Removal of Uranium(VI), Lead(II) at the Surface of TiO2 Nanotubes Studied by X-Ray Photoelectron Spectroscopy

A thin film of well-ordered anatase TiO₂ nanotubes prepared by anodic oxidation of titanium metal were synthesised and used as adsorbent medium for the purification of water from aqueous uranium and lead. The amount of subtracted metal ions was quantified by using X-ray photoelectron spectroscopy at the surface of the reacted TiO₂ surface. Batch experiments for the sorption of U and Pb at the surface of the titania substrate were carried out in separated solution equilibrated with air of uranyl acetate and lead nitrate, in the pH range 3?C9. For uranium, the experiments were also repeated in anoxic (N₂) atmosphere. The amount of metal ions adsorbed onto the titania medium was quantified by measurements of the surface coverage expressed in atomic percent, by recording high-resolution XPS spectra in the Ti2p, U4f and Pb4f photoelectron regions. Adsorption of the uranyl species in air atmosphere as a function of pH showed an adsorption edge near pH 4 with a maximum at pH 7. At higher pH the presence of very stable uranyl?Ccarbonate complexes prevented any further adsorption. Further adsorption increased until pH 8.5 was obtained when the uranyl solution was purged from dissolved CO₂. Lead ion showed a sorption edge at pH 6, with a maximum uptake at pH 8. The results showed that the uptake of uranium and lead on the selected titania medium is remarkably sensitive to the solution pH. This study demonstrates the reliability of this type of material for treating water polluted with heavy metals as well as leachates from radioactive nuclear wastes.     

Effective Removal of Toxic Heavy Metal Ions from Aqueous Solution by CaCO<Subscript>3</Subscript> Microparticles

Heavy metals are a common contaminant in water supplies and pose a variety of serious health risks to nearby human populations. A promising approach to heavy metal decontamination is the sequestration of heavy metal ions in porous materials; however, current technologies involve materials which can be difficult to synthesize, are high-cost, or are themselves potentially toxic. Herein, we demonstrate that rapidly synthesized calcium carbonate (CaCO₃) microparticles can effectively remove high quantities of Pb²⁺, Cd²⁺, and Cu²⁺ ions (1869, 1320, and 1293 mg per gram of CaCO₃ microparticles, respectively) from aqueous media. The CaCO₃ microparticles were characterized with powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and Brunauer–Emmett–Teller (BET) N₂ sorption–desorption. It was found that the Ca²⁺ ions of the microparticles were replaced by the heavy metal ions, leading to partially recrystallized nanoparticles of new compositional phases such as cerussite (PbCO₃). The adsorption, surface dissolution/re-precipitation, and nucleation/crystal growth mechanisms were determined by investigating the Ca²⁺ released, along with the changes to particle morphology and crystal structure. Importantly, this study demonstrates that the porous CaCO₃ microparticles performed well in a system with multiple heavy metal ion species: 100% of Cu²⁺, 97.5% of Pb²⁺, and 37.0% Cd²⁺ were removed from an aqueous solution of all cations with initial individual metal concentrations of 50 mg/L and 1.5 g/L of CaCO₃ microparticles. At this concentration, the CaCO₃ microparticles significantly outperformed activated carbon. These results help to establish CaCO₃ microparticles as a promising low-cost and scalable technology for removing heavy metal ions from contaminated water.

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THE SPECIFIC ADSORPTION OF DIVALENT Cd, Co, Cu, Pb, AND Zn ON GOETHITE

The specific adsorption of divalent Cd, Co, Cu, Pb, and Zn on goethite is measured as a function of pH. For each mole of heavy metal adsorbed approximately two moles of H+ ions are displaced from the interface. Using these results the heavy metal adsorption data are expressed as functions of the solution concentrations of both H+ and metal ions, and the interfacial reaction is described by the equation, ₂SH+M²⁺= S₂M+₂H⁺. The adsorption data are consistent with an electrochemical model of the simultaneous adsorption of H⁺ ions and divalent metal ions on to the oxide. The intrinsic affinities of the metal ions for the oxide surface increase in the order, Cd < Co < Zn < Pb < Cu. However, besides the affinity of the metal ion for the surface, the adsorption curves are considered to be influenced by surface charge, the adsorption density of the metal ions and their size. The analysis of the data in terms of H⁺ and M²⁺ ion adsorption is considered to be complementary to the hydrolysis model for heavy metal adsorption.     

