Environmental mineralogy: spectroscopic studies on ferrous saponite and the reduction of hexavalent chromium

A green-coloured phyllosilicate occurring on the walls of amygdaloidal cavities and along fractures in the Deccan Flood basalts at Killari, Maharashtra, India, has been identified as iron-rich saponite with a chemical composition [Na_0.60 K_0.40 Ca_0.47] {Mg_2.05Fe_3.95} (Si_6.45Al_1.55) O₂₀(OH)₄. In order to explore the possible application of this phyllosilicate for environmental management, we have carried out X-ray photon spectroscopic (XPS) and diffuse reflectance spectroscopic (DRS) measurements on the dichromate solutions, in both the untreated and treated form. The dichromate solution treated with the saponite samples show a remarkable capability not only to adsorb hexavalent chromium but also effect a reduction of hexavalent to trivalent chromium at an efficiency of 75%. These valence states of chromium were characterised unambiguously by XPS and DRS spectra collected at room temperature. Our studies show that Killari saponite is capable of reducing Cr (VI) to Cr (III). The ferrous saponite in Deccan Flood basalts could therefore be a useful mineral in environmental management in areas affected by Cr (VI) effluents.     

Commercial steel wool for reduction of hexavalent chromium in wastewater: batch kinetic studies and rate model

The present study aims at searching the potential of commercial grade steel wool in reducing hexavalent chromium in aqueous phase under batch mode. About 30 % of the initial hexavalent chromium was found to reduce within 2 h at a pH of 3. However, on testing the combined effects of different process parameters, namely the solution pH, wool loading, etc., the optimum batch parametric condition has been fixed. A moving boundary type kinetic model, which takes into account the effect of passivation along with the direct reduction mechanism to simulate the gross uptake profile of Cr(VI) from the bulk solution is proposed. The effective pore diffusivity of Cr(VI) in commercial steel wool was determined by a suitable global optimization technique. Additionally, the model is also capable to simulate the decline of active external surface area of the wool caused by passivation with time. A good match of the experimental data and model-simulated transient bulk concentration of Cr(VI) (under optimum parametric condition only) establishes the general validity of the proposed model.     

Biosorption of hexavalent chromium from aqueous solution onto pomegranate seeds: kinetic modeling studies

Hexavalent chromium has been proved to be the reason of several health hazards. This study aimed at evaluating the application of pomegranate seeds powder for chromium adsorption (VI) from aqueous solution. Chromium adsorption percentage (VI) increased with increasing the adsorbent dosage. Chromium adsorption capacity (VI), at pH = 2 and 10 mg/L initial metal concentration, decreased from 3.313 to 1.6 mg/g through increasing dosage of adsorbent from 0.2 to 0.6 g/100 ml. The adsorption rate increased through increase in chromium initial concentration (VI). However, there was a removal percentage reduction of chromium (VI). Chromium adsorption kinetics by different models (pseudo-first-order, modified pseudo-first-order, pseudo-second-order, Elovich, intraparticle diffusion, Boyd kinetic) was investigated as well. Studies on adsorption kinetic indicated that the experimental data were matched by pseudo-second-order model (R ² = 0.999) better. Obtained results demonstrated the pomegranate seeds can be used as an effective biomaterial and biosorbent for hexavalent chromium adsorption from aqueous solutions.     

Ionic solid-impregnated sulphate-crosslinked chitosan for effective adsorption of hexavalent chromium from effluents

