Arsenic chemistry in a groundwater aquifer from the Eastern Province of Saudi Arabia

Groundwater samples were collected from shallow aquifers underneath an industrial complex in the Eastern Province of Saudi Arabia. Arsenic (As) concentrations in the groundwater samples varied between 10^?8.6 and 10^?6.8 M (0.18 and 11.14 µg L^?1), with an average of 10^?7.5 M (2.19 μ L^?1). The analysis of variance for the analytical data showed that sampling locations had significantly affected As concentrations in the groundwater samples. Analytical and thermodynamic calculations showed that H₂ASO₄ ^? was the most predominant As species in acidic groundwater samples, and HAsO₄ ^2? was the most abundant species in alkaline groundwater samples. Concentrations of H₃AsO₄° and AsO₄ ^3? were too low to be important in this study. Reduced As chemical forms were also expected to be very low. All the groundwater samples were undersaturated with respect to the thermodynamic solubility isotherms of Ca₃(AsO₄)₂(c), Fe₃(AsO₄)₂(c), and Mn₃(AsO₄)₂(c) minerals. Lack of reliable thermodynamic data for these arsenates could be responsible for differences between the theoretical and measured concentrations of As in the shallow groundwater samples. The general trend in the distribution of HAsO₄ ^2? activities in the groundwater samples was parallel to that of the Ca₃(AsO₄)₂ solubility isotherm but different from those of Fe₃(AsO₄)₂(c), and Mn₃(AsO₄)₂(c). These data suggest that As concentrations in the groundwater samples were probably controlled by the precipitation and dissolution of Ca₃(AsO₄)₂ type mineral. A three step hypothesis for As interactions in groundwater/soil system is proposed that combines both solid phase formation and adsorption of As onto the solid colloidal surfaces. This hypothesis is expected to better represent As behavior in groundwater/soil environment.     

Solubility and Stability of Calcium Arsenates at 25∘C

The solubility and stability of calcium arsenates at 25 ^°C was determined by both precipitation and dissolution experiments. Ca₃(AsO₄)₂? 3H₂O(c), Ca₃(AsO₄)₂? 2¹/₄H₂O(c), Ca₅(AsO₄)₃(OH)(c) and Ca₄(OH)₂(AsO₄)₂? 4H₂O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ～ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca₃(AsO₄)₂? xH₂O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca₃(AsO₄)₂? 3H₂O(c), Ca₃(AsO₄)₂? 2¹/₄H₂O(c), Ca₅(AsO₄)₃(OH)(c) and Ca₄(OH)₂(AsO₄)₂? 4H₂O(c) were calculated as K _sp of 10^{? 21.14}(10^{? 20.01} ～ 10^{? 22.02}), 10^{? 21.40}(10^{? 20.08} ～ 10^{? 21.98}), 10^{? 40.12}(10^{? 37.53} ～ 10^{? 42.72}) and 10^{? 27.49}(10^{? 26.10} ～ 10^{? 28.91}), respectively. Correspondingly, the free energies of forming (Δ G _f ^o ) of these calcium arsenates were calculated to be ?3787.87 kJ/mol, ?3611.50 kJ/mol, ?5096.47 kJ/mol and ?4928.86 kJ/mol.     

Inorganic anion sorption and interactions with phosphate sorption by hydrous ferric oxide gel

The sorption of inorganic anions by hydrous ferric oxide gel (Fe gel) from 10 ^?1 M NaClO₄ at pH 6.5 decreased in the order: orthophosphate (H₂PO₄)>arsenate (H₂AsO₄) = selenite (HseO3) > silicate (H₄SiO₄) > molybdate (MoO²₄^?) > sulphate (SO²₄^?) > selenate(SeO²₄^?)>chloride (Cl^?) = nitrate (NO^?₃). When each anion was added to Fe gel with an equimolar quantity of H₂PO^?₄, there was no detectable effect of SO²₄^?, SeO²₄^?, Cl^?, and NO^?₃ on the amount of H₂PO^?₄ sorbed. Other anions depressed H₂PO₄ sorption in the order H₂AsO₄ >HseO₃ > H₄SiO₄ > MoO²₄. At the lowest level of anion addition (500 mmol kg ^?1), H₂PO₄ sorption was depressed by no more than 6% of the sorption level in the absence of a competing anion. In contrast, at the highest level of anion addition (1700 mmol kg-¹ of each), H₂AsO₄ decreased H₂PO₄ sorption by 44%. The sorption of SO₄^? was completely eliminated when this anion was added with equimolar amounts of H₂PO₄. The ability of anions to compete with H₂PO₄ for sorption sites could not be explained solely by the results obtained for the sorption of each anion alone. Thus, H₂AsO₄ was more competitive than H₂PO₄ when added together, even though more H₂PO₄ than H₂AsO₄ was sorbed when each anion was added alone. Although H₂PO₄ was sorbed in larger amounts, there is no evidence to suggest that H₂PO₄. H₂AsO₄, and HseO₃ were sorbed on different sites.     

