Speciation (including adsorbed species) of copper, lead, nickel and zinc in the Meuse River: Observed results compared to values calculated with a chemical equilibrium computer program

The adsorption of trace metals on sediments of the Meuse River was interpreted in terms of competition between metals and protons for surface sites. Surface constants (^*β₁^surf) were determined for Cu, Zn and Cd (10^−1.8, 10^−3.6 and 10^−3.7). The constants for Pb, Ni, Ca and Mg (10^−1.7, 10^−3.8, 10^−6.5 and 10^−5.2) were estimated using a correlation between hydrolysis and surface constants. A chemical equilibrium computer program in which surface sites (for adsorption reactions) are treated as conventional ligands was used to calculate the speciation of Cu, Pb, Ni and Zn in the Meuse River. Calculated values of the adsorbed/dissolved distribution agreed well with observed values, after some realistic data manipulation. This work indicates that dissolved trace metal concentrations in the Meuse River are controlled by adsorption and not by precipitation mechanisms. The relationship between organic matter and suspended matter greatly influences the adsorption of metals like Cu and Pb.     

Recovery of chromium(VI) from wastewaters with iron(III) hydroxide—I: Adsorption mechanism of chromium(VI) on iron(III) hydroxide

Chromium(VI) [Cr(VI)] is adsorbed as HCrO⁻₄ on iron(III) hydroxide at pH below 8.5. The Cr(VI) adsorption is suppressed by the presence of other anions such as SO²⁻₄ and SCN⁻, and enhanced by the presence of metal ions such as Cd(II) and Pb(II). The suppression is due to the competitive adsorption of other anions, depending upon the stability of their iron complexes. The enhancement is probably due to the increase in adsorption sites as a result of coprecipitation of metal ion with iron(III) hydroxide.     

The extent of reversibility of polychlorinated biphenyl adsorption

The extent of reversibility of PCB bonding to sediments has been characterized in studies on the partitioning behavior of a hexachlorobiphenyl isomer. Linear non-singular isotherms have been observed for the adsorption and desorption of 2.4.5.2′,4′,5′ hexachlorobiphenyl (HCBP) to 1100 ppm sediment suspensions. Partition coefficients, π(1 kg−1), for desorption from lake sediments (Saginaw Bay. Lake Huron. Michigan) are substantially greater (π_d 20.000–35.000) than those obtained for adsorption (π₄ 9000–14,000). HCBP was found to be more weakly adsorbed to montmorillonite (π_a − 3000, π_d 9000) and kaolinite (π_a 1000, π_d 3000) clay samples than to the natural sediment samples. Desorption results (π_d) for Saginaw Bay sediments were quite similar to π values (15,000–35.000) calculated from field measurements of aqueous and particulate PCB concentrations. For Saginaw Bay sediments and clay minerals partitioning appeared to be correlated both to sediment surface area and to sediment organic content. A regression analysis using both of these variables explained 90% of the observed variations. HCBP adsorption at 40 C (π_a 14,000) was significantly greater than at 1° C (π_a 6500) resulting in a calculated enthalpy of adsorption of +3.3 kcal mol⁻¹. Non-singular isotherm behavior was not found to be readily attributable to microbiological, kinetic or experimental effects. Evidence from consecutive desorption studies suggests that while HCBP adsorption may ultimately be reversible. release from sediments appeared to involve desorption along two distinct isotherms. These results have been interpreted in terms of possible similarities between the sorption properties observed in the distilled water systems of the present study and PCB bonding processes in natural water systems.     