Microwave-assisted tetrabutyl ammonium-impregnated sulphate-crosslinked chitosan was synthesized for enhanced adsorption of hexavalent chromium. The adsorbent obtained was extensively characterized using Fourier transform infrared, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray studies. Various isotherm models such as Langmuir, Freundlich and Dubinin–Radushkevich were studied to comprehend the adsorption mechanism of hexavalent chromium by the adsorbent. Maximum adsorption capacity of 225.9 mg g^?1 was observed at pH 3.0 in accordance with Langmuir isotherm model. The sorption kinetics and thermodynamic studies revealed that adsorption of hexavalent chromium followed pseudo-second-order kinetics with exothermic and spontaneous behaviour. A column packed with 1 g of adsorbent was found to give complete adsorption of Cr(VI) up to 900 mL of 200 mg L^?1 solution which discerns the applicability of the adsorbent material for higher sample volumes in column studies. The effective adsorption results were obtained due to both ion exchange and ion pair interaction of adsorbent with hexavalent chromium. Greener aspect of overall adsorption was regeneration of the adsorbent which was carried out using sodium hydroxide solution. In the present study, the regenerated adsorbent was effectively reused up to ten adsorption–desorption cycles with no loss in adsorption efficiency.     

Metallophilic fungi research: an alternative for its use in the bioremediation of hexavalent chromium

Contamination by hexavalent chromium has had a large impact on modern society and human health. This problem is a consequence of its great industrial applicability to several products and processes. Short-term exposure to hexavalent chromium can cause irritation, ulceration in skin and stomach and in addition to cancer, dermatitis, and damage to liver, renal circulation and nervous tissues, with even death being observed in response to long-term exposures. Many techniques have been used for the remediation of this pollutant, including physical and chemical approaches and, in more recent years, biological methods. Filamentous fungi isolated from contaminated sites exhibit a significant tolerance to heavy metal; hence, they are an important source of microbiota capable of eliminating hexavalent chromium from the environment. However, these microorganisms can do so in different ways, including biosorption, bioreduction, and bioaccumulation, among others. In this review, we explore several of the most documented mechanisms that have been described for fungi/hexavalent chromium interactions and their potential use in bioremediation.     

Efficient removal of hexavalent chromium from aqueous solutions using autohydrolyzed Scots Pine (Pinus Sylvestris) sawdust as adsorbent

In this work, a low-cost lignocellulosic adsorbent with high biosorption capacity is proposed, suitable for the efficient removal of hexavalent chromium from water and wastewater media. The adsorbent was produced by autohydrolyzing Scots Pine (Pinus Sylvestris) sawdust. The effect of the autohydrolysis conditions, i.e., pretreatment time and temperature, on hexavalent chromium biosorption was investigated using energy-dispersive X-ray spectroscopy (EDS) and UV–visible spectrophotometry. The Freundlich, Langmuir, Sips, Radke-Prausnitz, Modified Radke-Prausnitz, Tóth, UNILAN, Temkin and Dubinin-Radushkevich adsorption capacities and the rate constant values for pseudo-first- and pseudo-second-order kinetics indicated that the autohydrolyzed material exhibits significantly enhanced hexavalent chromium adsorption properties comparing with the untreated sawdust. The Freundlich’s adsorption capacity K _F increased from 2.276 to 8.928 (mg g^?1)(L mg^?1)^1/n , and the amount of hexavalent chromium adsorbed at saturation (Langmuir constant q _m) increased from 87.4 to 345.9 mg g^?1, indicating that autohydrolysis treatment at 240 °C for 50 min optimizes the adsorption behavior of the lignocellulosic material.     

Decontamination of hexavalent chromium and tri-ethyl phosphate stimulants through photacatalytic oxidation

In this paper, the photocatalytic decontamination of hexavalent chromium and tri-ethyl phosphate, two important wastewater contaminants, are studied by the ultraviolet / nano-titanium dioxide process. The pH value and synergic effect between the oxidation of tri-ethyl phosphate and the reduction of hexavalent chromium were investigated in different concentrations of tri-ethyl phosphate and hexavalent chromium. Furthermore, the effects of ultraviolet and nano-titanium dioxide were investigated in a solution which contained tri-ethyl phosphate and hexavalent chromium. Results of adsorptions showed that hexavalent chromium was adsorbed better in acidic pH while the better adsorption for tri-ethyl phosphate was occurred in alkalinity pH. The reduction rate of hexavalent chromium was higher in acidic solutions while it was obtained at natural pH for tri-ethyl phosphate. In co-adsorption of hexavalent chromium and triethyl phosphate pollutants, tri-ethyl phosphate slightly increased adsorption of hexavalent chromium, but hexavalent chromium had no influence on the adsorption of tri-ethyl phosphate on nano-titanium dioxide particles. In contrast, triethyl phosphate has an improving effect on the reduction reaction rate of hexavalent chromium which increases with the interaction of the concentration of tri-ethyl phosphate in mixture. The same is true for the oxidation rate of tri-ethyl phosphate.     