Short‐Term Effect of Lime,Phosphorus, and Iron Amendments on Water‐Extractable Lead and Arsenic in Orchard Soils

Abstract

Lead arsenate was extensively used to control insects in apple and plum orchards in the 1900s. Continuous use of lead arsenate resulted in elevated soil levels of lead (Pb) and arsenic (As). There are concerns that As and Pb will become solubilized upon a change in land use. In situ chemical stabilization practices, such as the use of phosphate‐phosphorus (P), have been investigated as a possible method for reducing the solubility, mobility, and potential toxicity of Pb and As in these soils. The objective of this study was to determine the effectiveness of calcium carbonate (lime), P, and iron (Fe) amendments in reducing the solubility of As and Pb in lead‐arsenate‐treated soils over time. Under controlled conditions, two orchard soils, Thurmont loam (Hapludults) and Burch loam (Haploxerolls), were amended with reagent‐grade calcium carbonate (CaCO₃), iron hydroxide [Fe(OH)₃], and potassium phosphate (KH₂PO₄) and incubated for 16 weeks at 26°C. The experimental results suggested that the inorganic P increased competitive sorption between H₂PO₄ ^? and dihydrogen arsenate (H₂AsO₄ ^?), resulting in greater desorption of As in both Thurmont and Burch soils. Therefore, addition of lime, potassium phosphate, and Fe to lead‐arsenate‐contaminated soils could increase the risk of loss of soluble As and Pb from surface soil and potentially increase these metal species in runoff and movement to groundwater.     

Controlling Acid Drainage in a Pyritic Mine Waste Rock. Part II: Geochemistry of Drainage

Mine waste rock can produce acid rock drainage (ARD) when constituent sulphide minerals (for example, pyrite) oxidize upon exposure to the atmosphere. Outdoor experiments were performed to test techniques for preventing and controlling ARD in a pyritic mine waste rock. The experiments involved lysimeter (plastic drum) experiments in which the crushed (25–50 mm particle sizes), amended and unamended waste rock was exposed to natural weather conditions (rain, drying, freezing and thawing) for 125 weeks. The amendments consisted of separately covering the waste rock with compacted soil, wood bark and water and mixing with limestone and phosphate rock at 1 and 3%. Waters draining the various rocks were collected and analyzed for acidity, pH, sulphate and metals. In general, concentrations of SO₄ ^2-, Fe, As, Cu, Al and Mg in the drainage from the control rock increased gradually in the first year, peaked in the second year and increased further in the third year, reflecting increasing acid generation with time. SO₄ ^2- displayed strong positive correlation (0.91 to 0.98) with Al, As, Cu, Fe and Mg.Concentrations of Zn, Mn and Cd reached their maximumin the second year. Geochemical analysis of thecomplete water quality data using the equilibriumspeciation model WATEQ4F suggested waste rockoxidation was most likely controlled by Fe³⁺. Al, SO₄ ^2- and Fe concentrations in thecontrol rock appeared to be controlled by alunite(KAl₃(SO₄)₂(OH)₆), jarosite(KFe₃(SO₄)₂(OH)₆) and amorphousferric hydroxide [(am)Fe(OH)₃] during the firstyear. Ion activity product data (log IAP) forFe³⁺ and OH^- generally ranged between –37and –34 in the first two years but decreased to –39and –40 in the third year, suggesting that amorphousferric hydroxides were beginning to crystallize intomore stable forms such as ferrihydrite (Fe[OH]₃)and goethite (FeOOH) in the third year. The addedlimestone lost its effectiveness after a while,probably because of precipitation of secondaryminerals on the limestone particles. The phosphaterock could not sustain the drainage pH above 6 andlost its effectiveness before the limestone did. Underthe conditions of the experiments, the soil cover didnot work as expected, probably because of sidewallpassage of oxygen and water. The water cover was themost effective control method, reducing the acidproduction rate data from 41 to only 0.08 mgCaCO₃ week^-1 kg^-1 waste rock. The wood bark was theworst performer and accelerated acid production by 170%.     

