Determination of Chromium and Zinc in Soil by Microwave Digestion and Flame Atomic Absorption Spectrometry

[Objective] The aim was to develop a rapid, simple method for determination of chromium and zinc in soil samples by flame atomic absorption spectrometry.[Method] The method for determination of Cr and Zn in soil by combined flame atomic absorption spectrometry and microwave digestion was used. [Result] The concentration curve was linear within the range of 0-0.8 mg/L for Cr and 0-0.8 mg/L for Zn, the detection limits of Cr and Zn was 0.0025 mg/L and 0.002 3 mg/L, respectively. Recoveries of 102.4%-103.2% for Cr and 97.7%-98.3% for Zn were obtained for there soil samples. [Conclusion] The proposed method has the advantages of simple operation, high sensitivity, and high efficiency; it was successfully used for determination of Cr and Zn in soil samples.

Catalysis of Dissolved and Adsorbed Iron in Soil Suspension for Chromium(VI) Reduction by Sulfide

The kinetics of Cr（Ⅵ） reduction by sulfide in soil suspensions with various pHs, soil compositions, and Fe（Ⅱ） concentrations was examined using batch anaeroblc experimental systems at constant temperature. The results showed that the reaction rate of Cr（Ⅵ） reduction was in the order of red soil 〈 yellow-brown soil 〈 chernozem and was proportional to the concentration of HCl-extractable iron in the soils. Dissolved and adsorbed iron in soil suspensions played an important role in accelerating Cr（Ⅵ） reduction. The reaction involved in the Cr（Ⅵ） reduction by Fe（Ⅱ） to produce Fe（ⅡI）, which was reduced to Fe（Ⅱ） again by sulfide, could represent the catalytic pathway until about 70% of the initially present Cr（Ⅵ） was reduced. The catalysis occurred because the one-step reduction of Cr（Ⅵ） by sulfide was slower than the two-step process consisting of rapid Cr（Ⅵ） reduction by Fe（Ⅱ） followed by Fe（Ⅲ） reduction by sulfide. In essence, Fe（Ⅱ）/Fe（Ⅲ） species shuttle electrons from sulfide to Cr（Ⅵ）, facilitating the reaction. The effect of iron, however, could be completely blocked by adding a strong Fe（Ⅱ）-complexing ligand, 1,10-phenanthroline, to the soil suspensions. In all the experiments, initial sulfide concentration was much higher than initial Cr（Ⅵ） concentration. The plots of In e[Cr（Ⅵ）] versus reaction time were linear up to approximately 70% of Cr（Ⅵ） reduction, suggesting a first-order reaction kinetics with respect to Cr（Ⅵ）. Elemental sulfur, the product of sulfide oxidation, was found to accelerate Cr（Ⅵ） reduction at a later stage of the reaction, resulting in deviation from linearity for the In c[Cr（Ⅵ）] versus time plots.

Chemical Behavior of Chromium in Soils: Ⅲ. Mechanism of Cr(Ⅵ) Adsorption by Soils

An analysis of Cr (Ⅵ)-sorbed surface of the soils by using a scanning electron microscope and an electron probe microscope has proved that aluminium is the chief element affecting Cr (Ⅵ) adsoption. As the ionic strength of the solution increased, the amounts of Cr (Ⅵ) adsorbed by goethite and soils decreased. Cr (Ⅵ) adsorption was greatly depressed in the presence of SO42-, WO42-, MoO42-, HPO42- and H2PO4- which competed for anion adsorption sites. The depressing extent of these anions was found to follow the sequence: HPO42-, H2PO4-> MoO42-> WO42-> SO42-> > Cl-, NO3-. The amounts of Cr (Ⅵ) desorption varied with different extractants. 0.5 M NH4F and 0.1 M KH2PO4 could be regarded as the best extractants for Cr (Ⅵ) sorbed in the soils. The mechanism of Cr (Ⅵ) adsorption by goethite and soils seemed to be similar to that of phosphate. Basically, Cr (Ⅵ) was adsorbed through specific adsorption and could not be desorbed by Cl- and NO3-. Cr (Ⅵ) adsorption on goethite released OH-. There was a significant correlation between the amounts of Cr (Ⅵ) adsorbed and pH variation in goethite suspension.