Cr(VI) Adsorption and Desorption on Soils and Biosorbents

In this study, high surface area porous carbons were synthesized by chemical activation using petroleum coke as the precursor and KOH as the activation agent. The pore structure of the as-synthesized activated carbons was characterized by nitrogen adsorption, and their CO₂ sorption capacities were measured by a magnetic suspension balance at 1 and 10 bar, respectively. The effects of activated carbon preparation parameters (preheating temperature, preheating time, activation time, heating rate during the pyrolysis, and particle size of the precursor) on porous texture, CO₂ adsorption capacity, and CO₂/N₂ selectivity of the activated products were investigated. It has been found that at 1 bar, the CO₂ adsorption capacity is determined by the micropore contribution, i.e., the ratio between micropore surface area and Brunauer–Emmett–Teller (BET) surface area of the sorbents, while at 10 bar, CO₂ adsorption capacity is related to the BET surface area of the activated products. The maximum CO₂ adsorption uptake of 15.1 wt% together with CO₂/N₂ selectivity of 9.4 at 1 bar were obtained for a sample activated at 700 °C indicating its high potential in the capture of CO₂.     

Cuprous Oxide-Modified Diatomite Waste from the Brewery Used as an Effective Adsorbent for Removal of Organic Dye: Adsorption Performance,Kinetics and Mechanism Studies

Cuprous oxide-modified diatomite waste (Cu₂O-DW) as a low-cost and effective adsorbent was prepared via a hydrothermal route combined with acid-alkali treatment. The microstructure and surface properties of the obtained Cu₂O-DW composite was characterized by Brunauer-Emmett-Teller, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The adsorption behaviors of three different types of dyes such as cationic dye methyl blue (MB), anionic dye acid orange (AO), and reactive dye reactive yellow (RY) onto the as-prepared Cu₂O-DW were investigated. Several experimental parameters such as contact time, adsorbent dosage, initial dye concentration, and initial pH values were systematically estimated. The experimental results indicated that as-prepared Cu₂O-DW have a better adsorption performance for MB, AO, and RY. Moreover, the kinetic and isotherm models were also used to account for the adsorption mechanism of dye molecules onto Cu₂O-DW. The results demonstrated that three different dyes are all fitted well with pseudo-first-order kinetic model. Additionally, the Langmuir and Freundlich isotherm model is more suitable for describing the adsorption process of RY and MB on the as-prepared Cu₂O-DW, respectively, and the AO adsorption is propitious to the D-R isotherm model. The value of adsorption energy (E?<?8 kJ mol^?1) confirmed that the physical adsorption is dominator during the adsorption process. The findings of the study demonstrated that the synthesized Cu₂O-DW composite can be a promising adsorbent for the removal of organic dyes from wastewater and it provided a sustainable development method for cycling the diatomite waste from the brewery.     

不同土地利用方式土壤对铜、镉离子的吸附解吸特征

采用一次平衡法对Cu²⁺、Cd²⁺在城市及城郊农田、林地、草地3种土地利用方式土壤中的吸附解吸过程进行比较研究, 结果表明: Cu²⁺、Cd²⁺在3种土地利用方式土壤中的吸附量均随平衡液浓度的增加而增大, Cu²⁺、Cd²⁺在农田土壤上的吸附量均高于林地和草地土壤。分别用Langmuir和Freunlich两种等温吸附方程对吸附过程进行拟合, 3种土壤对Cu²⁺的吸附过程运用Langmuir方程拟合效果好, 而对Cd²⁺的吸附过程运用Freunlich方程拟合效果更好。Cu²⁺在3种土壤的解吸量大小顺序为农田>林地>草地, Cd²⁺在3种土壤的解吸量大小顺序为农田>草地>林地。两种离子在3种土壤中的动态吸附是个快速反应的过程, 随时间延长, 吸附反应趋于平衡。运用双常数函数方程和Elovich方程能较好地拟合重金属在土壤上的吸附动力学过程。Cu²⁺、Cd²⁺的吸附与土壤黏粒含量、有机质含量、CEC和pH均有关。     