Arsenic Chemistry in Soils: An Overview of Thermodynamic Predictions and Field Observations

Published information, both theoretical and experimental, on As chemical behavior in soils is reviewed. Because of many emission sources, As is ubiquitous. Thermodynamic calculations revealed that As(V) species (HAsO ₄ ^2- >H₂AsO ₄ ^- at pH 7) are more abundant in soil solutions that are oxidized more than pe+pH>9. Arsenic is expected to be in As(III) form (HAsO ₂ ⁰ =H₃AsO ₃ ⁰ >AsO ₂ ^- =H₂AsO ₃ ^- at pH 7) in relatively anoxic soil solutions with pe+pH<7. Adsorption on soil colloids is an important As scavenging mechanism. The adsorption capacity and behavior of these colloids (clay, oxides or hydroxides surfaces of Al, Fe and Mn, calcium carbonates, and/or organic matter) are dependent on ever-changing factors, such as hydration, soil pH, specific adsorption, changes in cation coordination, isomorphous replacement, crystallinity, etc. Because of the altering tendencies of soil colloids properties, adsorption of As has become a complex, empirical, ambiguous, and often a self contradicting process in soils. In general, Fe oxides/hydroxides are the most commonly involved in the adsorption of As in both acidic and alkaline soils. The surfaces of Al oxides/hydroxides and clay may play a role in As adsorption, but only in acidic soils. The carbonate minerals are expected to adsorb As in calcareous soils. The role of Mn oxides and biogenic particles in the As adsorption in soils appears to be limited to acidic soils. Kinetically, As adsorption may reach over 90% completion in terms of hours. Precipitation of a solid phase is another mechanism of As removal from soil solutions. Thermodynamic calculations showed that in the acidic oxic and suboxic soils, Fe-arsenate (Fe₃(AsO₄)₄)₂) may control As solubility, whereas in the anoxic soils, sulfides of As(III) may control the concentrations of the dissolved As in soil solutions. In alkaline acidic oxic and suboxic soils, precipitation of both Fe- and Ca-arsenate may limit As concentrations in soil solutions. Field observations suggest that direct precipitation of discrete As solid phases may not occur, except in contaminated soils. Chemisorption of As oxyanions on soil colloid surfaces, especially those of Fe oxide/hydroxides and carbonates, is believed to a common mechanisms for As solid phase formation in soils. It is suggested that As oxyanions gradually concentrate on colloid surfaces to a level high enough to precipitate a discrete or mixed As solid phase. Arsenic volatilization is another As scavenging mechanism operating in soils. Many soil organisms are capable of converting arsenate and arsenite to several reduced forms, largely methylated arsines which are volatile. These organisms may generate different or similar biochemical products. Methylation and volatilization of As can be affected by several biotic (such as type of organisms, ability of organism for methylation, etc.) and abiotic factors (soil pH, temperature, redox conditions, methyl donor, presence of other ions, etc.) factors. Information on the rate of As biotransformations in soils is limited. In comparison to the biologically assisted volatilization, the chemical volatilization of As in soils is negligible.     

Arsenate Displacement from Fly Ash in Amended Soils

Arsenic (As) is the biggest environment contaminant in most of the soils where fly ash is applied. Usually, it is not mobile and strongly adsorbed on to soil particles. However, in gypsum and phosphorus amended soils As may be much more mobile. A study in repacked columns was conducted to determine whether or not As becomes mobile when Ca(H₂PO₄)₂and CaSO₄are used as leaching solutions, and to compare the competitive interactions between PO₄-AsO₄and SO₄-AsO₄. Arsenic concentration in leachate was found to be approximately ten times greater when Ca(H₂PO₄)₂was used to leach the columns as compared to CaSO₄. A maximum concentration of 800 μg As L^-1was found in the leachate in this case, which is much higher than the groundwater limit of 50 μg L^-1for drinking water established by the United States Environmental Protection Agency. In fly ash, the portion of arsenate non-specifically adsorbed is believed to be much lower than that of specifically adsorbed. Sulfate anions were able to displace only non-specifically adsorbed arsenate. In this case the concentration of As in leachate was found to be within acceptable limits. On the other hand, phosphate can compete with arsenate for all available adsorption sites, non-specific and specific. Phosphate displacement of both forms of arsenates increases As mobility in both control and fly ash treatments.     