Equilibrium, thermodynamic, and kinetic of Cr(VI) adsorption using a modified and unmodified bentonite clay

The capacities of natural and modified Brazilian bentonite samples as adsorbents to remove hexavalent metal chromium were investigated under several conditions in batch and column methods. The raw material, Ca-bentonite, was modified by anchorament of 3-aminopropyltrietoxisilane （APS） and 3,2- aminoethylaminopropyltrimetoxisilane （AEAPS） in the surface of bentonite sample： This type of new occurrence of bentonite is suitable as a raw material for adsorption process. Adsorption behavior of three bentonite types was strongly depending on pH of adsorbate solution, contact time adsorbent/adsorbate, and initial concentration of Cr（VI）. The results were confirmed by column method and reveals that the adsorption process of materials accorded by the Redlich-Peterson, Sips, Dubinin-Radushkevich, and tang- muir isotherm models. The exothermic entbalpic values reflected a favorable energetic process for chro- mium ions anchored in the material surfaces. The negative Gibbs free energy results supported the spontaneity of three adsorption reactions with Cr（VI） ions.

Microbial extraction of nickel from chromite overburdens in the presence of surfactant

The effect of surfactant polyoxyethylenesorbitan monolaurate （Tween-20） on the nickel bioleaching from pre-treated chromite overburden （COB）, Sukinda with fungal strain Aspergillus niger, was examined in shake flasks. Along with the nickel recovery from COB by the fungal bioleaching, the effect of surfactant on the growth of the A. niger was also investigated. Results show that the addition of surfactant in low concentration was favorable for the recovery of nickel from pre-treated COB. Normally, the carbon source （sucrose） in the culture medium was utilized by the A. niger for its cellular metabolism and organic metabolites （bio acids） were produced, which were responsible for the bioleaching of minerals. However, the addition of surfactant （Tween-20） accelerated the rate of sucrose consumption by the fungi, and thus enhancing the extraction of nickel from pre-treated COB. During the study, around 39% nickel extraction was achieved in A. niger mediated bioleaching performed at 2% pulp density of pre-treated COB at 30 °C, in the presence of surfactant whereas only 24% nickel was extracted without surfactant.

Study on the adsorption of Cr( Ⅵ) onto landfill liners containing granular activated carbon or bentonite activated by acid

The adsorption capacity of landfill liners containing granular activated carbon (GAC), or bentonite activated by acid, for Cr(VI) was investigated by batch testing. The results show that both GAC and activated bentonite could be used as sorptive amendments for trapping Cr(VI) in landfill liners. The Cr(VI) sorption to GAC and activated bentonite is much greater than Cr(VI) sorption to natural clay. The adsorption capacity of Cr(VI) onto all the soils increases with increasing temperature; adsorption capacity is also significantly influenced by soil-solid concentration. As the soil-solid concentration increases the adsorption capacity first decreases logarithmically, but then stabilizes when the soil-solid concentration exceeds a critical value (e.g. 400 g/L). Permeability tests were conducted in the laboratory. The results indicate that the hydraulic conductivity of landfill liners containing GAC or activated bentonite can meet the engineering requirement of 1 nm/s. One-dimensional transport simulations for Cr(VI) were performed to evaluate the effect of GAC and activated bentonite on landfill liners. The results of the simulations indicate that landfill liners containing GAC, or activated bentonite, significantly retard the transport of Cr(VI) relative to a conventional clay liner.

Food-Security Indexes Related to Combined Pollution of Chromium and Phenol in Soil-Rice Systems

The pot-culture method combined with chemical and statistical analyses was used to get basic data for determination of the food-security indexes related to combined pollution of Cr and phenol in soil-rice systems.Regression analyses according to the experimental result indicated that the critical concentrations of Cr andphenol in brown rice were 0.37 and 0.33 mg kg-1, respectively, under the condition of Cr-phenol combined pollution.

Phytoextraction of Nickel and Chrome in Paddy Soil Amended With Municipal Solid Waste and Sewage Sludge

A field study was conducted in the paddy soil amended with municipal solid waste （MSW） and sewage sludge （SS） to find out total, DTPA extractable nickel （Ni） and chrome （Cr） in soil and their uptake by rice plant as split-plot arrangement based on randomized complete block design with three replications. The results indicated that the most available Ni （2.10 ppm） measured under 3 years application of 40 ton.ha~ MSW＋I/2 chemical fertilizer （CF）. Also, with 3 application of 40 ton-ha＂l MSW concluded the highest available Cr. The most accumulation of Ni in root （14.51 ppm） and grain （8.89 ppm） happened under 3 years application of 40 ton.ha-1 SS and 20 ton ha-1 MSW＋I/2 CF, respectively. With application of 40 ton.ha-1 SS and 40 ton SS＋1/2 CF, respectively happened the maximum Cr uptake by root and grain （12.43 and 3.65 ppm, respectively）.

