Determination of the complexing properties of drinking waters toward copper(II) and aluminium(III) by ligand titration

The complexing capacity of some drinking waters for aluminium(III) and copper(II) is determined by a ligand titration with metal ions based on the use of complexing resins. The resins used in the titration are the iminodiacetic resin Chelex 100, the carboxylic resin Amberlite CG50 and the anionic exchange resin AG1X8. They allow the detection of ligands forming complexes of different stability with the metal ions used for the titration, since they have different sorbing properties. After equilibration with the resin, the concentration of the free metal ion in solution is evaluated from the concentration of sorbed metal ion and from the quantity K^*, which is the ratio of the concentration of the metal ion sorbed on the resin to the free metal ion in solution. It strongly depends on the conditions, but it can be evaluated, at the considered conditions, from the sorption equilibria of the metal ion on the resin. The concentration of the ligands in solution and the conditional stability constant are obtained from the Ruzik linearization procedure. Very strong ligands of copper(II) and aluminium(III) were detected in a tap water sample at concentrations ranging from 10⁻⁷ to 10⁻⁶ mol kg⁻¹, and forming complexes having conditional complexation constants K_cI=2.3×10¹⁷ (pH=6.77) and 4.5×10¹⁶ (pH=6.24), respectively, for copper(II) and aluminium(III). Weaker ligands were detected using the less strongly sorbing resins Amberlite CG50 and AG1X8, but at a concentration equal to that of the strong ligands. This was ascribed to the presence of competing metals in solution, not sorbed by the weak resins. Two other drinking waters had completely different complexing properties both towards copper(II) and aluminium(III), containing much weaker ligands.     

Observations on the use of iron(II) complexing agents to fractionate the total filterable iron in natural water samples

Analysis of filtered natural water samples using Fe(II) complexing agents (bathophenanthrolinedisulphonic acid, 2,2′-dipyridyl, ferrozine, TPTZ) and acetic acid-sodium acetate buffer (pH 4.6) showed that the absorbance of the iron(II) complex increased with time both in the presence and the absence of a reducing agent (ascorbic acid, hydroxylammonium chloride). Exposure of the samples to 0.1 M HCl (pH 1.3) for 1 h prior to the addition of the complexing reagents resulted in a stable iron concentration which was designated the acid-extractable fraction of the total filterable iron. The results have implications for the complexometric determination of iron fractions other than total iron after acid digestion.     

Removal of free and complexed heavy-metal ions by sorbents produced from fly (Musca domestica) larva shells

Fly larva shells (FLS) are formed as a side product in the biological treatment of organic wastes, and chitin and chitosan produced from the FLS have been used as sorbents for heavy-metal ions. Sorbents are characterised by FT-IR measurements and pH-potentiometric titration and by determination of their surface area, and the content of main elements (C, N, P, S) and ashes. Free metal ions are sorbed best (up to 0.5-0.8 mmol g(-1)) onto chitin and chitosan. The sorption ability for free metal ions of chitin decreases in the order Fe(III) > Cu(II) (Pb(II) > Zn(II). > Ni(II) > Mn(II) and that of chitosan decreases in the order Cu(II) > Mn(II) > Ni(II) > Zn(II) > Pb(II) > Fe(III). The complexed metal ions are sorbed by the FLS up to 0.2-0.4mmol g(-1). The sorption ability for metal ions and ligands depends on pH, concentration of complexed metal ions and the ligand species in the solution. Glycine has the retarding effect on the sorption of Ni(II) and Cu(II) ions, and EDTA enhances the Cu(II) ion sorption. Ni(II) and glycine sorption obeyed the Langmuir isotherm. The observed sorption data show the promising potentialities of the FLS for the heavy-metal removal from the solutions, containing strong complexing agents. Mechanisms for the removal of free and complexed metal ions by chitin, chitosan and the FLS have been discussed.     