Removal of Cu(II) Ions from Aqueous Solution by Magnetic Chitosan-Tripolyphosphate Modified Silica-Coated Adsorbent: Characterization and Mechanisms

A magnetic chitosan-modified Fe₃O₄@SiO₂ with sodium tripolyphosphate adsorbent (MTPCS) was synthesized by surface modification of Fe₃O₄@SiO₂ with chitosan using sodium tripolyphosphate (STPP) as the cross-linker in buffer solution for the adsorption of Cu(II) ions from aqueous solution. The structure and morphology of this magnetic nanoadsorbent were examined by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), BET surface area measurements, Fourier transform infrared spectrometer (FTIR), and X-ray photoelectron spectroscopy (XPS). The effects of initial pH, adsorbent amount, and initial concentration of heavy metal ions were investigated by batch experiments. Moreover, adsorption isotherms, kinetics, and thermodynamics were studied to understand the mechanism of adsorbing metal ions by synthesized MTPCS. The results revealed that adsorption kinetics was best depicted by the pseudo-second-order rate mode and intraparticle-diffusion models. The adsorption isotherm fitted well to the Langmuir model. Moreover, thermodynamic study verified the adsorption process was endothermic and spontaneous in nature. The maximum adsorption occurred at pH 5 ± 0.1, and the adsorbent could be used as a reusable adsorbent with convenient conditions.     

Removal of Dyes from Water Using a TiO2 Photocatalyst Supported on Black Sand

A TiO₂ photocatalyst was prepared by depositing silica and titanium dioxide on the surface of black sand that made the photocatalyst recoverable using a magnetic field. The magnetic photocatalyst was used to remove six aqueous dyes from water and the removal was attributed to both adsorption and photocatalytic oxidation. Removal by adsorption was more noticeable with the cationic dyes than with the anionic dyes. The difference was related to the electrostatic interaction between the charged dye molecular and the silica-occupied surface of the photocatalyst. Removal by photocatalytic oxidation occurred with anionic dyes, while it was not appreciable with cationic dyes. It was postulated that photocatalytic oxidation might have happened with cationic dyes as well, but the strong adsorption made the photocatalytic oxidation undetectable.     

Adsorption of Methylene Blue from Aqueous Solution onto Perlite

Adsorption of methylene blue from aqueous solutionsonto unexpanded and expanded perlite samples activatedby H₂SO₄ and NaCl solutions has beeninvestigated, to assess the possibility of usingperlite for removing cationic dyes from aqueoussolutions. The effects of pH and temperature of dyesolution on the adsorption capacities have beenevaluated. The experimental data were correlatedreasonably well by the Langmuir adsorption isothermand the isotherm parameters (Q _m and K) have beencalculated. The removal efficiency (P) anddimensionless separation factor (R) have shown thatperlite can be used for removal of methylene blue fromaqueous solutions, but unexpanded perlite is more effective.     

Suitability of Mn-Oxihydroxides from Karst Caves as Filter Material for Drinking Water Treatment in Gunung Sewu,Indonesia (6 pp)

