-41 and the optimal pH range for the extraction by the modified materials was pH 4-7. Foreign ions, especially Cl- had some effect on the extraction efficiency of the modified silica gel and the modified MCM-41. The adsorption behavior of both adsorbents could be described by a Langmuir model at 298 K, and the maximum adsorption capacity of the modified silica gel and MCM-41 at pH 3 was 0.79 and 0.70 mmol g(-1), respectively. The modified MCM-41 showed a larger Langmuir constant than that of the modified silica gel, indicating a better ability for Hg(II) ion adsorption. The results indicate that the structure of the materials affects the adsorption behavior. These materials show a potential for the application as effective and selective adsorbents for Hg(II) removal from water.     

Removal of aniline from aqueous solution by PVC-CDAE ligand-exchanger

The adsorption of aniline from aqueous solutions onto cobalt(II)-poly(vinyl chloride)-carboxylated diaminoethane (PVC-CDAE) resin has been studied using a mini-column apparatus at 25 ± 0.1 °C. First of all, experimental data obtained from the breakthrough curves were tested by using the Scatchard plot analysis, to have a preliminary prediction about the types of interaction of the resin with aniline. Our aim was to determine the model which best describes the experimental data. The aspect of the Scatchard plot indicated that the aniline adsorption did not follow the Langmuir model and the presence of two types of binding sites for aniline on the resin. However, the dynamics of aniline uptake were represented by the Freundlich model reasonably well. The kinetics of aniline adsorption from aqueous solution on the cobalt(II)-PVC-CDAE have also been tested using continuous column runs and rate-controlling step of the process was determined. In this study, homogeneous diffusion model was adapted to a column system to describe the change in the aniline concentration at the column exit beginning from breakthrough point as a function of time. Kinetic studies revealed that the rate-controlling step of the aniline adsorption was predominately film diffusion controlled rather particle diffusion.     

Efficient removal of Reactive Black 5 from aqueous media using glycidyl methacrylate resin modified with tetraethelenepentamine

Glycidyl methacrylate/methelenebisacrylimide resin loaded with tetraethelenepentamine ligand was prepared and investigated. The adsorption characteristics of the obtained resin towards Reactive Black 5 (RB5) from aqueous solutions at different experimental conditions were established by means of batch and column methods. The mechanism of interaction between RB5 and resin's active sites was discussed. The resin showed high affinity for the adsorption of RB5 where an uptake value of 0.63 mmol/g was reported for the obtained resin, at 25 °C. The kinetics and thermodynamic behavior of the adsorption reaction were also defined. These data indicated an endothermic spontaneous adsorption process and kinetically followed the pseudo-second order model. Breakthrough curves for the removal of RB5 were studied at different flow rates and bed heights. The critical bed height for the studied resin column was found to be 0.764 cm at flow rate of 8 mL/min. The adsorbed dye was eluted from the investigated resin effectively. Regeneration and durability of the loaded resin towards the successive resin were also clarified.     

Adsorption properties of crosslinked carboxymethyl-chitosan resin with Pb(II) as template ions

In the current study, we attempt to find a new adsorbent material based on chitosan to improve adsorption selectivity for heavy metals, so that the crosslinked N,O-carboxymethyl-chitosan resin with Pb(II) as template ions (crosslinked CMC template) were synthesized by using CMC adsorbed Pb(II) ions crosslinked with glutaraldehyde. The effects on adsorption capacities such as amount of crosslinking agent, degree of substitution of CMC, pH value of the initial solution and adsorption time were investigated. The adsorption experiments demonstrated the crosslinked CMC template has high adsorption selectivity for Pb(II) ions in solution containing single metal ions or coexistence of three metals ions of Cu(II), Zn(II) and Pb(II). Furthermore, it was investigated that the crosslinked CMC template has a good reusability and stability as compared to CMC. Finally, we studied the adsorption mechanism by FTIR spectra and XPS analysis. The results reveal that the adsorption of crosslinked CMC template and CMC for Pb(II) is a chelation process.     