The removal of hexacyanoferrate(II) and (III) ions in dilute aqueous solution by activated carbon

The removal of iron cyano-complex ions [hexacyanoferrate(II) and (III) ions] in a dilute aqueous solution by activated carbon was investigated. The maximum adsorption of iron cyano-complex ions on activated carbon occurred at pH around 3. The hexacyanoferrate(III) ion was more adsorbable than the hexacyanoferrate(II) ion. Activated carbon promoted the oxidation of hexacyanoferrate(II) ion to (III) ion with dissolved oxygen in an acidic solution and the reduction of hexacyanoferrate(III) ion to (II) ion in an alkaline solution. The iron cyano-complex ion adsorbed on activated carbon could be eluted with higher concentrated acidic and alkaline solutions. The degree of elution decreased with an increase in potassium hydroxide concentration, since parts of the iron cyano-complexes on activated carbon were decomposed to form the iron hydroxide and the hexacyanoferrate(II) ion with an alkaline solution. The behavior of iron cyano-complexes in the presence of activated carbon, in the lower pH range (pH < 1) and at higher temperatures (80°C), was discussed.     

Use of complexing agents for effective ion-exchange separation of Co(II)/Ni(II) from aqueous solutions

Cation-exchange separation of Co(2+)/Ni(2+) from aqueous solutions using water-soluble complexing agents of ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), and citrate was experimentally studied at 298 K. Experiments were carried out as a function of initial aqueous pH (1.0-6.0), concentration of total metals (1.5-45.0 mol/m(3)), the concentration ratio of two metals (0.1-10) and of complexing agent to the total metals (0-1). It was shown that the exchange selectivity strongly depended on solution pH and was not completely related to the affinity of any metal with the complexing agents. When a certain level of complexing agent was present, highly effective separation could be achieved at an appropriate pH range (for an equimolar metal solution, e.g., pH 2-3 with EDTA and NTA as well as pH>3 with IDA and citrate). The application potential of this method was highlighted for the separation of Co(2+) from binary mixtures in the presence of trace amount of Ni(2+) due to its high selectivity and the smaller amount of the complexing agents needed.     

Comparative and competitive adsorption of Cu(II), Cd(II) and Pb(II) onto Sepia pharaonis endoskeleton biomass from aqueous solutions

In this research, the possibility of simultaneous removal of lead, cadmium and copper divalent ions from water samples through the use of Sepia pharaonis endoskeleton powder (SPEB) as bio‐material, was investigated. The bio‐sorbent was characterised by Fourier transform infrared spectrum (FT‐IR), atomic force microscopy (AFM) and X‐ray fluorescence (XRF). The different factors affecting the bio‐sorption process were studied. Langmuir and Freundlich isotherm models were applied to analyse the experimental data. The kinetic studies showed that the pseudo‐second order model kinetics were compatible with the investigated systems. It was found that under optimal conditions, this bio‐sorbent was efficient in the uptake of these heavy metal ions from both mono and multi‐metal solutions, and high removal percentages were achieved. This study verified the potential ability of SPEB as an efficient natural adsorbent for removal of Pb(II), Cd(II) and Cu(II) ions from river, tap and mineral water samples.     

Desorption characteristics of Cr(III), Mn(II), and Ni(II) in contaminated soil using citric acid and citric acid-containing wastewater

Heavy metal-contaminated soil and wastewater have been attracting an increasing amount of attention due to the potential threat to the surrounding environment and human health. Thus, in this study, citric acid (CA) and citric acid-containing wastewater (CACW) were selected for an evaluation of the influence of the contamination level of the soil, the concentration of citric acid, the contact time, the soil pH, and the ionic interaction on the desorption characteristics of three heavy metals (i.e., Cr(III), Mn(II), and Ni(II)). According to the experimental results, a high concentration of citric acid, an acidic condition, a low level of contamination, and a lengthy contact time were found to be beneficial for desorbing the heavy metals from the contaminated soil. Based on the experimental and calculated results, the H⁺ ions and organic ligands made substantial contributions to the release and adsorption of the heavy metals. The metal ions on the low selectivity sorption sites were leached out earlier than those on the high selectivity sorption sites. The removal percentages of Cr(III), Mn(II), and Ni(II) using CA with a contact time of 6?h were 39.9%, 77.0%, and 62.8%, respectively. By using the CACW as a desorbent, the removal percentages of Cr(III), Mn(II), and Ni(II) with a contact time of 6?h reached 21.4%, 26.9%, and 63.4%, respectively. This suggests a promising practical application of CACW for removing heavy metals from contaminated soil.     