Origin and mobility of hexavalent chromium in North-Eastern Attica,Greece

An integrated framework that is comprised of field surveys of groundwater, surface water and soils, laboratory process experiments and hydrologic and geochemical modeling is used to identify the origin (anthropogenic versus geogenic sources), fate and transport of hexavalent Cr in Tertiary and Quaternary deposits of Oropos plain in Greece. Groundwater and soils were analyzed in May 2008 and exhibited considerable Cr concentrations. Mineralogical analysis and micro-XRF analysis of the heavy soil fractions (metallic components) showed Cr bearing phases like chromites, Cr-silicate phases with positive correlation between Si, Al, Fe and Cr soil concentrations. Column experiments showed the Cr(VI) desorption ability of soils, e.g. concentration of 20 μg L⁻¹ was detected after the application of 50 mm of rain. The groundwater model simulated the variability of Cr concentrations emanating from both anthropogenic and geogenic sources, successfully using rate constants obtained from the laboratory experiments, e.g. 4.24 nM h⁻¹ for serpentine soil and 0.77 nM h⁻¹ for soil in alluvial deposits. The mineralogical and geochemical results support a geogenic origin for Cr in soils and groundwater of Oropos plain while modeling results suggest that contaminants transported by Asopos River have affected only the upper layers of the subsurface in the vicinity of the river. The framework can be used to establish background concentrations or clean up levels of Cr-contaminated soils and groundwater.     

Hexavalent chromium removal from aqueous solutions by using low-cost biological wastes: equilibrium and kinetic studies

The batch removal of hexavalent chromium from aqueous solutions using almond shell, activated sawdust, and activated carbon, which are low-cost biological wastes under different experimental conditions, was investigated in this study. The influences of initial concentration, adsorbent dose, adsorbent particle size, agitation speed, temperature, contact time, and pH of solution were investigated. The adsorption was solution pH dependent and the maximum adsorption was observed at a solution pH of 2.0. The capacity of chromium adsorption under equilibrium conditions increased with the decrease in particle sizes. The equilibrium was achieved for chromium ion after 30?min. Experimental results showed that low-cost biosorbents are effective for the removal of pollutants from aqueous solution. The pseudo-second-order kinetic model gave a better fit of the experimental data as compared to the pseudo-first-order kinetic model. Experimental data showed a good fit with the Freundlich isotherm model. Changes in the thermodynamic parameters, including Gibbs free energy (??G_o), enthalpy (??H_o), and entropy (??S_o), indicated that the biosorption of hexavalent chromium onto almond shell, activated sawdust, and activated carbon was feasible, spontaneous, and endothermic in the temperature range 28?C50?°C.     

Modified biomass of Phanerochaete chrysosporium immobilized on luffa sponge for biosorption of hexavalent chromium

Phanerochaete chrysosporium, a white rot basidiomycete, was immobilized over Luffa cylindrica sponge discs, treated with 0.1 N HCl and its potentiality for the removal of hexavalent chromium [Cr(VI)] from water was investigated in both batch and in up-flow fixed-bed bioreactor. The acid treatment of biomass increased the uptake capacity and percentage removal of Cr(VI) from 33.5 to 46.5 mg g^?1 and 67 to 92 %, respectively. Maximum uptake of Cr(VI) was achieved at pH 2, temperature 40 °C after 100 min of contact time. The Cr(VI) sorption on the biomass was better explained by Langmuir isotherm. Thermodynamic studies indicated that the process was spontaneous and endothermic. Sorption kinetic study showed that pseudo-second-order model best correlates the Cr(VI) sorption on the biomass as compare to pseudo-first-order kinetic model. The performance of fixed-bed bioreactor was evaluated at different bed heights (5, 15 and 25 cm) and flow rates (1.66, 4.98 and 8.33 mL min^?1) by using bed depth service time model. Response surface methodology statistical method was applied for optimizing the process parameters. FTIR analysis showed that amino groups were mainly involved in adsorption of Cr(VI).     