Heavy metal ions and organic dyes removal from water by cellulose modified with maleic anhydride

A novel type of adsorbent (CM) was synthesized by cellulose modified with maleic anhydride to remove heavy metal ions and organic dyes in this work. The synthesized adsorbent was characterized by FTIR, SEM, TGA and XRD. The degree of carboxyl group of CM was found to be 2.7 mmol g⁻¹ by the titration method. The adsorption of Hg(II) ions as heavy metal ions while basic fuchsine, methylene blue and crystal violet as organic dyes by CM was investigated. The influence of different experimental parameters such as pH, contact time, temperature on removal process was evaluated. The results indicated that the CM has a good adsorption capacity for Hg(II). The maximum adsorption capacity of Hg(II) was found to be 172.5 mg g⁻¹, and the adsorption process was described by Freundlich isotherm model of adsorption well. The process of basic fuchsine, methylene blue and crystal violet adsorbed by CM was also studied and the adsorption removal rate of those organic dyes was 88.10, 98.47 and 92.85 % under the optimum conditions, respectively. The adsorption process was depicted by the Langmuir isotherm model more correctly.     

Kinetics of mercury ions removal from synthetic aqueous solutions using by novel magnetic p(GMA-MMA-EGDMA) beads

Poly(glycidylmethacrylate-methylmethacrylate), p(GMA-MMA-EGDMA), magnetic beads were prepared via suspension polymerization in the presence of ferric ions. The epoxy groups of the beads were converted into amino groups via ring opening reaction of the ammonia and, the aminated magnetic beads were used for the removal of Hg(II) ions from aqueous solution in a batch experiment and in a magnetically stabilized fluidized bed reactor (MFB). The magnetic p(GMA-MMA-EGDMA) beads were characterized with scanning electron microscope (SEM), FT-IR and ESR spectrophotometers. The optimum removal of Hg(II) ions was observed at pH 5.5. The maximum adsorption capacity of Hg(II) ions by using the magnetic beads was 124.8+/-2.1 mgg(-1) beads. In the continuous MFB reactor, Hg(II) ions adsorption capacity of the magnetic beads decreased with an increase in the flow-rate. The maximum adsorption capacity of the magnetic beads in the MFB reactor was 139.4+/-1.4 mgg(-1). The results indicate that the magnetic beads are promising for use in MFB for removal of Hg(II) ions from aqueous solution and/or waste water treatment.     

Influence of sulfur-crosslinking in vulcanized rubber chips on mercury(II) removal from contaminated water

The adsorption of Hg(II) by natural rubber chips was investigated. First, the effect of chip size (5 mmx5 mm and 10 mmx10 mm) on the adsorption kinetics was studied. The pseudo-second-order modeling was found to explain the kinetics well. The smaller chips had higher adsorption rate so they were used for the rest of the research. Next the effects of sulfur, zinc oxide and carbon black on the adsorption capacity of Hg(II) at equilibrium conditions were investigated. The effect of sulfur was studied through different standard vulcanizing systems. The amount of zinc oxide was varied to be 3, 4 and 5 part per hundred parts of rubber (phr) while the carbon black (N-330) loading was varied to be 0, 30 and 50 phr, respectively. It was found that adsorption capacity increased with the degree of crosslink density, generated by sulfur reacting with rubber molecules. In addition, the adsorption capacities of various amounts of zinc oxide corresponded with their crosslink densities while the addition of carbon black seemed to obstruct Hg(II) adsorption.     

