Evaluation of the Combined Cr(VI) Removal Capacity of Sawdust and Sawdust-Immobilized Acinetobacter haemolyticus Supplied with Brown Sugar

The purpose of this study is to evaluate the combined Cr(VI) removal capacities of nonliving (untreated rubber wood sawdust, URWS) and living biomass (URWS-immobilized Acinetobacter haemolyticus) in a continuous laboratory scale downward-flow two column system. Synthetic solutions of Cr(VI) between 237 and 320 mg L^?1 were mixed with 1 g L^?1 brown sugar in a nonsterile condition. Final Cr(VI) of between 0 and 1.6 mg L^?1 indicate a Cr(VI) removal capacity of 99.8–100%. The bacterial Cr(VI) reduction capacity increased with column length. This study shows the feasibility of using the two column system consisting of living (bacteria) and nonliving biomass (URWS) as a useful alternative treatment for Cr(VI) contamination in the aqueous system.     

Sorption Characteristics of Chromium and Its Phytotoxic Effect with or without City Compost on Maize and Spinach in a Vertisol of Central India

Abstract

Phytotoxicity, due to chromium [Cr (VI)] additions from low to very high levels in a swell–shrink clayey soil (Haplustert), in maize and spinach was studied in a pot culture experiment. Six levels of Cr (VI) (0, 5, 10, 25, 50, and 75 mg kg^?1 soil) for maize and five levels for spinach (0, 2, 5, 10, and 25 mg kg^?1 soil) were applied singly and in combination with two doses (0 and 20 t ha^?1) of city compost. At levels of more than 75 mg Cr (VI) kg^?1 soil for maize there was virtually no growth after germination, whereas 25 mg Cr (VI) kg^?1 soil hindered the germination of spinach crop. Initial symptoms of Cr (VI) toxicity appeared as severe wilting of the tops of treated plants. Maize plants suffering from severe Cr (VI) toxicity had smaller roots and narrow brownish red leaves covered with small necrotic spots. In spinach, severe chlorosis was observed in leaves. Higher levels of Cr (VI) inhibited the growth and dry‐matter yield of the crops. However, application of city compost alleviated the toxic effect of Cr (VI). The concentration of Cr (VI) in plant parts increased when Cr (VI) was applied singly but decreased considerably when used in combination with city compost. There was evidence of an antagonistic effect of Cr (VI) on other heavy‐metal (Mn, Cu, Zn, and Fe) concentrations in plant tops. Thus, when Cr (VI) concentration increases, the concentration of other beneficial metals decreases. Chromium (VI) concentration in maize roots ranged from traces (control) to 30 mg kg^?1and were directly related to soil Cr (VI) concentration. At 25 mg Cr (VI) kg^?1 soil, yield of maize was reduced to 41% of control plants, whereas in spinach, 10 mg Cr (VI) kg^?1 soil caused a 33% yield reduction. Experimental results revealed that the maize top (cereal) is less effective in accumulating Cr (VI) than spinach (leafy vegetables). Laboratory studies were also conducted to know Cr (VI) sorption capacity of a swell–shrink clayey soil with and without city compost, and it was found that Cr (VI) sorption reaction was endothermic and spontaneous in nature.     

Critical Influence of Culture Medium and Cr(III) Quantification Protocols on the Interpretation of Cr(VI) Bioremediation by Environmental Fungal Isolates

The influence of culture medium composition on chromium(VI) quantification according to diphenylcarbazide (DPC) colorimetric determination was evaluated. Considering the eventual biospeciation of Cr(VI) as a mechanism of microbial bioremediation, the possibility to quantify Cr(III) in culture medium was also explored. Yeast nitrogen base (YNB) was identified as the least interferent culture medium for Cr(VI) quantification by DPC and it was applied to compare different strategies for Cr(III) oxidation. The most appropriate oxidation protocol consisted in the reaction with 80 mM KIO₄ at room temperature for 30 min prior to DPC. Parameters like basal culture medium (vitamins + salts + oligoelements), C and N source were systematically evaluated, either independently or in combination. Results demonstrated that C source was the most interferent culture medium component, being the use of sucrose preferable to glucose. A medium arbitrarily named as YNB′ (YNB without amino acids and ammonium sulfate plus 50 g L^?1 sucrose and 0.6 g L^?1 (NH₄)₂SO₄) was defined for Cr(VI)-amended fungal cultures. Kinetics of growth, Cr(VI) removal, and nutrient consumption for isolates A. pullulans VR-8, filamentous fungus PMF-1, and Lecythophora sp. NGV-1 were obtained. The order of Cr(VI) removal efficiency was as follows: A. pullulans VR-8 > Lecythophora sp. NGV-1 > filamentous fungus PMF-1, and a similar trend was observed for biomass yield and nutrients consumption. Studies on biospeciation by means of the selected Cr(III) oxidation protocol were unsuccessful, leading to Cr(VI) values much lower than expected. It revealed that this kind of protocols should be cautiously evaluated when studying microbial Cr(VI) bioremediation.     