Analysis of sulfur compounds in acid sulfate soils and other recent marine soils

Abstract

A refined scheme for the semi micro chemical analysis of sulfur fractions in soils is presented. Pyrite is analyzed, as iron, after extraction in HNO₃. Non‐pyrite iron is excluded by a pretreatment with HF/H₂SO₄. Water‐soluble sulfate and jarosite [KFe₃(SO₄)₂(OH)₆], the other dominant sulfur fractions in acid sulfate soils, are analyzed turbidimetrically, as sulfate, after successive extractions by EDTA.3Na (water soluble plus exchangeable SO₄) and by hot 4 M HCl (jarosite). These methods are simpler, less bulky and more specific than most existing procedures.

Introduction of elemental sulfur analysis permits estimation of organic sulfur fraction as well. Sums of individual sulfur fractions agree well with separate total sulfur determinations.

The proposed analysis of pyrite permits also distinction of the components Fe₂O₃, FeO and FeS₂ in soils and rocks².     

Matrix-Based Fertilizers Reduce Pesticide Leaching in Soil

The presence of pesticides in groundwater has been documented in several large-scale studies and numerous small-scale investigations. Pesticide leaching through soil has been identified as a major cause for the occurrence of these chemicals in surface and groundwater. We developed matrix-based fertilizers (MBFs) that have been shown to reduce N and P leaching. We tested the efficacy of the ionic bonds in the MBFs to reduce 2,4-dichlorophenoxyacetic acid (2,4-D), metolachlor, thiophanate methyl, carbaryl, diazinon, and malathion leaching in soil columns. After 7 days 2,4-D, thiophanate methyl, carbaryl, and malathion did not leach in sufficient quantities to determine if the MBF fertilizers reduced leaching compared with the control and the slow-release fertilizer Polyon®. The MBF fertilizers leached from five to 30 times less metolachlor than the control and Polyon® treatment. Treatments with MBF fertilizers leached from two to 72 times less diazinon than the control treatment. The MBF fertilizer treatment leached from eight to 268 less diazinon than columns receiving Polyon®. The MBF formulations allow compounds with both anionic and cationic charges to bind with the Al(SO₄)₃ 3H₂O and/or Fe₂(SO₄)₃ 3H₂O-lignin-cellulose matrix. When pesticides are added to the soil amended with matrix-based fertilizers, the ion exchange matrix will likely bind the metolachlor and diazinon to the Al(SO₄)₃ 3H₂O and/or Fe₂(SO₄)₃ 3H₂O-starch-cellulose-lignin matrix thereby substantially reducing leaching. The MBFs could be used to limit both nutrients and pesticide leaching from agricultural fields.     

Dry Deposition of Sulfur Containing Species to the Water Surface Sampler at Two Sites

Sulfate dry deposition increases the deteriorating effects on environment. Sulfate can be deposited from atmosphere to water via both particulate (SO₄ ² :sulfate)and a gas(SO ₂:sulfurdioxide)form.In this research, the fluxes of gaseous(SO ₂)and particulate(SO ₄ ²)species were measured employing a water surface sampler(WSS)and glass fiber filters(GFFs)ontheknife?edge surrogate surface(KSSs)in the campus of Uludag University and the city of Bursa, Turkey.Sampling program was conducte dinter mittently between September2004and March2005.Average to talsulfate fluxes measured with the WS Satthe Uludag University campus and in the city of Bursa were58 ± 41and235 ± 43?mgm ^?2 d ^?1, respectively.The to talsulfate fluxe smeasure dat Bursa were highe rand this was probably due to greater sulfur containing species in it satmosphere.The dry deposition of gas eous SO ² flux was calculated by sub tracting the particulate flux collected with the KSS s from the total flux(particulate sulfate plus SO ₂ flux)obtained by the WSS.Anautomatic SO ₂ analyzer was used concurrently to measure the ambient concentration of gas eous SO ₂. The average SO₂ gas fluxes and ambient SO ₂ concentrations were18 ± 28and54 ± 48?mgm ^?2 day ^?1 and11 ± 7and49 ± 14?μgm ^?3 for the campus and the city, respectively.The measured gaseous SO ₂ fluxes and ambient concentrations were used to calculate the mass transfer coefficient.The calculated MTC values for the campus and the city were0.8 ± 1.0and1.2 ± 1.1?cms ^?1, respectively.The sevalues wereinag reement with previously reported dry deposition velocities for SO ₂.     