Kinetic analysis of palladium(II) adsorption process on condensed-tannin gel based on redox reaction models

We have developed a novel recovery system of palladium (Pd) from wastes such as spent catalysts or scraps, using tannin gel particles synthesized from condensed-tannin molecules. The Pd(II) ionic species are reduced to metallic Pd(0) on the network of the tannin gel: a two-electron transfer from the tannin gel to Pd(II). The kinetic study of the electron transfer was performed with a multiple reaction model containing an intermediate step (formation of a ligand-substituted Pd(II)-tannin inner sphere complex), resulting in a better fit with the experimental results than with the single reaction model (outer sphere redox reaction), which means that the inner sphere redox mechanism is an appropriate reaction model for the Pd(II) adsorption process. Because the intermediate is included in the adsorption amount, the adsorption process can be divided into two steps: fast adsorption by the ligand substitution at the initial stage and slow adsorption by the subsequent redox reaction after the ligand substitution reaches an equilibrium state, with different adsorption rates between the Pd(II) ionic species (PdCl(+)>PdCl(2)>PdCl(3)(-),PdCl(4)(2-)).     

Removal of free and chelated Cu(II) ions from water by a nondispersive solvent extraction process

The possibility of extraction of free and the ethylenediaminetetraacetic acid (EDTA)-chelated Cu(II) ions from water through a microporous hollow fiber to an organic phase containing extractants LIX64N and Aliquat 336 (a quaternary amine) was studied, in which Cu(II) was simultaneously back-extracted across another hollow fiber to a stripping solutions containing HCl. Experiments were carried out at different flow rates of feed (1.4-4.3 cm3/s), organic (1.2-4.1 cm3/s), and stripping phases (1.4-4.3 cm3/s), and temperatures (298-318 K). It was shown that the extraction rate increased with decreasing feed phase flow rate, but the effect of flow rates of organic and stripping phases was not pronounced under the ranges studied. Mechanism of the mass transfer at each step was discussed and the effect of mutual interaction of the two extractants in the organic phase was also examined. The application potentials of this novel extraction process for this subject appeared to be technically promising.     

Kinetic modeling of liquid-phase adsorption of reactive dyes and metal ions on chitosan

The rates of adsorption of three commercial reactive dyes and Cu(II) from water in the absence and presence of complexing agents using chitosan were measured at 30 degrees C. Three simplified kinetic models, i.e., pseudo-first-order, pseudo-second-order, and intraparticle diffusion, were tested to investigate the adsorption mechanisms. It was shown that the adsorption of reactive dyes and Cu(II) in the absence of complexing agents could be best described by the intraparticle diffusion model, whereas that of Cu(II) in the absence of complexing agents such as EDTA, citric acid, and tartaric acid by the pseudo-second-order equation. Kinetic parameters of the three models and the normalized standard deviations between the measured and predicted results were also calculated and discussed.     