The mechanisms of reduction of hexavalent chromium by green rust sodium sulphate: Formation of Cr-goethite

The molecular-level processes that control green rust sodium sulphate (GR_Na,SO4) reaction with chromate were studied using high-resolution techniques. Changes in solid morphology, structure and composition were observed with atomic force microscopy, transmission electron microscopy and X-ray diffraction. The results suggest the following mechanisms: Chromate replaces sulphate in the GR interlayer and is reduced by Fe(II). Formation of sparingly soluble Cr(III)-solid blocks further chromate entry, but Cr(VI) reduction continues at the GR solid/solution interface. Electron transfer from the centre of the GR crystals to the surface facilitates rapid reaction. Less stable zones of the reacted GR_Na,SO4 dissolve and amorphous Cr(III),Fe(III)-solid forms. During equilibration, Cr-substituted goethite evolves in association with remaining GR_Na,SO4, fed by material from the amorphous phase and dissolving oxidised GR. In contrast, previous Cr(VI) experiments with the carbonate form of GR, GR_CO3, have suggested only reaction and deposition at the surface. From the perspective of environmental protection, these results have important implications. Goethite is sparingly soluble and the inclusion of Cr(III) as a solid-solution makes it even less soluble. Compared to Cr adsorbed at the surface of an Fe(III)-phase, Cr(III) incorporated in goethite is much less likely to be released back to groundwater.     

FeSO4稳定铬污染土的铬赋存形态及浸出特性研究

采用硫酸亚铁（FeSO4）对Cr(VI)污染土进行稳定化处理。研究了Fe(II)/Cr(VI)摩尔比和养护龄期对污染土稳定过程中的铬赋存形态及浸出特性的影响规律。结果表明：随着Fe(II)/Cr(VI)摩尔比和养护龄期的增加,Cr(VI)和总Cr的浸出浓度降低,稳定土中Cr(VI)的含量降低,当摩尔比为3时,Cr(VI)和总Cr的浸出浓度均低于我国《危险废弃物鉴别标准 浸出毒性鉴别》（GB/T50853―2007）的限值;当摩尔比为10时,稳定土中Cr(VI)的含量低于我国《土壤环境质量标准》（GB15618―2008）中工业和商业用地的限值（30 mg/kg）;当摩尔比为20时,低于居住用地限值（5 mg/kg）。形态提取试验结果表明：FeSO4改变稳定土中铬的赋存形态,可促使铬从弱酸态向可还原态和可氧化态转化,而对残渣态的铬影响不大。Cr(VI)的浸出浓度与稳定土中的Cr(VI)含量均存在指数函数关系,且浸出试验不能全面、客观地评价铬污染土稳定效果。     

污染场地六价铬迁移转化机制与数值模拟研究

随着全球工业化迅猛发展,土壤和地下水六价铬污染日益严重。基于某铁合金厂铬渣场地现场调查与采样分析,开展铬渣场地土样吸附、渗透和弥散试验,研究六价铬在粉质黏土土样中的吸附特性和迁移规律,建立考虑对流-弥散-吸附的六价铬迁移三维动力学模型,结合数值软件获取污染源位于场地上、下游时地下水中六价铬迁移分布特征,并揭示弥散度?和分配系数 对六价铬时空分布的影响。试验结果表明,粉质黏土对六价铬吸附符合Langmuir等温吸附模型,最大吸附量为466.6 mg/kg;蒸馏水和160 mg/L 六价铬溶液入渗下粉质黏土渗透系数约为6.5×10–7～6.7×10–7 cm/s,1 000 mg/L六价铬溶液的渗透系数增大至4.4×10–6 cm/s;粉质黏土水动力弥散系数D为1.4×10–4 m2/d,计算得到阻滞因子 为4.2～10。数值模拟结果表明,场地下游受到六价铬污染时,即使不考虑分子扩散作用,上游仍有被污染的风险,污染程度取决于含水层的弥散度;考虑含水层对六价铬吸附时,土体分配系数越大,六价铬污染羽分布范围越小,在预测地下水中六价铬浓度分布时应重点考虑六价铬吸附等转化过程。     