Background, Aim and Scope   Manganese oxides are widely known as highly efficient heavy metal adsorbing materials. Therefore the use of natural materials containing Mn-oxihydroxides for the treatment of drinking water containing heavy metals in developing countries could be a cheap but nevertheless practical method. In the Karst area of Gunung Sewu, Java, Indonesia, people use the underground water resources as drinking water, which are endangered by heavy metal pollution. Local household filters have been considered for heavy metal elimination, to guarantee high quality drinking water from the Karst water. The aim of this study was to examine the suitability of mineral deposits containing Mn-oxihydroxides found in the Karst cave Bribin as filter material. Materials and Methods: Batch experiments were performed with the selected divalent cations Cd2+, Zn2+ and Ni2+ to test the adsorption efficiency of the Mn-rich deposits from Bribin to immobilize dissolved heavy metals. Results: Maximum adsorption capacities of 285 μmol/g for Cd2+, 217 μmol/g for Zn2+, and 178 μmol/g for Ni2+ were achieved after 24 h reaction time. These values are higher than literature values of Ni, Cd and Zn adsorption to other synthetic and natural manganese oxides. Discussion: Kinetic data from the batch experiments suggest that metal fixation occur in two steps; fast surface adsorption followed by a slower intercalation of the metal ions into the Mn-oxihydroxide lattice. Multi-element analysis of blank samples and samples under competitive conditions implies no desorption of metals from Mn-oxihydroxides. Conclusions: The sorption kinetics and capacities achieved in these experiments suggest that Mn-oxihydroxides from Bribin could be an effective filter material for drinking water purification. Recommendations and Perspectives: Nevertheless, reducing conditions in a filter system must be avoided to prevent Mn-oxides being reductively dissolved and the sorbed heavy metals mobilized.     

Water Remediation Using Calcium Phosphate Derived From Marine Residues

The processing of natural resources in marine and freshwater ecosystems, directly operated by industries related to maritime sector, contributes annually to several million tons of waste. The reuse and economic recovery of this waste would be very desirable and profitable, either economically or environmentally. In this work, the remediation of hazardous divalent metal ions from aqueous solutions using biological apatites derived from marine residues was addressed. The biological apatite (calcium phosphate particles) was produced by heat treatment of fish bones. Experimental sorption studies of kinetics and equilibrium of the metal ions as well as an evaluation of competitive sorption behavior for lead immobilization were carried out in batch operation mode. The efficiency and mechanisms of lead sorption on two different particle sizes of calcium phosphate from aqueous solution were investigated. The results showed a high adsorption capacity of the biosorbent for Pb²⁺ (above 370 mg/g_ads.), in opposition to Cd²⁺ and Zn²⁺. For the case of low initial concentrations of the metal ion, reducing the biosorbent particles size increases the sorption rate. It was possible to verify that lead immobilization proceeds with a rapid surface complexation of the lead on the sorption sites before partial dissolution of calcium phosphate and formation of pyromorphite-like compounds. By the selectivity experiments, performed using binary systems—Pb/Cd and Pb/Zn—it was evidenced a competitive process between the divalent ions, which leads to a considerable decrease on the adsorption capacity of the adsorbent material for all the metals.     

铁锰镁离子改性生物炭对溶液硝态氮的吸附性能研究

为突破生物炭对硝态氮吸附的局限性,以花生壳为原料,在600℃条件下热解制备生物炭(BC),分别用FeCl₃、MnCl₂、MgCl₂对其进行金属负载改性(BC-Fe、BC-Mn和BC-Mg),设计批量吸附试验,结合扫描电镜(SEM)、傅里叶变换红外光谱(FTIR)和X射线衍射(XRD)等进行表征分析。结果表明,铁、锰、镁离子改性使生物炭的比表面积增大6.67~12.16倍,孔容增加3.30~6.00倍,并显著增强了对硝态氮($\text{NO}_{3}^{-}$-N)的吸附性能(P<0.05),吸附量较BC增加11.5%~17.1%,BC-Fe、BC-Mn和BC-Mg对$\text{NO}_{3}^{-}$-N的最大吸附量分别为41.58、39.04、39.58 mg·g^-1,铁、锰、镁离子与炭的最佳质量比分别为0.80、0.20、0.20,铁离子改性效果最好;酸性条件有利于改性生物炭对$\text{NO}_{3}^{-}$-N的吸附,吸附动态符合Langmuir方程(R²=0.935~0.961),吸附过程符合准一级动力学方程(R²=0.971~0.980)。综上,通过金属离子改性,增大了生物炭的比表面积和孔容,优化了表面结构。此外,改性生物炭表面的含氧官能团和金属离子能通过形成氢键或静电作用吸附$\text{NO}_{3}^{-}$-N,进而增强对$\text{NO}_{3}^{-}$-N的吸附能力。本研究结果为生物炭吸附材料的制备及吸附性能优化提供了理论依据。     