Matrix Based Fertilizers Reduce Nitrogen and Phosphorus Leaching in Greenhouse Column Studies

We tested the efficacy of matrix based fertilizer formulations (MBF) that reduce NH₄, total phosphorus (TP), total reactive phosphorus (TRP) and dissolved reactive phosphorus (DRP) in leachate. The MBF formulations cover a range of inorganic N and P in compounds that are relatively loosely bound (MBF1) to more moderately bound (MBF2) and more tightly bound compounds (MBF3) mixed with Al(SO₄)₃ H₂O and/or Fe₂(SO₄)₃ and with the high ionic exchange compounds starch, chitosan and lignin. Glomus interadicies, a species of arbuscular mycorrhizal fungal spores that will form mycorrhizae in high nutrient environments, was added to the MBF formulations to increase plant nutrient uptake. When N and P are released from the inorganic chemicals containing N and P the matrix based fertilizers likely bind these nutrients to the Al(SO₄)₃ H₂O and/or Fe₂(SO₄)₃ starch–chitosan–lignin matrix. We tested the efficacy of the MBFs to reduce N and P leaching compared to Osmocote^® 14-14-14, a slow release fertilizer (SRF) in sand filled columns in a greenhouse study. SRF with and without Al and Fe leached 78–84% more NH₄, 58–78% more TP, 20–30% more TRP and 61–77% more than MBF formulations 1, 2, and 3 in a total of 2.0 liters of leachate after 71 days. The concentration and amount of NO₃ leached among SRF and MBF formulations 1 and 2 did not differ. The SRF treatment leached 34% less NO₃, than MBF3. Total plant weight did not differ among fertilizer treatments. Arbuscular mycorrhizal infection did not differ among plants receiving SRF and MBF formulations 1, 2 and 3. Although further greenhouse and field testing are called for, results of this initial investigation warrant further investigation of MBFs.     

Hydrochemistry of Waters of Volcanic Rocks: The Case of The Volcanosedimentary Rocks of Thrace, Greece

This work is referred to the characterization of the environmental hydrochemistry in the broader Sapes area – Thrace region, on the basis of physico-chemical properties of surface and groundwaters occurring in the volcanosedimentary formations of this area, where gold mining activities are planned to operate. Volcanic rocks are considerably altered where they are in contact with hydrothermal solutions. Aquifers are formed within these formations. Surface and ground waters are strongly metalliferous and their hydrochemical facies present similar but complex water types. Certain characteristic chemical types are the following: Ca-Mg-HCO₃-SO₄, Ca-Mg-SO₄-HCO₃. Ca-SO₄, Ca-Mg-SO₄. Ca-Na-Cl-HCO₃, Na-Cl. A small majority of the water samples present the following order of anion dominance HCO₃ ^? > SO₄ ^2? > Cl^?. Calcium is the dominant cation. Bicarbonates and sulfate ions are the dominant anions. The order of dominance for the heavy metals in surface and ground waters is as follows: Fe > Mn > Zn > Ni > Cu. The saturation index of waters regarding minerals is low. Computer simulation indicates that calcite and dolomite are common minerals in all water samples which are saturated in respect to quartz and argillaceous-siliceous minerals. The most pronounced property of waters is their acidic character. The high metal concentrations are related to water with low pH. Sulfide minerals control the low pH values of waters which is an important control factor for the evolution of the water chemical composition. The abundance of sulfates is attributed to the dissolution of the minerals pyrite (FeS₂) and alunite (KAl₃(SO₄)₂(OH)₆. The water–mineral interactions are responsible for the chemical composition of waters. Water quality problems can be successfully handled by the use factor analysis. 17 chemical parameters can be substituted by five factors which successfully represent the hydrochemical processes as well as their geographic distribution. Volcanic rocks in the study area have the potential to produce acid drainage.     