Sorption and accumulation of cadmium in the sediment of the Tama River

Cadmium contents in the water and the sediment samples collected from the Tama River and several branches were measured. Cadmium (above 0.005mgl⁻¹) was detected in only four of the water samples, while the sediment samples showed cadmium content of 1.0–9.8 μg g⁻¹ dry sediment. Cadmium concentration in the sediments of the main stream was correlated against ignition loss of the samples and it was found that 1 g of ignition loss (organic matter) corresponded to 35 μg of cadmium.The batch adsorption experiment in the laboratory using an aqueous solution of cadmium for 14 sediment samples with a higher concentration of cadmium indicated that the amount adsorbed by the sediment is highly dependent on the ignition loss. The amount adsorbed on unit mass of ignition loss q_IL could be correlated by a Freundlich-type equilibrium relation as where C is the equilibrium concentration in the aqueous phase ranging between 7 × 10⁻³ and 10 mg l⁻¹, while k_IL and n are equilibrium constants.The adsorption rate measurement showed that the intraparticle diffusion coefficient of cadmium in the sediment was about 1.1 × 10⁻⁶ cm²s⁻¹, which is of a reasonable order of magnitude assuming the pore diffusion mechanism inside the particle.The results suggest that suspended solid particles of high organic content in flowing water contribute significantly to the transport of cadmium along the river.     

Toxicity of lipid-soluble copper(II) complexes to the marine diatom Nitzschia closterium: Amelioration by humic substances

Algal assays, using the marine diatom Nitzschia closterium, have established that humic acid (5 mg kg⁻¹) can ameliorate the toxicity of the lipophilic complex Cu(oxine)₂ (3 × 10⁻⁸ mol l⁻¹ in unsupplemented seawater). The toxicity of Cu(PAN)₂ is not ameliorated [PAN = 1-(2-pyridylazo)-2-napthol]. In conjunction with previous visible absorption spectrophotometry and polarographic measurements it was established that humic acid sequesters copper(II) from the hydrophobic complexes, releasing a ligand molecule. The copper(II) toxicity may be ameliorated provided the ligand itself is not toxic.Fulvic acid was significantly less effective in ameliorating toxicity. Because of the significant competition from Ca(II) and Mg(II) in seawater, it is inferred that humic substances may be more effective in ameliorating toxicity of hydrophobic copper complexes in fresh water.     

Adsorption of Cu(II) to schwertmannite and goethite in presence of dissolved organic matter

Sorption processes involving secondary iron minerals may significantly contribute to immobilisation of metals in soils and surface waters. In the present work the effect of dissolved organic matter (DOM) from a concentrated bog-water on the adsorption of Cu(II) onto schwertmannite (Fe₈O₈(OH)₆SO₄) and goethite (α-FeOOH) has been studied. The acid/base behaviour of DOM up to pH 6 was explained by assuming a diprotic acid with a density of carboxylate groups of 6.90 μeq (mg C)⁻¹. The resulting acidity constants, recalculated to zero ionic strength were and .The uptake of DOM to schwertmannite and goethite was highest at low pH although adsorption was significant also under mildly alkaline conditions. Adsorption to the two minerals was similar although at high pH more DOM was adsorbed to schwertmannite than to goethite.DOM enhanced the adsorption of Cu(II) at moderately low pH in the goethite system but there was no effect of DOM in the case of schwertmannite. The presence of Cu(II) resulted in a decreased adsorption of DOM to goethite at weakly acidic pH and increased adsorption at high pH. In the case of schwertmannite, Cu(II) did not affect DOM uptake.     

Impact of dissolved organic matter on colloid transport in the vadose zone: deterministic approximation of transport deposition coefficients from polymeric coating characteristics

Although numerous studies have been conducted to discern colloid transport and stability processes, the mechanistic understanding of how dissolved organic matter (DOM) affects colloid fate in unsaturated soils (i.e., the vadose zone) remains unclear. This study aims to bridge the gap between the physicochemical responses of colloid complexes and porous media interfaces to solution chemistry, and the effect these changes have on colloid transport and fate. Measurements of adsorbed layer thickness, density, and charge of DOM-colloid complexes and transport experiments with tandem internal process visualization were conducted for key constituents of DOM, humic (HA) and fulvic acids (FA), at acidic, neutral and basic pH and two CaCl₂ concentrations. Polymeric characteristics reveal that, of the two tested DOM constituents, only HA electrosterically stabilizes colloids. This stabilization is highly dependent on solution pH which controls DOM polymer adsorption affinity, and on the presence of Ca⁺² which promotes charge neutralization and inter-particle bridging. Transport experiments indicate that HA improved colloid transport significantly, while FA only marginally affected transport despite having a large effect on particle charge. A transport model with deposition and pore-exclusion parameters fit experimental breakthrough curves well. Trends in deposition coefficients are correlated to the changes in colloid surface potential for bare colloids, but must include adsorbed layer thickness and density for sterically stabilized colloids. Additionally, internal process observations with bright field microscopy reveal that, under optimal conditions for retention, experiments with FA or no DOM promoted colloid retention at solid-water interfaces, while experiments with HA enhanced colloid retention at air-water interfaces, presumably due to partitioning of HA at the air-water interface and/or increased hydrophobic characteristics of HA-colloid complexes.     