Determination of ammonia removal from aqueous solution and volumetric mass transfer coefficient by microwave-assisted air stripping

In this study, the removal of ammonia from synthetically prepared ammonia solution at pH 11 was investigated by using microwave radiation heating. Initially, conventional and microwave radiation heating were compared with respect to ammonia removal efficiency and overall volumetric mass transfer coefficient at five different temperatures. Overall volumetric mass transfer coefficient was calculated from the material balance for ammonia at unsteady-state condition. The effects of temperature, initial ammonia concentration, air flow rate, stirring speed, and microwave radiation power on both ammonia removal efficiency and overall volumetric mass transfer coefficient in liquid phase were also examined. The results of the experiments revealed that microwave-assisted air stripping allowed to the higher ammonia removal efficiency and overall volumetric mass transfer coefficient value compared to the conventional heating air stripping. Additionally, temperature and air flow rate were determined as the most substantial parameters affecting both ammonia removal efficiency and overall volumetric mass transfer coefficient value.     

Photocatalytic reduction of hexavalent chromium with commercial Fe/Ti oxide catalyst under UV and visible light irradiation

The photoreduction efficiency of toxic hexavalent chromium into non-toxic trivalent chromium was studied using local low-cost material and modern technology. The materials involved different iron–titanium oxide nanopowders synthesized via simple hydrothermal–hydrolysis process. X-ray diffraction and high-resolution transmission electron microscope were employed to study the structural properties of the as-prepared samples. The effects of molar ratio (Fe/Ti) and hydrothermal temperature on spectroscopic properties have been investigated using Fourier transform infrared FT-IR spectroscopy. The photocatalytic performance of hexavalent chromium was systematically studied at various conditions including initial concentration of Cr(VI), hydrothermal temperature and Fe/Ti ratios of mixed iron–titanium oxide powders. It has been found that the highest photoreduction efficiencies of Cr(VI) were 95.7 and 86.2% at initial concentrations 10 and 60 ppm of Cr(VI), respectively. The synthesized mixed Fe₂O₃–TiO₂ photocatalyst exhibited higher efficiency of about 88% under visible light irradiation. The as-prepared mixed oxide catalyst exhibited high photocatalytic conversion efficiency and recycling stability in comparison with different commercial catalysts.     

Calculation of mass transfer coefficients for dolomitization models

Reversible mass transfer associated with alteration of carbonate rocks can be modeled in terms of a pore fluid migrating across a temperature field slowly enough that chemical equilibrium is maintained. These types of models yield both the spatial distribution of the alteration and the absolute amounts of minerals dissolved and precipitated. In order to compute the total mass transfer involved, it is necessary to obtain mass transfer coefficients for each mineral. This paper presents detailed examples of mass transfer in closed systems for the system calcite-dolomite-solution over the temperature range 0–200°C. It is shown that the mass transfer coefficients for the reversible process can be obtained from thermodynamic data and elementary solubility calculations. The connection between these reversible coefficients and the more familiar irreversible mass transfer coefficients is shown to be a simple derivative relation. Calculations are presented for two cases: calcite-ordered dolomite solution and calcite-disordered dolomite solution in both pure water and brines with Ca/Mg ratios similar to seawater. The large mass transfer coefficients obtained for the brines relative tot he pure water is attributed to the fact that the presence of both calcite and dolomite in a system fixes the Ca/Mg cation ratio in the solution as a function of temperature. Dolomitization occurring in nature likely involves both reversible and irreversible processes and accurate interpretation of field data requires the ability to distinguish between these two cases.     