Adsorptive removal of Cu(II) from aqueous solutions using collagen-tannin resin

The collagen-tannin resin (CTR), as a novel adsorbent, was prepared via a reaction of collagen with black wattle tannin and aldehyde, and its adsorption properties to Cu(II) were systematically investigated, including pH effect, adsorption equilibrium, adsorption kinetics, and column adsorption. The adsorption capacity of Cu(II) on CTR was pH-dependent, and it increased with the increase of solution pH. The adsorption isotherms were well described by Langmuir isotherm model with correlating constant (R(2)) higher than 0.99. The adsorption capacity determined at 303 K was high up to 0.26 mmol/g, which was close to the value (0.266 mmol/g) estimated from Langmuir equation. The adsorption capacity was increased with the increase of temperature, and thermodynamic calculations suggested that the adsorption of Cu(II) on CTR is an endothermic process. The adsorption kinetics were well fitted by the pseudo-second-order rate model. Further column studies suggested that CTR was effective for the removal of Cu(II) from solutions, and more than 99% of Cu(II) was desorbed from column using 0.1 mol/L HNO(3) solution. The CTR column can be reused to adsorb Cu(II) without any loss of adsorption capacity.     

Immobilization of aqueous Hg(II) by mackinawite (FeS)

As one of the major constituents of acid volatile sulfide (AVS) in anoxic sediments, mackinawite (FeS) is known for its ability to scavenge trace metals. The interaction between aqueous Hg(II) (added as HgCl(2)) and synthetic FeS was studied via batch sorption experiments conducted under anaerobic conditions. Due to the release of H(+) during formation of hydrolyzed Hg(II) species which is more reactive than Hg(2+) in surface adsorption, the equilibrium pH decreased with the increase in Hg(II)/FeS molar ratio. Counteracting the loss of FeS solids at lower pH, the maximum capacity for FeS to remove aqueous Hg(II) was approximately 0.75 mol Hg(II) (mol FeS)(-1). The comparison of X-ray power diffraction (XRPD) patterns of synthetic FeS sorbent before and after sorption showed that the major products formed from the interaction between FeS and the aqueous Hg(II) were metacinnabar, cinnabar, and mercury iron sulfides. With the addition of FeS at 0.4 g L(-1) to a 1 mM Hg(II) solution with an initial pH of 5.6, Fe(2+) release was approximately 0.77 mol Fe(2+) per mol Hg(II) removed, suggesting that 77% of Hg(II) was removed via precipitation reaction under these conditions, with 23% of Hg(II) removed by adsorption. Aeration does not cause significant release of Hg(II) into the water phase.     

Polyrhodanine modified anodic aluminum oxide membrane for heavy metal ions removal

Polyrhodanine was immobilized onto the inner surface of anodic aluminum oxide (AAO) membrane via vapor deposition polymerization method. The polyrhodanine modified membrane was applied to remove heavy metal ions from aqueous solution because polyrhodanine could be coordinated with specific metal ions. Several parameters such as initial metal concentration, contact time and metal species were evaluated systematically for uptake efficiencies of the fabricated membrane under continuous flow condition. Adsorption isotherms of Hg(II) ion on the AAO-polyrhodanine membrane were analyzed with Langmuir and Freundlich isotherm models. The adsorption rate of Hg(II) ion on the membrane was obeyed by a pseudo-second order equation, indicating the chemical adsorption. The maximum removal capacity of Hg(II) ion onto the fabricated membrane was measured to be 4.2 mmol/g polymer. The AAO-polyrhodanine membrane had also remarkable uptake performance toward Ag(I) and Pb(II) ions. Furthermore, the polyrhodanine modified membrane could be recycled after recovery process. These results demonstrated that the polyrhodanine modified AAO membrane provided potential applications for removing the hazardous heavy metal ions from wastewater.     