The removal of chromium(VI) from dilute aqueous solution by activated carbon

The removal of chromium(VI) by activated carbon, filtrasorb 400, is brought by two major interfacial reactions: adsorption and reduction. Chemical factors such as pH and total Cr(VI) that affect the magnitude of Cr(VI) adsorption were investigated. The adsorption of Cr(VI) exhibits a peak value at pH 5–6. The particle size of carbon and the presence of cyanide species do not change the magnitude of chromium removal. The reduced Cr(VI), e.g. Cr(III) is less adsorbable than Cr(VI).The free energy of specific chemical interaction, ΔG^chem was computed by the Gouy-Chapman-Stern-Grahame model. The average values of ΔG^chem are −5.57 RT and −5.81RT, respectively, for Cr(VI) and CN. These values are significant enough to influence the overall magnitude of Cr(VI) and CN adsorption. Results also indicate that HCrO⁻₄ and Cr₂O²⁻₇ are the major Cr(VI) species involved in surface association.     

Adsorption from Brazilian soils of Cu(II) and Cd(II) using cattle manure vermicompost

The potential of cattle manure vermicompost and Brazilian soils (whole soils and soils incubated with vermicompost) was assessed for adsorption of heavy metals such as Cu(II) and Cd(II) from aqueous solutions. Experimental data have been fitted to Langmuir and Freundlich isotherms to obtain the characteristic parameters of each model, with R ² values from 0.89 to 0.99. Based on the maximum adsorption capacity obtained from the Langmuir isotherm the affinity of the studied metals for the vermicompost and soils have been established as Cu(II) > Cd(II). The values of the separation factor, R _L, which has been used to predict affinity between adsorbate and adsorbent were between zero and 1, indicating that sorption was very favourable for Cu(II) and Cd(II) in synthetic solution. Addition of vermicompost to soils resulted in higher distribution coefficient, K _d, as compared with whole soils. The thermodynamic parameter, the Gibbs energy changes, was calculated for each system and the negative values obtained confirm that the adsorption processes are spontaneous. The ΔG° values for the substrates were between ?2.630±1.41 kJ mol^?1 and ?13.700±1.250 kJ mol^?1. Adsorption tests from multimetal systems confirm the affinity order obtained in the individual metal tests. The adsorption capacity for Cu(II) measured in individual tests is not reduced by the presence of Cd(II). There is also desorption of Cu(II) and Cd(II) previously bound to vermicompost, whole soils and soils incubated with vermicompost by DTPA. The experiment indicates the importance of cattle manure vermicompost and oxisol amended with vermicompost in relation to Cu(II) and Cd(II) adsorption from aqueous solution.     

Adsorption of Cu(II) from industrial liquid waste on Na-bentonite and Al-PILC following a full factorial design

The paper reports a study of the performance of Maghnia bentonite in a purified and modified state for the removal of Cu(II) from industrial liquid waste in the region of Oran (North West Algeria). Bentonite was firstly treated to produce a Na-bentonite, then modified with an aluminum solution containing molar ratio OH/Al of 1.8 and finally calcined at 450 °C. The polymer [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ formed in solution was adsorbed by surface complexation on the bentonite, which is known to have a high capacity to fix metal cations. The prepared materials were characterized by DRX, BET and EDX. In order to find the optimum conditions, a full factorial design of 2⁴ allowed us to determine the main effects and interactions of the factors studied: pH, mass of materials, contact time and temperature. The results obtained show that the best rate of adsorption of copper requires a pH = 10, a mass = 0.8 g, a stirring time = 80 min, and a temperature = 25 °C. The adsorption capacity of treated bentonite increased considerably from 4.147 mg/g for Na-bentonite to 7.173 mg/g for pillared aluminum bentonite. This shows the strong adsorption of copper compared with Na-bentonite, caused by its high surface area.     