Fällungsprodukte im system Eisen(II)-sulfat und trinatrium-phosphat im pH-bereich von 2,5 bis 13,0

The precipitation of Fe(II)-phosphates and -hydroxides by interactions of Fe(II)-sulphate and Na₃PO₄ in aqueous solutions of pH 2–13 at room temperature has been investigated. The results of potentiometric titration were similar to those reported in literature. On the other hand, no stoichiometric compound has been found among the precipitation products in the whole pH-range, as for example, FeHPO₄, Fe₃(PO₄)₂ and Fe(OH)₂. All precipitation products contain PO₄-groups and OH-groups in various proportions depending on the pH during their formation. FeHPO₄ may be regarded as a hypothetical initial compound, when precipitation begins at about pH 2 and Fe(OH)₂ as the end-product at very high pH-values, respectively. Fe₃(PO₄)₂ does not occur except in the form of partially hydrolyzed species in a neutral milieu. All these compounds form a continuous system of non-stoichiometric phosphates and hydroxydes, respectively. The situation is even more complicated with sodium present in solutions, and this is not due to surface adsorption.The composition of the precipitates plotted against the pH-values do show some peculiarities, which clearly divide the “field of precipitation” into two regions: that of non-stoichiometric phosphates and that of non-stoichiometric hydroxides. The transition point has been defined as a pH-value at which PO₄- and OH-groups in the precipitates are interchangable most easily. This occurs at a pH of 8,8. The composition of this “transition product” may be written as FeOH(PO₄)_0,33.     

CHLOROSIS IN WILD PLANTS: IS IT A SIGN OF IRON DEFICIENCY?

ABSTRACT

Chlorosis in crops grown on calcareous soil is mainly due to iron (Fe) deficiency and can be alleviated by leaf application of soluble Fe²⁺ or diluted acids. Whether chlorosis in indigenous plants forced to grow on a calcareous soil is also caused by Fe deficiency has, however, not been demonstrated. Veronica officinalis, a widespread calcifuge plant in Central and Northern Europe, was cultivated in two experiments on acid and calcareous soils. As phosphorus (P) deficiency is one of the major causes of the inability of many calcifuges to grow on calcareous soil we added phosphate to half of the soils. Leaves in pots with the unfertilized and the P-fertilized soil, respectively, were either sprayed with FeSO₄ solution or left unsprayed. Total Fe, P, and manganese (Mn) in leaves and roots and N remaining in the soil after the experiment were determined. In a second experiment, no P was added. Leaves were either sprayed with FeSO₄ or with H₂SO₄ of the same pH as the FeSO₄ solution. Degree of chlorosis and Fe content in leaves were determined. Calcareous soil grown plants suffered from chlorosis, which was even more pronounced in the soils supplied with P. Newly produced leaves were green with Fe spray but leaves that were chlorotic before the onset of spraying did not totally recover. H₂SO₄ spray even increased chlorosis. This demonstrated that chlorosis was due to Fe deficiency. As total leaf Fe was similar on acid and calcareous soil, it was a physiological Fe deficiency, caused by leaf tissue immobilization in a form that was not metabolically “active”. Iron in the leaves was also extracted by 1,10-phenanthroline, an Fe chelator. In both experiments, significant differences between leaves from acid and calcareous soil were found in 1,10-phenanthroline extractable Fe but not in total leaf Fe, when calculated on a dry weight basis. Differences in 1,10-phenanthroline extractable Fe were more pronounced when calculated per unit dry weight than calculated per leaf area, whereas the opposite condition was valid for total leaf Fe.     

Effect of Fe,Mn or Al Compounds on Humification of Three Types of Plant Residues during Thermal Incubation