Speciation of chromium in soil inoculated with Cr(Ⅵ)-reducing strain,Bacillus sp.XW-4

Cr(Ⅵ)-amended soil was inoculated with Cr(Ⅵ)-reducing strain,Bacillus sp.XW-4 and incubated at 28 ℃in an incubator. Cr(Ⅵ)reduction,available Cr and Cr fractionin soils were studied.The results show that addition of Bacillus sp.XW-4 can promote Cr(Ⅵ)reduction,but inoculation of this strain has a negative effect on the decrease of available Cr content in soil.In controls (without this strain)amended with 100 and 200 mg/kg of Cr(Ⅵ),Cr(Ⅵ)contents decrease to about 41 and 92 mg/kg respectively after incubation of 4 d,while in soil inoculated with XW-4,Cr(Ⅵ)contents decrease to about 18 and 60 mg/kg,respectively.The content of available Cr in soils with inoculation of XW-4 is higher than that in controls.Chromium is partitioned into water soluble Cr,exchangeable Cr,precipitated Cr,Cr bound to organics and residual Cr.The highest content of Cr is observed in residual form and water soluble Cr is not detected for all treatments after 42 d,but the soils inoculated with Bacillus sp.XW-4 contain higher content of exchangeable Cr and lower content of precipitated Cr than the soil without the inoculum.Inoculation of Bacillus sp.XW-4 can increase chromium activity in soils.

Soil Explorations and Groundwater Monitoring to Evaluate Subsurface Contamination Due to Chromium in District Kasur, Pakistan

Due to improper tannery wastewater management in district Kasur Pakistan, groundwater has been reported to be highly contaminated. It was aimed to find out the extent up to which subsurface has contaminated due to chromium in areas adjacent to the tannery units. Eight （8） soil bores were conducted up to the depth of 30.5 meters and soil samples were tested for total and hexavalent chromium concentrations retained in soil by aqua regia digestion at the every depth of 1.5 meters. Afterwards monitoring wells were installed in these eight （8） bores so as to monitor chromium concentrations in the groundwater on monthly basis. The main source of contamination was considered to be the four （4） drains carrying tanneries effluent therefore samples were collected from these drains so as to observe seasonal variation in chromium concentration.

Environmental Factors Affecting Chromium-Manganese Oxidation-Reduction Reactions in Soil

Disposal of chromium （Cr） hexavalent form, Cr（Ⅵ）, in soils as additions in organic fertilizers, liming materials or plant nutrient sources can be dangerous since Cr（Ⅵ） can be highly toxic to plants, animals, and humans. In order to explore soil conditions that lead to Cr（Ⅵ） generation, this study were performed using a Paleudult （Dystic Nitosol） from a region that has a high concentration of tannery operations in the Rio Crande do Sul State, southern Brazil. Three laboratory incubation experiments were carried out to examine the influences of soil moisture content and concentration of cobalt and organic matter additions on soil Cr（Ⅵ） formation and release and manganese （Mn） oxide reduction with a salt of chromium chloride （CrCl3） and tannery sludge as inorganic and organic sources of Cr（Ⅲ）, respectively. The amount of Cr（Ⅲ） oxidation depended on the concentration of easily reducible Mn oxides and the oxidation was more intense at the soil water contents in which Mn（Ⅲ/Ⅳ） oxides were more stable. Soluble organic compounds in soil decreased Cr（Ⅵ） formation due to Cr（Ⅲ） complexation. This mechanism also resulted in the decrease in the oxidation of Cr（Ⅲ） due to the tannery sludge additions. Chromium（Ⅲ） oxidation to Cr（Ⅵ） at the solid/solution interface involved the following mechanisms： the formation of a precursor complex on manganese （Mn） oxide surfaces, followed by electron transfer from Cr（Ⅲ） to Mn（Ⅲ or Ⅳ）, the formation of a successor complex with Mn（Ⅱ） and Cr（Ⅵ）, and the breakdown of the successor complex and release of Mn（Ⅱ） and Cr（Ⅵ） into the soil solution.

Chromium Adsorption in Different Mineralogical Fractions from Subtropical Soils

Safe application of chromium （Cr）-containing organic industrial wastes to soil requires considering the ability of the soil to adsorb Cr. In this study, the maximum Cr adsorption capacity was assessed for the bulk samples and their clay and iron-free clay fractions of four subtropical soils differing in mineralogy. To this end, the samples were supplied with Cr（Ⅲ） nitrate solutions at pH 4.5 or 5.5. The results of Cr（Ⅲ） adsorption fitted to a Freundlich equation and the adsorption capacity was positively correlated with soil organic matter and iron oxide contents. The clay fractions adsorbed more Cr per unit mass than the bulk soils and the iron-free clay fractions. The Cr（Ⅲ） adsorption capacity increased with increasing soil pH due to more charges on adsorbing surfaces. Our results suggest that the soils rich in organic matter and iron oxides and having a pH above 4.5 are suitable for application of Cr（Ⅲ）-loaded industrial wastes.

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