Procion Green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) composite membranes for heavy metal removal

The effective removal of toxic heavy metals from environmental samples still remains a major topic of present research. Metal-chelating membranes are very promising materials as adsorbents when compared with conventional beads because they are not compressible, and they eliminate internal diffusion limitations. The purpose of this study was to evaluate the performance of a novel adsorbent, Procion Green H-4G immobilized poly(hydroxyethylmethacrylate (HEMA)/chitosan) composite membranes, for the removal of three toxic heavy metal ions, namely, Cd(II), Pb(II) and Hg(II) from aquatic systems. The Procion Green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) composite membranes were characterized by elemental analysis, scanning electron microscopy and Fourier transform infrared (FTIR) spectroscopy. The immobilized amount of the Procion Green H-4G was calculated as 0.018+/-0.003 micromol/cm(2) from the nitrogen and sulphur stoichiometry. The adsorption capacity of Procion Green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) composite membranes for selected heavy metal ions from aqueous media containing different amounts of these ions (30-400mg/l) and at different pH values (2.0-6.0) was investigated. The amount of Cd(II), Pb(II) and Hg(II) adsorbed onto the membranes measured at equilibrium, increased with time during the first 45 min and then remained unchanged toward the equilibrium adsorption. The maximum amounts of heavy metal ions adsorbed were 43.60+/-1.74, 68.81+/-2.75 and 48.22+/-1.92 mg/g for Cd(II), Pb(II) and Hg(II), respectively. The heavy metal ion adsorption on the pHEMA/chitosan membranes (carrying no dye) were relatively low, 6.31+/-0.13 mg/g for Cd(II), 18.73+/-0.37 mg/g for Pb(II) and 18.82+/-0.38 mg/g for Hg(II). Competitive adsorption of the metal ions was also studied. When the metal ions competed with each other, the adsorbed amounts were 12.74+/-0.38 mg Cd(II)/g, 28.80+/-0.86 mg Pb(II)/g and 18.41+/-0.54 mg Hg(II)/g. Procion Green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) membranes can be regenerated by washing with a solution of nitric acid (0.01 M). The percent desorption achieved was as high as 95%. These novel membranes are suitable for repeated use for more than five adsorption/desorption cycles without any considerable loss in adsorption capacity. Adsorption equilibria were well described by Langmuir equation. It can be concluded that Procion Green H-4G immobilized poly(hydroxyethylmethacrylate/chitosan) membranes may effectively be used for the removal of Cd(II), Pb(II) and Hg(II) ions from aqueous solutions.     

Melamine-formaldehyde-NTA chelating gel resin: Synthesis, characterization and application for copper(II) ion removal from synthetic wastewater

A new chelating resin was synthesised by anchoring nitrilotriacetic acid (NTA) to melamine during the melamine-formaldehyde gelling reaction in the presence of water, using acetone and guaiacol as a porogen mixture. This technique gives a porous chelating gel resin capable of removing heavy metals from wastewater. FT-IR, XRD, elemental analysis, surface area and water regain measurements were conducted for characterization of the new chelating gel resin. A comprehensive adsorption study (kinetics isotherm, and thermodynamics) of Cu(II) removal from synthetic acidic aqueous solutions by adsorption on this resin was conducted regarding the effects of time, temperature, initial pH and copper(II) initial concentration.     

Eggshell nano-particle potential for methyl violet and mercury ion removal: Surface study and field application

A nano-scale sorbent was produced from eggshell wastes for sorption of Hg(II) and methyl violet (MV) from aqueous solutions and real wastewaters. The properties of the nano-particles were fully determined using SEM, DLS, FTIR, XRD, BET, TGA, AFM, EDAX, mapping, and TEM analyses. The adsorbent structure mainly contained carbonate and silica. The effects of influential parameters including temperature, contact time, initial contaminants concentration, sorbent dose, and initial pH on the removal efficiency were investigated. The maximum sorption efficiency of Hg(II) and MV occurred at pH of 6 and 9 and temperatures of 25 °C and 55 °C, respectively. Freundlich model could be interpreted the equilibrium data of the sorption process of both contaminants. The maximum sorption capacity of Hg(II) and MV using eggshell nano-particles was obtained as 116.27 mg/g and 123.45 mg/g, respectively. The dynamic behavior of the process was studied using two kinetic models. The sorption system performance was also examined and t_1/2 were determined as 4.34 min for Hg(II) and 4.97 min for MV. The sorption process of Hg(II) and MV was exothermic and endothermic, respectively. Effective sorption after seven cycles and successful treatment of landfill leachate and textile wastewater with eggshell nano-particles confirms its adequacy.     