Efficient arsenic(V) removal from water by ligand exchange fibrous adsorbent

This study is an efficient arsenic(V) removal from contaminated waters used as drinking water in adsorption process by zirconium(IV) loaded ligand exchange fibrous adsorbent. The bifunctional fibers contained both phosphonate and sulfonate groups. The bifunctional fiber was synthesised by graft polymerization of chloromethylstyrene onto polyethylene coated polypropylene fiber by means of electron irradiation graft polymerization technique and then desired phosphonate and sulfonate groups were introduced by Arbusov reaction followed by phosphorylation and sulfonation. Arsenic(V) adsorption was clarified in column methods with continuous flow operation in order to assess the arsenic(V) removal capacity in various conditions. The adsorption efficiency was evaluated in several parameters such as competing ions (chloride and sulfate), feed solution acidity, feed flow rate, feed concentration and kinetic performances at high feed flow rate of trace concentration arsenic(V). Arsenic(V) adsorption was not greatly changed when feed solutions pH at 3.0-7.0 and high breakthrough capacity was observed in strong acidic area below pH 2.2. Increasing the flow rate brings a decrease both breakthrough capacity and total adsorption. Trace level of arsenic(V) (0.015 mM) in presence of competing ions was also removed at high flow rate (750 h⁻¹) with high removal efficiency. Therefore, the adsorbent is highly selective to arsenic(V) even in the presence of high concentration competing ions. The adsorbent is reversible and reusable in many cycles without any deterioration in its original performances. Therefore, Zr(IV) loaded ligand exchange adsorbent is to be an effective means to treat arsenic(V) contaminated water efficiently and able to safeguard the human health.     

Influence of fulvic acid on the removal of trace concentrations of cadmium(II), copper(II), and zinc(II) from water by alum coagulation

The effectiveness of Cu²⁺, Cd²⁺ and Zn²⁺ removal from solution by alum coagulation was measured with fulvic acid present and absent. A factorial experimental design and analysis of variance were used to determine the effect on metal ion removal of the individual variables pH, metal ion concentration, alum concentration and fulvic acid concentration and their combinations. The variable levels model water treatment plant conditions. Metal ion losses up to 96% for Cu²⁺. 59% for Cd²⁺ and 82% for Zn^z+ were measured in the presence of fulvic acid. In its absence the maximum metal ion losses observed were 93%, 14% and 53% for Cu²⁺, Cd²⁺ and Zn²⁺ respectively. Fulvic acid enhances metal ion removal under most experimental conditions. The practical implication of the results is that strong complexes between natural water organic matter and metal ions enhance their removal by the alum coagulation process.     

Sonochemical destruction of free and metal-binding ethylenediaminetetraacetic acid

This study focused on the sonochemical degradation of ethylenediaminetetraacetic acid (EDTA) and chromium-EDTA complexes. Degradation of the copper(II)-EDTA complex was also investigated as a comparison metal complex. A 90% degradation of a 150-microM EDTA solution with continuous O2-bubbling was shown for the 20-kHz system in approximately 3 h (kpseudo-first order = 1.22 x 10(-2) min-1) and less than 1 h for the 354-kHz system (kpseudo-first order = 5.42 x 10(-2) min-1). These results are consistent with the higher concentrations of hydrogen peroxide found in the higher frequency system and an expected oxidation of EDTA in bulk solution. The presence of a chelated metal decreased the rate of degradation at both frequencies. Cr(III)-EDTA degraded the slowest, supporting the theory that the extremely slow ligand exchange rate of chromium is the determining factor in how fast degradation by hydroxyl radical can occur. The 354-kHz system showed a 17% decrease in the original 150-microM Cr(III)-EDTA complex after 3 h of sonication. All of the chromium from the degraded EDTA complex existed as a combination of oxidized Cr(VI) and possibly small amounts of a new Cr(III)-organic complex (Cr(III)-Y). The 20-kHz system showed a similar extent of degradation (16%) after 3 h of sonication, despite lower hydroxyl radical production. Fifty percent of the chromium from the degraded EDTA complex was found as free Cr3+ ion, with the remaining 50% existing as both Cr(III)-Y and Cr(VI). Varying degrees of bulk oxidation, near-bubble thermolysis, and perhaps different degradation pathways at the two frequencies are responsible for these differences.     