Uptake and release of zinc by aquatic bryophytes (Fontinalis antipyretica L. ex. Hedw.)

The zinc uptake and posterior release by an aquatic bryophyte—Fontinalis antipyretica L. Ex Hedw.—was experimentally studied in laboratory exposing the plants to different zinc concentrations in the range, 1.0–5.0 mg l⁻¹, for a 144 h contamination period, and then exposed to metal-free water for a 120 h decontamination period. The experiments were carried out in perfectly mixed contactors at controlled illumination, using mosses picked out in February 1997, with a background initial zinc concentration of 263 mg g⁻¹ (dry wt.). A first-order mass transfer kinetic model was fitted to the experimental data to determine the uptake and release constants, k₁ and k₂, the zinc concentration in mosses at the end of the uptake period, C_mu, and at the equilibrium, for the contamination and decontamination stages, C_me and C_mr, respectively. A bioconcentration factor, BCF=k₁/k₂ (zinc concentration in the plant, dry wt./zinc concentration in the water) was determined. A biological elimination factor defined as BEF=1−C_mr/C_mu was also calculated. BCF decreases from about 4500 to 2950 as Zn concentration in water increases from 1.05 to 3.80 mg l⁻¹. BEF is approximately constant and equal to 0.80. Comparing Zn and Cu accumulation by Fontinalis antipyretica, it was concluded that the uptake rate for Zn (145 h⁻¹) is much lower than for Cu (628 h⁻¹) and the amount retained by the plant decreased by a factor of about seven.     

Iron and manganese oxides in Finnish ground water treatment plants

Large amounts of ochreous precipitates are formed on aeration of Fe containing Finnish ground waters during purification for drinking purposes. Sixty-four precipitates were characterized chemically and mineralogically. X-ray diffraction (XRD) indicated that the Fe-rich precipitates consist mainly of a poorly ordered ferrihydrite (5 Fe₂O₃ · 9 H₂O) which only has 2–3 of the 6 XRD lines characteristic of better ordered ferrihydrite. The surface area ranges between 325 and 433 m² g⁻¹ corresponding to a particle size of 5 nm. The ferrihydrites contain 3–7% Si strongly associated with the ferrihydrite as indicated by an i.r. absorption band at 960–975 cm⁻¹ which is associated to Fe-O-Si bonds. Si-containing ferrihydrite typically forms by rapid oxidation of ground waters with 1–23 mg 1⁻¹ Fe and 7–12 mg 1⁻¹ Si at pH 6–7. Very similar products formed in a simulation experiment in which artificial ground water with 20 mg 1⁻¹ Fe was oxidized in the presence of 12 mg 1⁻¹ Si. A1 < 4 mg 1⁻¹ Si lepidocrocite (γ-FeOOH) was formed showing that Si in the system prevents the formation of the more stable and better crystallized FeOOH forms. A transformation of 2-line ferrihydrite to better ordered ferrihydrite or goethite with time is indicated. The Mn-oxide birnessite was identified in black precipitates formed in one plant.     

The adsorption of Cu, Mn and Zn by iron oxyhydroxide in model estuarine solutions

Chemical models have been used to examine the adsorption behaviour of Cu, Mn and Zn onto freshly precipitated iron oxyhydroxide as a function of pH and salinity. Solutions of varying salinity (S = 0, 5, 10, 15‰) containing 0.05 mM Fe and 1.5 μM of each metal were allowed to attain equilibrium under carefully controlled conditions. The solutions were analysed for the dissolved metals (passing a 0.45 μm filter) over the pH range 3.5–10. The pH of the onset of adsorption followed the order Cu < Zn < Mn and it occurred at pHs well below the hydrolysis of the cations in the bulk solution. Hypotheses on the metal uptake involving coulombic interactions between the precipitate and the metal species in solution did not fit the observations and it was concluded that a detailed knowledge of surface complexation reactions was required, as was shown for Cu. The adsorption isotherms for Cu were independent of salinity, on the other hand, those for Zn and Mn showed an increase in the pH of the adsorption edge with increases in salinity. It was suggested that the major cations, Ca²⁺ and Mg²⁺, were probably co-adsorbed and competition from these species for adsorption sites increased with increasing salinity. The results reported here are useful for the development of predictive models for the fate of trace metals injected into estuarine systems where iron compounds may be freshly precipitated.     