Enhanced Heavy Metal Sorption by Surface-Oxidized Activated Carbon Does Not Affect the PAH Sequestration in Sediments

We examined the sorption of heavy metals and polycyclic aromatic hydrocarbons (PAHs) to surface-oxidized activated carbon (AC) and its effect on the distribution of those compounds in sediments. Created surface oxygen groups on AC enhanced the sorption of copper, which is superior in sorption competition, in the marine sediments. In case of cadmium, aqueous chemistry altered by AC addition, such as pH, has greater impact on the bioavailability according to the result of a sequential extraction combined with the pore water concentration measurements. Oxidized AC exhibited 2.3 times more adsorption of reduced bioavailable copper while 23% of bioavailable cadmium was adsorbed onto unmodified AC. No significant changes in BET surface area, pore volume, and AC/water distribution coefficient (K _AC) of PAHs were observed with surface-oxidized AC. The largest difference in K _AC after the oxidation was only 0.14 log unit. Consequently, freely dissolved aqueous concentrations of PAHs were reduced by more than 96% for all tested ACs in a week despite the increased Cu sorption on AC. This indicates that enhanced metal sorption by surface oxidation of AC is less significant in controlling bioavailability of PAHs in sediments than particle size or sorbent dose.     

THE HIGH- AND LOW-ENERGY PHOSPHATE ADSORBING SURFACES IN CALCAREOUS SOILS

The two-surface Langmuir equation was used to study P adsorption by 24 calcareous soils (pH 7.2-7.6; 0.8-24.2 per cent CaCO₃) from the Sherborne soil series, which are derived from Jurassic limestone. High-energy P adsorption capacities (x″_m) ranged from 140–345 μg P/g and were most closely correlated with dithionite-soluble Fe. Hydrous oxides therefore appear to provide the principal sites, even in calcareous soils, on which P is strongly adsorbed (x″_m 6–51 ml/μg P). The low-energy adsorption capacities (x″_m) ranged from 400–663 μg P/g and were correlated with organic matter contents and the total surface areas of CaCO₃ but not with per cent CaCO₃, pH, or dithionite-soluble Fe. Total surface areas of CaCO₃ in the soils ranged from 4.0 to 8.5 m²/g soil. Low-energy P adsorption capacities agree reasonably with values (100 pg P/m²) for the sorption of phosphate on Jurassic limestones but phosphate was bonded much less strongly by soil carbonates (k″= 0.08–0.45 ml/μg P) than by limestones (k～10.0 ml/μg P). Low-energy P adsorption in these soils is tentatively ascribed to adsorption on sites already occupied by organic anions (and probably also by bicarbonate and silicate ions) which lessen the bonding energy of co-adsorbed P.     

Sorption of cupric,dichromate and arsenate ions in some New Zealand soilds

Concentrated solutions of copper (Cu²⁺), dichromate (Cr₂O^2? ₇) and aresenate (AsO₄ ^3?) ions (CCA solutions) are used extensively in the New Zealand timber preservation industry. These ions are therefore, potential soil pollutants at timber treatment sites. Sorption of these three ions was examined by the surface and sub-surface horizons of two free-draining New Zealand soils over a range of soil solution pH values. Copper sorption by both soils increased substantially with increasing pH and was greater in the surface compared with the sub-surface horizons. Less dichromate was sorbed than the other two ions and wa similar in both surface and sub-surface horizons for each soil. Dichromate sorption increased with decreasing pH. Arsenate sorption from solutions containing all three ions was not greatly different to influenced by changes in soil solution pH. Arsenate sorption was generally greater in the sub-surface horizons of both soils. Sorption from solutions containing all three ions was not greatly different to sorption from solutions containing the single metal ions. Sorption behaviour for each ion is related to its chemistry and the soil chemical properties of each horizon. Results suggest that in the event of soil contamination by CCA solution, the immediate leaching potential of the initial ions species present would increase in the following order: Cu²⁺ < HAsI ₄ ^? ? Cr₂O ₇ ^2? .     