Graphene oxide nanosheets modified with trithiocyanuric acid for extraction and enrichment of Pb (II) and Cu (II) ions in seawater

Trithiocyanuric acid (TTC) is a robust chelating ligand for extracting divalent and univalent heavy metals from water samples. In this method, graphene oxide nanosheets (GO) was modified with TTC, to produce a new nano‐adsorbent (GO‐TTC), for extraction and preconcentration of Pb (II) and Cu (II) ions from seawater samples. The concentrated analytes were measured by a flame atomic absorption spectrometry. The detection limits of the method for Pb (II) and Cu (II) ions were 0.32 ng/mL and 0.13 ng/mL, respectively. The relative standard deviations of analytes for six successive extractions were below 1.64%. The maximum adsorption capacity of GO‐TTC for Pb (II) and Cu (II) ions was 750 µg/g and 460 µg/g, respectively. The recoveries of the spiked analytes in artificial and natural seawater samples were above 90%, pointing out that the method is not affected by the major coexisting ions in seawater matrices.     

The behaviour of technetium during microbial reduction in amended soils from Dounreay, UK

Radioactive technetium-99 forms during nuclear fission and has been found as a contaminant at sites where nuclear wastes have been processed or stored. Here we describe results from microcosm experiments containing soil samples representative of the UKAEA site at Dounreay to examine the effect of varying solution chemistry on the fate of technetium during microbial reduction. Analysis of a suite of stable element redox indicators demonstrated that microbial activity occurred in a range of microcosm experiments including unamended Dounreay sediments, carbonate buffered sediments, and microcosms amended with ethylenediaminetetraacetic acid (EDTA) a complexing ligand used in nuclear fuel cycle operations. During the development of anoxia mediated by indigenous microbial populations, TcO4- was removed from solution in experiments. In all cases, the removal of TcO4- from solution occurred during active microbial Fe(III)-reduction when Fe(II) was growing into the microcosms. Tc removal was most likely via reduction of TcO4- to poorly soluble Tc(IV) which is retained on the sediments. The potential stability of Tc associated with the soil to remobilisation via complexation with EDTA was examined as reduced Tc-labelled sediments were contacted with a de-oxygenated EDTA solution. No remobilisation of Tc(IV) in the presence of EDTA was observed.     

Degradation of 2,4,5-trichlorophenoxyacetic acid by a novel Electro-Fe(II)/Oxone process using iron sheet as the sacrificial anode

A novel electrochemically enhanced advanced oxidation process for the destruction of organic contaminants in aqueous solution is reported in this study. The process involves the use of an iron (Fe) sheet as sacrificial anode and a graphite bar as cathode. In the oxidation process, once an electric current is applied between the anode and the cathode, a predetermined amount of Oxone is added to the reactor. Ferrous ions generated from the sacrificed Fe anode mediate the generation of highly powerful radicals (SO₄^•−) through the decomposition of Oxone. The coupled process of Fe(II)/Oxone and electrochemical treatment (Electro-Fe(II)/Oxone) was evaluated in terms of 2,4,5-Trichlorophenoxyacetic acid degradation in aqueous solution. Various parameters were investigated to optimize the process, including applied current, electrolyte and Oxone concentration. In addition, low solution pH facilitates the system performance due to the dual effects of weak Fenton reagent generation and persulfate ions generation, whereas the system performance was inhibited at basic pH levels through non-radical self-dissociation of Oxone and the formation of ferric hydroxide precipitates. Furthermore, the active radicals involved in the Electro-Fe(II)/Oxone process were also identified. The Electro-Fe(II)/Oxone process demonstrates a very high 2,4,5-T degradation efficiency (over 90% decay within 10 min), which justifies the novel Electro-Fe(II)/Oxone a promising treatment process for herbicide removal in water.     