Development and characterization of ferrihydrite-modified diatomite as a phosphorus adsorbent

A novel phosphorus adsorbent, ferrihydrite-modified diatomite was developed and characterized in this study. The ferrihydrite-modified diatomite was made through surface modification treatments including NaOH treatment and ferrihydrite deposition on raw diatomite. In the NaOH treatment, surface SiO₂ of diatomite was partially dissolved in the NaOH solution. The dissolved Si contributed to form the stable 2-line ferrihydrite which deposited into the macropores and mesopores of diatomite. Blocking macropores and larger mesopores of diatomite with 0.24 g Fe/g of 2-line ferrihydrite resulted in a specific surface area of 211.1 m²/g for the ferrihydrite-modified diatomite, which is 8.5-fold increase than the raw diatomite (24.77 m²/g). The surface modification also increased the point of zero charge (pH_PZC) values to 10 for the ferrihydrite-modified diatomite from 5.8 for the raw diatomite. Because of the increased surface area and surface charge, the maximum adsorption capacity of ferrihydrite-modified diatomite at pH 4 and pH 8.5 was increased from 10.2 mg P/g and 1.7 mg P/g of raw diatomite to 37.3 mg P/g and 13.6 mg P/g, respectively.     

Conditions required for the precipitation of aluminium in acidic natural waters

Laboratory and field studies were carried out in order to define the conditions necessary for the precipitation of Al in natural waters of pH 4–6. It is concluded that if precipitation does occur it involves the formation of Al(oxy)hydroxide, not aluminosilicates or basic aluminium sulphates. The solubility product of the Al(oxy)hydroxide is highly temperature dependent (ΔH = −30.5 kcal⁻¹). It is also sensitive to concentrations of SO₄²⁻ and, more markedly, humic substances (HS); both of these decrease solubility, HS by more than an order of magnitude at a concentration of approx. 5 mg l⁻¹ (equivalent to approx. 2.5 mg l⁻¹ dissolved organic carbon). A semi-empirical equation is proposed that allows the prediction of the effective solubility product at different temperatures and at humic concentrations in the range 0–7 mg l⁻¹. Of the 113 natural water samples analysed, only one was calculated to be oversaturated with respect to Al(oxy)hydroxide.     

Sorption of organics on clay and synthetic humic-clay complexes simulating aquifer processes

Sorption/partition of several organic solute (contaminants) of a wide range of hydrophobicities was studied on clay and on clay-humic complexes representing aquifer-soil systems. The role of the mineral and of the organic (humic) fractions was elucidated and a model considering both fractions in the sorption process was proposed. The adsorption constants on humic (organic fraction), K_∝, were 8–20 times higher than on “pure” clay, K_m. But with soils with low to medium organic fractions (ƒ_∝ < 0.05) the contribution of the clay mineral to adsorption was quite significant, in spite of the fact that half of the sorption sites on the mineral surface were blocked by the humic. In the range of very low organic content in aquifer soil a non-linear pattern going through a minimum is observed between the overall partition coefficient and soil-organic fraction, transforming to the familiar linear relationship at higher ƒ_∝s. Both the K_∝ and K_m followed the linear-free energy relationship to the octanol-water partition coefficient K_ow.     