N<Subscript>2</Subscript>O,CO<Subscript>2</Subscript>, Production,and C Sequestration in Vineyards: a Review

Even if it is less polluting than other farm sectors, grape growing management has to adopt measures to mitigate greenhouse gas (GHG) emissions and to preserve the quality of grapevine by-products. In viticulture, by land and crop management, GHG emissions can be reduced through adjusting methods of tillage, fertilizing, harvesting, irrigation, vineyard maintenance, electricity, natural gas, and transport until wine marketing, etc. Besides CO₂, nitrous oxide (N₂O) and methane (CH₄), released from fertilizers and waste/wastewater management are produced in vineyards. As the main GHG in vineyards, N₂O can have the same harmful action like large quantities of CO₂. Carbon can be found in grape leaves, shoots, and even in fruit pulp, roots, canes, trunk, or soil organic matter. C sequestration in soil by using less tillage and tractor passing is one of the efficient methods to reduce GHG in vineyards, with the inconvenience that many years are needed for detectable changes. In the last decades, among other methods, cover crops have been used as one of the most efficient way to reduce GHG emissions and increase fertility in vineyards. Even if we analyze many references, there are still limited information on practical methods in reducing emissions of greenhouse gases in viticulture. The aim of the paper is to review the main GHG emissions produced in vineyards and the approached methods for their reduction, in order to maintain the quality of grapes and other by-products.     

Seasonal Dynamics of Soil CO<Subscript>2</Subscript> Concentration and CO<Subscript>2</Subscript> Fluxes from the Soil of the Former Lake Texcoco,Mexico

Seasonal changes of the soil CO₂ concentration and the rate of CO₂ fluxes emission from the soil formed on the sediments of the former Lake Texcoco, which occupied a significant part of the Mexico Valley until the mid-17th century, were studied. The soils (Fluvic Endogleyic Phaeozems) were characterized by a low CO₂ fluxes rate, which is related to their high alkalinity. The mean values of soil respiration were 6.0–14.1 mg C/(m² h) depending on vegetation type, which corresponds to 60–157 g C/(m² yr). The contribution of plants to the CO₂ fluxes insignificantly varied by seasons and depended on the species composition of vegetation. The soil CO₂ concentration and soil respiration in eucalypt (Eucalyptus globulus Labill.) plantation were two times higher than those in the grass–subshrub area, the ground cover of which consisted of Distichlis spicata (L.) Greene and Suaeda nigra (Raf.) J.F. Macbr. species. This can be related to the significant volumes of gas production during the respiration of eucalypt roots and associated rhizosphere community. The contribution of the root systems of grass cover to the soil CO₂ fluxes in eucalypt plantation slightly varied within the year and was equal to 24% on the average. In the grass–subshrub area, its value varied from 41% in the cold season to 60% in the warm season. The spatial variability of soil CO₂ concentration and its flux rate to the atmosphere was due to the differences in plant species composition and hydrothermal conditions, and their temporal trend was closely related to the seasonal accumulation of plant biomass and soil temperature.     