Nitrite reduction with hydrous ferric oxide and Fe(II): Stoichiometry, rate, and mechanism

Fe(II)/Fe(III) oxide is an important redox couple in environmental systems. Recent studies have revealed unique characteristics of Fe(II)/Fe(III) oxide and reactions with oxidizing or reducing agents. Nitrite was used as an oxidizing agent in this study in order to probe details of these reactions and hydrous ferric oxide (HFO) was used as the Fe(III) oxide phase. Abiotic nitrite reduction is a significant global producer of nitric oxide (a catalyst for production of tropospheric ozone) and nitrous oxide (a greenhouse gas and contributor to stratospheric ozone depletion). All experiments were conducted at pH 6.8 using a strictly anoxic environment with mass-balance measurements for Fe(II). Oxidation of Fe(II) was negligible in the absence of HFO. The reaction was fast in the presence of HFO and was described by d[Fe(II)]/dt = −k_overall [Fe(II)_diss] [Fe(II)_solid-bound] [NO₂⁻] (k_overall = 2.59 × 10⁻⁷ μM⁻² min⁻¹) for Fe(II)/Fe(III) molar ratios less than 0.30. The reaction was inhibited for higher Fe(II)/HFO ratios. The concentration of solid-bound Fe(II) was constant after an initial equilibration period and the reaction stopped when dissolved Fe(II) was depleted even though substantial solid-bound Fe(II) and nitrite remained. The results regarding rate-dependence and conservation of solid-bound Fe(II) and inhibition of reaction at high Fe(II)/Fe(III) ratios were similar to our earlier results for the Fe(II)/HFO/O₂ system [Park, B., Dempsey, B.A., 2005. Heterogeneous oxidation of Fe(II) on ferric oxide at neutral pH and a low partial pressure of O₂. Environmental Science and Technology 39(17), 6494-6500.].     

Removal of Cu(II) and Cd(II) from aqueous solution by seafood processing waste sludge

Dried waste slurry generated in seafood processing factories has been shown to be an effective adsorbent for the removal of heavy metals from dilute solutions. Characterization of the sludge surface with scanning electron microscope and X-ray microanalyzer were carried out to evaluate the components on the sludge surface that are related to the adsorption of metal ions. Aluminum and calcium, as well as organic carbon are distributed on the surface of sludge. Alkalimetric titration was used to characterize the surface acidity of the sludge sample. The surface acidity constants, pKa1s and pKa2s, were 5.80 and 9.55, respectively. Batch as well as dynamic adsorption studies were conducted with 10(-5) to 5 x 10(-3) M Cu(II) and Cd(II). A surface complexation model with the diffuse layer model successfully predicted Cu(II) and Cd(II) removals in single metal solutions. Predictions of sorption in binary-adsorbate systems based on single-adsorbate data fits represented competitive sorption data reasonably well over a wide range of conditions. The breakthrough capacity found from column studies was different for each metal ion and the data reflect the order of metal affinity for the adsorbent material very well.     

Fabrication and optimization of Cu(II) ion selective membrane electrode

In this study, the fabrication, performance characteristics and application of a Cu(II) ion selective pyridine based thorium(IV) phosphate membrane electrode are studied. The membrane electrode exhibited a fast response time of 10 s, the wide linear response in the concentration of 1 × 10^–1 to 1 × 10^–7 M of Cu (II) ions with a slope of 27.60 mV/decade change in concentration and a lifetime of 4 months. The potentiometric response revealed that the potentials are independent of pH in the wide range of 3.0–6.5. It was also used as an indicator electrode in the potentiometric titration of Cu(II) ions using ethylenediamine tetraacetic acid, disodium salt.     

Removal of Cu(II), Ni(II), and Co(II) from aqueous solutions using layered double hydroxide intercalated with EDTA

A sample of a layered double hydroxide intercalated by EDTA has been synthesized and its chemical formula [Zn₄Al₂(OH)₁₂](EDTA) · 8H₂O was determined. The possibility of applying such sorbent for the extraction of Cu(II), Ni(II), and Co(II) from aqueous solutions was investigated. Comparative investigation of the sorption capacity of carbonate and chelate forms of layered double hydroxides was performed. It is shown that the degree of extraction of metals on sorbent [Zn₄Al₂(OH)₁₂](EDTA) · 8H₂O completely correlates with the stability of complex compounds of these metals in the solution.