The effect of three inorganic minerals on the humification of three types of plant residues was determined by employing a model thermal incubation experiment. The plant residues consisiting of rice (Oryza sativa) straw, broadleaf tree (a mixture of oak/beech, Quercus serrata, Q. dentata, Q. acutissima etc.) sawdust and Japanese cedar (Cryptomeria japonica) sawdust were each mixed with Fe, Mn and Al in the form of hydroxides, oxides and sulfates. Humic materials were extracted after incubation and their composition was analyzed using a mixed solution of 0.02 M Na₄P₂O₇ and 0.1 M NaOH. The pH values of the samples after a longer duration of the incubation period were all less than 5.0, with the lowest value of 2.16 for a sample incubated with Al₂(SO₄)₃, except for the values of the samples incubated with MnO₂, which ranged from 4.75 to 6.0. The ΔlogK values decreased with the increase of the duration of the incubation period, whereas the RF values increased, as well as the amount of humus extracted (HE) and percentage of humic acid (PQ). Whereas most of the samples were identified as Type B and Type Rp humic acids, Type A humic acid was formed in all the plant residues incubated with Al₂(SO₄)₃, FeO(OH) and MnO₂ after ?180 d of incubation period. Moreover, the degree of humification of the plant residues was observed in the order of broadleaf tree > rice straw > Japanese cedar. It can be concluded that the inorganic compounds Al₂(SO₄)₃, FeO(OH) and MnO₂ contributed to the acceleration of the humification process of plant residues during the thermal incubation. The effect of Al₂(SO₄)₃ may be associated with the increase in the reactivity with other components in the system due to its high solubility, whereas FeO(OH) and MnO₂ may be involved in a reduction-oxidation reaction during the incubation. The browning and/or blackening of the plant residues were similar to the production of melanoidin which led us to consider that the mechanism involved in the study was similar to that of the Maillard reaction.     

Heavy Metals in Wastewater: The Effect of Electrolyte Composition on the Precipitation of Cadmium(II) Using Lime and Magnesia

The effect of effluent composition on the efficiency of hydroxide precipitation of Cd(II), using both lime and magnesia as precipitants, has been modelled by the solubility domain approach in order to provide wastewater effluent treatment limits, and has been experimentally validated. Common anionic species such as Cl^?, SO₄ ^2? and CO₃ ^2? have been treated. Solubility domain calculations were based on those phases that were found to determine metal solubility for systems representing the upper and lower limits of potential effluent chemical compositions. The isolated phases were found to resemble their mineralized counterparts, although in several cases exhibited a lower degree of structural order. Experimentally determined Cd(II) solubilities were generally encompassed within the calculated solubility domains, indicating that solubility domain predictions provide effluent treatment quality assurance ranges for the hydroxide precipitation process. The formation of gypsum (CaSO₄.2H₂O) and calcite (CaCO₃) at higher SO₄ ^2? and CO₃ ^2? concentrations as secondary precipitates using lime as the precipitant, and the hydroxy-sulfate Mg₂(OH)₂SO₄.xH₂O, nesquehonite (MgCO₃.4H₂O), hydromagnesite [Mg₅(OH)₂(CO₃)₄.4H₂O] and brucite [Mg(OH)₂] when employing magnesia was shown to have little effect on the observed residual Cd(II) solubility, although Mg(OH)₂ did promote β-Cd(OH)₂ formation in the Mg²⁺-containing Cd(II)/Cl^? and Cd(II)/SO₄ ^2? systems.     

Co-Ion Effect on Cr3+ Sorption by Amberlyst-15(H+)

Cr³⁺ sorption on strong acid exchanger Amberlyst-15(H⁺) is studied as a function of time and temperature using CrCl₃.6H₂O and [Cr₄(SO₄)₅(OH)₂] solutions. The rate is found to be governed by a mixed diffusion for both the solutions and faster for Cl^1? solution than SO₄ ^2?. The exchange capacities are found to be higher for Cl^1? system than SO₄ ^2?. From the rate constant values, the energies of activation are calculated using the well-known Arrhenius equation. Equilibrium data is explained with the help of the Langmuir equation. The Langmuir parameters are also found to be higher for exchange from the chloride solutions. Various thermodynamic parameters (??H^o, ??S^o, and ??G^o) for Cr³⁺ exchange on the resin are calculated. The ??G^o values are found to be negative while ??H^o and ??S^o are positive for both the Cr³⁺/Cl^1? and Cr³⁺/SO₄ ^2? systems. It is suggested that in case of Cl^1? solutions, the metal is exchanged as Cr³⁺, while in case of SO₄ ^2? solutions, the metal exchanging specie is CrSO₄ ⁺.     