The Impact of Diesel Oil Pollution on the Hydrophobicity and CO<Subscript>2</Subscript> Efflux of Forest Soils

The contamination of soil with petroleum products is a major environmental problem. Petroleum products are common soil contaminants as a result of human activities, and they are causing substantial changes in the biological (particularly microbiological) processes, chemical composition, structure and physical properties of soil. The main objective of this study was to assess the impact of soil moisture on CO₂ efflux from diesel-contaminated albic podzol soils. Two contamination treatments (3000 and 9000 mg of diesel oil per kg of soil) were prepared for four horizons from two forest study sites with different initial levels of soil water repellency. CO₂ emissions were measured using a portable infrared gas analyser (LCpro+, ADC BioScientific, UK) while the soil samples were drying under laboratory conditions (from saturation to air-dry). The assessment of soil water repellency was performed using the water drop penetration time test. An analysis of variance (ANVOA) was conducted for the CO₂ efflux data. The obtained results show that CO₂ efflux from diesel-contaminated soils is higher than efflux from uncontaminated soils. The initially water-repellent soils were found to have a bigger CO₂ efflux. The non-linear relationship between soil moisture content and CO₂ efflux only existed for the upper soil horizons, while for deeper soil horizons, the efflux is practically independent of soil moisture content. The contamination of soil by diesel leads to increased soil water repellency.     

Zusammensetzung und Speziierung der Bodenlösung eines mit Schwermetallen belasteten kalkreichen Bodens

Composition and Speciation of Soil Solution collected in a Heavy Metal polluted calcareous Soil Close to a brass foundry, which had emitted heavy metal containing dusts for over 80 years, soil water was collected in the topsoil (18 cm) and in the subsoil (40 cm) of a severely polluted Calcic Fluvisol by means of polyethylen suction cups over a period of 2 years. The total metal content of the topsoil (extracted with 2M HNO₃ at 100 °C for 2 hours) was 38 nmol g^?1, 24 μmol g^?1, and 25 μmol g^?1 for Cd, Cu, and Zn, respectively. The mean heavy metal concentrations of the soil solution were 0.5 mol L^?1, 300 nmol L^?1, and 200 nmol L^?1 in the topsoil and 0.6 nmol L^?1, 90 nmol L^?1, and 30 nmol L^?1 in the subsoil for Cd, Cu, and Zn, respectively. Solubility calculations showed that the soil solutions were undersaturated with respect to heavy metal carbonates as well as to hydroxides. It seems that the heavy metal concentration is determined by sorption processes rather than by precipitation. The composition of the soil solution has been shown to be governed by the presence, of calcite, by the soil temperature and by the partial pressure of CO₂ in the soil air. The pCO₂ in the soil air (in both depths) has been estimated at 2 mbar during the winter term and at 20 mbar during the summer term. A corresponding increase of the concentration of macroelements (Ca, Mg, Na) as well as of total dissolved carbonate and of dissolved organic matter (DOC) has been measured in the summer half year. No significant seasonal variations of the heavy metal concentrations were detected and no correlations with concentrations of other components could be found.     

Removal of Cu2+ and Cd2+ from Aqueous Solution by Bentonite Clay Modified with Binary Mixture of Goethite and Humic Acid

Pre-modification of bentonite clay with goethite, humic acid, and a binary mixture of goethite and humic acid reagents increased its cation exchange capacity from 95 to 105.32, 120.4, and 125.8 meq/100 g of bentonite clay, respectively. The effective pre-modification of bentonite clay with goethite, humic acid, and goethite–humic acid reagents was confirmed from their Fourier transform infrared spectra which suggested that modification was effective on the AlAlOH and Si–O sites for goethite and humic acid modification and AlAlOH for goethite–humic acid modification. The presence of 0.001 M NaNO₃ electrolyte increased the adsorption capacity of bentonite clay. Temperature was observed to favor the adsorption of Cu²⁺ and Cd²⁺ onto both the raw and modified bentonite clay samples. The goethite–humic acid-modified bentonite gave the best adsorption capacity of ≈10 and 16 mg/g at 30 and 50°C, respectively, for both metal ions. The inner sphere complexation mechanism was suggested for the adsorption of both metal ions onto the modified adsorbents. Modifying bentonite clay with a binary mixture of goethite and humic acid reduced the selectivity of bentonite clay for either Cu²⁺ or Cd²⁺. Preadsorbed goethite and humic acid on bentonite clay will further reduce the mobility of heavy metal ions in soils and in aquatic environments.