Detoxification of seawater used for gas scrubbing in the steel industry

Cyanide ion present in seawater after scrubbing blast furnace and coke ovens gases can be removed by sedimentation of hexacyanoferrate complexes followed by oxidation of residual cyanide with Caro's acid. Zinc ion is removed at the same time by adsorption on the hexacyanoferrate/hydrous ferric oxide precipitate.Sulphide is precipitated as ferrous sulphide, then oxidised by atmospheric oxygen. At 25°C and using an Fe/CN ratio of 1·00, initial concentrations of 50 mg l⁻¹ of CN⁻ and 10 mg l⁻¹ of Zn²⁺ in seawater are reduced to 5–7 mg l⁻¹ and 0·1 mg l⁻¹. Subsequent treatment with H₂SO₅/CN = 1·2 reduces the [CN⁻] to 0·1 mg l⁻¹.Treatment of a combined blast furnace/coke ovens effluent ([CN⁻] = 24 mgl⁻¹, [Zn²⁺] = 4·0 mgl⁻¹) with Fe/CN = 1·5 reduced [CN⁻] to 0·2 mg l⁻¹ and [Zn²⁺] to <0·1 mgl⁻¹. Subsequent treatment with H₂SO₅/CN = 2·0 reduced [CN⁻] to 0·2 mg l⁻¹. The process operates best in the pH range 7–9 and so is not affected by the buffer characteristics of seawater.     

A study of the sorptive properties of surface sediments in a landfill project region

Surface sediment samples were collected from 3 regions of a river island (a landfill area, a foreshore zone and a swamp segment) and the adsorptive properties of the - 180 μm and “clay” fractions were examined by studying the effects of concentration, pH and complex formation on the uptake of copper or zinc ions. The fine sand fraction contained basic material which raised the pH of aqueous suspensions to 8, at which value near total precipitation of added metal ions occurred. Uptake by the clay fractions (at pH 5) conformed to a Langmuir type adsorption isotherm, yielding parameters of similar order to values reported for soil clays and clay minerals. Using constant initial concentrations the amount of Cu or Zn adsorbed increased with pH until the threshold for precipitation (7) was exceeded. Both uptake and precipitation were inhibited in the presence of ligands which formed stable anionic complexes (e.g. NTA, EDTA, citric acid). The foreshore and swamp samples contained organic matter which retained part of the added metal ion as a solute in alkaline media. Treatment of these clay fractions with hydrogen peroxide did not destroy all the organic content, but it did change adsorptive behaviour.Extraction studies with 0.01 M nitric acid, 0.1 M oxalic acid, 0.1 M ammonium acetate, and 0.01 M EDTA showed that both the clay fractions and fine sands contained measurable amounts (0.05–40 mmol kg⁻¹) of the metal ions Pb, Cd, Cu and Zn, but because of the basic nature of the sediments, adjacent waters had solution levels which remain well within acceptable limits.The overall results have been used to predict factors likely to promote formation of high leachate levels.     

The removal of mercury(II) from dilute aqueous solution by activated carbon

The results of preliminary screening tests comparing the total Hg(II) removal capacity of 11 different brands of commercial activated carbon indicated that a very high percent (99–100%) total Hg removal was attained by all types of activated carbon especially at pH 4–5; the percent total Hg(II) removal decreased with pH's 4–5 except activated carbons Nuchar SA and SN which maintained a relatively high percent (>90%) total Hg(II) removal capacity at all pH values. Experiments were then conducted to reveal the mechanisms of Hg(II) removal by Nuchar SA (a powdered carbon). The results show that total Hg(II) removal was brought by two mechanisms: the adsorption and reduction. In order to investigate the kinetics of these two reactions, volatilization by bubbling N₂ gas at high flow rate was used to remove the Hg(g) product of the reduction reaction. It was noted that both the adsorption and the reduction/volatilization reactions were highly pH-dependent; at pH approx. <3–4 or > approx. 9–10 the extent of reduction/volatilization reaction superceded the adsorption reaction; whereas in the mid-pH region adsorption reaction dominated the total Hg(II) removal. The rate of adsorption reaction is very fast, reaching equilibrium in a few minutes; the rate of reduction/volatilization follows a linear √t expression. The reduction reaction is more significant with Filtrasorb 400 (H-type carbon) than Nuchar SA (L-type carbon). In the presence of strong chelating agent, ethylenediaminetetraacetate (EDTA), the total Hg(II) removal decreases due partly to the formation of less adsorbably mercuric(II)-EDTA complexes.