Ferrihydrite in soils

Ferrihydrite—an ephemeral mineral—is the most active Fe-hydroxide in soils. According to modern data, the ferrihydrite structure contains tetrahedral lattice in addition to the main octahedral lattice, with 10–20% of Fe being concentrated in the former. The presence of Fe tetrahedrons influences the surface properties of this mineral. The chemical composition of ferrihydrite samples depends largely on the size of lattice domains ranging from 2 to 6 nm. Chemically pure ferrihydrite rarely occurs in the soil; it usually contains oxyanion (SiO1₄^4-, PO₄^3-) and cation (Al³⁺) admixtures. Aluminum replace Fe³⁺ in the structure with a decrease in the mineral particle size. Oxyanions slow down polymerization of Fe³⁺ aquahydroxomonomers due to the films at the surface of mineral nanoparticles. Si- and Al-ferrihydrites are more resistant to the reductive dissolution than the chemically pure ferrihydrite. In addition, natural ferrihydrite contains organic substance that decreases the grain size of the mineral. External organic ligands favor ferrihydrite dissolution. In the European part of Russia, ferrihydrite is more widespread in the forest soils than in the steppe soils. Poorly crystallized nanoparticles of ferrihydrite adsorb different cations (Zn, Cu) and anions (phosphate, uranyl, arsenate) to immobilize them in soils; therefore, ferrihydrite nanoparticles play a significant role in the biogeochemical cycle of iron and other elements.     

Abstract

In a greenhouse, radish (Raphanus sativus L.), corn (Zea mays L.), soybean (Glycine max Merr), and wheat (Triticum aestivum L.) were grown in soil‐based medium with captan at 60 mg/kg and truban at 30 mg/kg and with different levels of N from (NH₄)₂SO₄ or NaNO₃. Growth of radish, soybean, and corn was restricted by NH₄‐N compared with NO₃‐N. Captan and truban stunted growth of radish and soybean. As NH₄‐N or NO₃‐N fertilizer increased, the concentration of Ca and Mg in all plants decreased, and the percentage of K in corn, soybean, and wheat increased. Application of captan and truban increased all cation concentrations in corn, wheat, and soybean but decreased Ca concentration in radish. The amount of residual NH₄‐N in the medium supplied with (NH₄)₂SO₄ was increased by application of captan or truban. Captan increased the residual NO₃‐N in the medium treated with NaNO₃. Chemical names used: captan, (N‐(trichloro)methylthio)‐4‐cyclo‐hexene‐l, 2‐dicarboximide); truban, (5‐ethoxy‐3‐trichloromethyl‐l, 2, 4,‐thiadiazole).     

Iron-cyanide complexes in soil under varying redox conditions: speciation, solubility and modelling

The distribution of iron‐cyanide complexes between ferrocyanide, [Fe^II(CN)₆]^4–, and ferricyanide, [Fe^III(CN)₆]^3–, in soils on contaminated sites depends on the redox potential, E_H. We carried out microcosm experiments in which ferrocyanide (20 mg l^?1) was added to an uncontaminated moderately acidic subsoil (pH 5.2), and varied the E_H of the soil suspension between 200 and 700 mV over up to 109 days. Ferrocyanide and ferricyanide were analysed by capillary isotachophoresis. At redox potentials ranging from 400 to 700 mV, small amounts of iron‐cyanide complexes were adsorbed, and ferrocyanide was almost completely oxidized to ferricyanide. Decreasing E_H to 200 mV led to nearly complete removal of iron‐cyanide complexes from solution, and the complexes were not mobilized after subsequent aeration (E_H > 350 mV). Under weakly to moderately reducing conditions (E_H ≈ 200 mV), iron‐cyanide complexes were removed from solution by precipitation, which occurred, presumably in the form of e.g. Fe₂[Fe^II(CN)₆], Fe₄[Fe^II(CN)₆]₃ or Mn₂[Fe^II(CN)₆], after reductive dissolution of Mn and Fe oxides. Four different sets of geochemical model calculations were carried out. The species distribution between ferrocyanide and ferricyanide in solution was predicted reliably under varying pH and redox conditions when iron‐cyanide complex concentrations and Fe concentrations, excluding Fe bound in iron‐cyanide complexes, were used in model calculations. In model calculations on the fate of iron‐cyanide complexes in soil, adsorption reactions must be considered, especially under oxidizing conditions. Otherwise, the calculated iron‐cyanide complex concentrations are larger than those actually measured.