氨基磷酸螯合树脂(D418)高效去除Cu(II)的性能与机制

为阐明氨基磷酸螯合树脂(D418)在水体中高效去除Cu(II)的作用机制,通过吸附实验系统考察了pH、离子强度、接触时间、温度等因素对D418树脂去除Cu(II)的影响,并通过吸附动力学模型、等温吸附模型和位点能量分布理论分析其去除机制。研究结果表明:Cu(II)溶液初始pH=9.00时,Cu(II)最大去除率达到97.20%,且Zeta电位变化对Cu(II)去除率影响符合Boltzmann模型。离子强度在0~0.10 mol/L增加,有利于促进D418树脂去除Cu(II)。根据线性相关系数大小比较,D418树脂吸附Cu(II)过程最符合颗粒内扩散模型和Sips模型。以Sips模型计算热力学参数和吸附位点能量分布,D418树脂对Cu(II)的去除为自发进行的吸热过程。Cu(II)先占据D418树脂高能量位点,再占据低能量位点。基于XPS和FTIR数据分析,D418树脂去除Cu(II)的机制主要是静电吸引、化学沉淀和内层络合作用。      

The adsorption of Cd(II) ions on sulphuric acid-treated wheat bran

The adsorption of Cd(II) ions which is one of the most important toxic metals by using sulphuric acid-treated wheat bran (STWB) was investigated. The effects of solution pH and temperature, contact time and initial Cd(II) concentration on the adsorption yield were studied. The equilibrium time for the adsorption process was determined as 4 h. The adsorbent used in this study gave the highest adsorption capacity at around pH 5.4. At this pH, adsorption capacity for an initial Cd(II) ions concentration of 100 mg/L was found to be 43.1 mg/g at 25 degrees C for contact time of 4 h. The equilibrium data were analysed using Langmuir and Freundlich isotherm models to calculate isotherm constants. The maximum adsorption capacity (qmax) which is a Langmuir constant decreased from 101.0 to 62.5 mg/g with increasing temperature from 25 to 70 degrees C. Langmuir isotherm data were evaluated to determine the thermodynamic parameters for the adsorption process. The enthalpy change (deltaH(o)) for the process was found to be exothermic. The free energy change (deltaG(o)) showed that the process was feasible. The kinetic results indicated that the adsorption process of Cd(II) ions by STWB followed first-order rate expression and adsorption rate constant was calculated as 0.0081 l/min at 25 degrees C. It was observed that the desorption yield of Cd(II) was highly pH dependent.     

An investigation into the sorption of heavy metals from wastewaters by polyacrylamide-grafted iron(III) oxide

An adsorbent for heavy metals was synthesized by introducing carboxylate functional group into polyacrylamide-grafted hydrous iron(III) oxide. The product exhibits a very high adsorption potential for Pb(II), Hg(II) and Cd(II). The removal of metal ions by adsorption on adsorbent has been found to be contact time, concentration, pH and temperature dependent. The process follows first-order reversible kinetics. The intraparticle diffusion of metal ions through pores in the adsorbent was shown to be the main rate-limiting step. The optimum pH range for the removal of metal ions was found to be 5.0-6.0. The thermodynamic parameters such as free energy change, enthalpy change and entropy change have been calculated to predict the nature of adsorption. The adsorption data were fitted using the Langmuir equation and maximum adsorption for each metal was estimated using their respective Langmuir equation constants. The method was applied for synthetic wastewaters. NaCl regeneration has been tried for several cycles with a view to recover the adsorbed metal ions and also to restore the sorbent to its original state.