Retention studies of chromium (VI) from aqueous solution on the surface of a novel carbonaceous material

In the present study, the retention capacity of carbonaceous material obtained from the diesel engine exhaust mufflers for Cr(VI) removal has been investigated. The physicochemical properties such as density, pH of aqueous slurry, pH at point of zero charge, ash content, moisture content, volatile matter, surface area, scanning electron microscopy and electron dispersive spectroscopy of the carbonaceous material were determined. The capacity of adsorbent for removal of Cr(VI) from aqueous solution was observed under different experimental condition like contact time, initial concentration of metal ions, pH and temperatures on the adsorption capacity of the adsorbent. Maximum adsorption of Cr(VI) ions was found at low pH. The adsorption process was found to follow second-order kinetics. The rate constant was evaluated at different temperatures along with other thermodynamic parameters like activation energy, Gibbs free energy change, enthalpy change and entropy change. Both Langmuir and Freundlich isotherms were used to describe the adsorption equilibrium of carbonaceous material at different temperatures. Langmuir isotherm shows better fit than Freundlich isotherm at given conditions. The result shows that low-cost carbonaceous material from diesel engine exhaust mufflers can be efficiently used for wastewater treatment containing Cr(VI) ions.     

Utilization of a comparison of curvatures for land surface segmentation

Utilization of a new geomorphometric variable for land surface segmentation — the angle of absolute curvatures — is a main goal of the paper. The angle of absolute curvatures is defined as the difference between the orientation of maximal curvature (field independent) and the orientation of the greater of the profile or the tangential curvature. Land-forms separated by three types of borders (A, B, C) can be delimited from the field of angles of absolute curvatures. Borders of A type are connected with a local extreme of slope. Borders of B and C type are connected with a change to the priority of either profile or tangential curvature, as shown in computation, respectively. Fields of altitude, slope, profile curvature, tangential curvature and rotor curvature are reflected by an algorithm. Distinct borders in the field of the angles of absolute curvatures are connected with a sudden change of value and with zero isolines in the previously mentioned fields. Spatially closed entities generated by this proposed algorithm are considered to be a variant of the elementary forms of the land surface. The quality of information generated by this algorithm depends on the size of the grid mesh of the input digital elevation model. The algorithm in its current state is suitable for locating the borders of some elementary forms in the first stage of geomorphology mapping.     

Biotransformation potential of hexavalent chromium by Bacillus pumilus-S4, Pseudomonas doudoroffii-S5 and Exiguobacterium-S8 in association with hydrophytes

Three chromium-resistant bacteria Bacillus pumilus-S4, Pseudomonas doudoroffii-S5 and Exiguobacterium-S8 were isolated from chromium-contaminated wastewater/soil and could resist very high concentrations of potassium chromate in Luria agar (up to 25 mg ml^?1) and acetate minimal medium (2 mg ml^?1). The strains showed growth at diverse pH and temperatures and could resist multiple heavy metals. Pseudomonas doudoroffii-S5 reduced (8.27 mg hexavalent chromium 24 h^?1) at a lower initial potassium chromate concentration (100 μg ml^?1), but overall more chromate (28.4 mg hexavalent chromium 24 h^?1) was reduced at a higher initial concentration (1,000 μg ml^?1). The addition of various heavy metals (zinc sulphate, copper sulphate, and manganese sulphate at 50 μg ml^?1) in the chromium reduction media did not significantly affect the hexavalent chromium reduction potential of these isolates. The chromium removal/detoxification potential of these strains increased when used in conjunction with hydrophytes Eichornia crassipes and Pistia stratiotes. Interestingly, the whole process runs automatically with less energy input, that is, the bacterial strains support the growth of plant while in turn the plant releases exudates that help bacterial growth.