Effect of Metals on Decolorization of Reactive Blue HERD by <Emphasis Type="Italic">Comamonas</Emphasis> sp. UVS

Comamonas sp. UVS was able to decolorize Reactive Blue HERD (RBHERD) dye (50 mg L^?1) within 6 h under static condition. The maximum dye concentration degraded was 1,200 mg L^?1 within 210 h. A numerical simulation with the model gives an optimal value of 35.71?±?0.696 mg dye g^?1 cell h^?1 for maximum rate (V_max) and 112.35?±?0.34 mg L^?1 for the Michaelis constant (K_m). Comamonas sp. UVS has capability of decolorization of RBHERD in the presence of Mg²⁺, Ca²⁺, Cd²⁺, and Zn²⁺, whereas decolorization was completely inhibited by Cu²⁺. Metal ions also affected the levels of biotransformation enzymes during decolorization of RBHERD. Comamonas sp. UVS was also able to decolorize textile effluent with significant reduction in COD. The biodegradation of RBHERD dye was monitored by UV–vis spectroscopy, FTIR spectroscopy, and HPLC.     

Organic Matter Effects on the Cr(VI) Removal Efficiency and Tolerance of <Emphasis Type="Italic">Typha domingensis</Emphasis>

The removal efficiency and tolerance of Typha domingensis to Cr(VI) in treatments with and without organic matter (OM) addition were evaluated in microcosm-scale wetlands. Studied Cr(VI) concentrations were 15 mg L^?1, 30 mg L^?1, and 100 mg L^?1, in treatments with and without OM addition, arranged in triplicate. Controls (without neither metal nor OM addition—without metal with OM addition) were disposed. Cr(VI) was removed efficiently from water in all treatments. OM addition enhanced significantly Cr(VI) and total Cr removals from water. In the treatments with OM addition, significantly higher Cr concentrations were found in sediment than the treatments without OM addition. Plants of the treatments without OM addition showed significantly higher Cr concentrations in tissues but lower biomass increase than the treatments with OM addition. The highest Cr concentrations in tissues were observed in submerged parts of leaves, followed by roots. According to SEM analysis, in the 100 mg L^?1 treatments, the highest Cr accumulation was observed in the epidermis of old leaves. Although Cr(VI) produced changes in root morphology, the OM addition favored the plant growth. In T. domingensis, root morphological plasticity is an important mechanism to improve metal tolerance and Cr uptake in wetland systems minimizing the environmental impact.     

Chromium Speciation in Water by Sorption on Calcite and Determination by Electrothermal Atomic Absorption Spectrometry

Abstract

A rapid, sensitive, and accurate method for the separation and speciative determination of chromium (Cr)(VI) and Cr(III) in water samples has been developed using sorption as the separation technique in conjunction with final determination by electrothermal atomic absorption spectroscopy (ETAAS). The present method, where granular calcite is used as selective sorbent, separates Cr(III) with retention values up to 99%, resulting in high accuracy determination of Cr(VI). Total Cr was likewise determined by ETAAS after an efficient reduction of Cr(VI) to Cr(III) using ascorbic acid as reducing agent, deriving Cr(III) concentration from the difference between total Cr and Cr(VI). The parameters of the separation technique, solution pH (4.5–5.5), solution flow rate through the calcite column (0.14–0.42 mL min^?1), and calcite column internal diameter (1.5–3.0 cm), were evaluated. Best results were achieved with pH of 5.5, flow rate of 0.42 mL min^?1, and column internal diameter of 1.5 cm. Optimum determination conditions were found using magnesium nitrate [Mg(NO₃)₂] as chemical modifier, pyrolysis, and atomization temperatures of 1400 and 2200°C, respectively. In such conditions, the detection limits (n=10) were 1.5 and 0.8 µg L^?1 for Cr(III) and Cr(VI), respectively.     

Remediation of Metal Contaminated Soil by Organic Metabolites from Fungi I—Production of Organic Acids

Investigations were made on living strains of fungi in a bioremediation process of three metal (lead) contaminated soils. Three saprotrophic fungi (Aspergillus niger, Penicillium bilaiae, and a Penicillium sp.) were exposed to poor and rich nutrient conditions (no carbon availability or 0.11 M d-glucose, respectively) and metal stress (25 µM lead or contaminated soils) for 5 days. Exudation of low molecular weight organic acids was investigated as a response to the metal and nutrient conditions. Main organic acids identified were oxalic acid (A. niger) and citric acid (P. bilaiae). Exudation rates of oxalate decreased in response to lead exposure, while exudation rates of citrate were less affected. Total production under poor nutrient conditions was low, except for A. niger, for which no significant difference was found between the poor and rich control. Maximum exudation rates were 20 µmol oxalic acid g^?1 biomass h^?1 (A. niger) and 20 µmol citric acid g^?1 biomass h^?1 (P. bilaiae), in the presence of the contaminated soil, but only 5 µmol organic acids g^?1 biomass h^?1, in total, for the Penicillium sp. There was a significant mobilization of metals from the soils in the carbon rich treatments and maximum release of Pb was 12% from the soils after 5 days. This was not sufficient to bring down the remaining concentration to the target level 300 mg kg^?1 from initial levels of 3,800, 1,600, and 370 mg kg^?1in the three soils. Target levels for Ni, Zn, and Cu, were 120, 500, and 200 mg kg^?1, respectively, and were prior to the bioremediation already below these concentrations (except for Cu Soil 1). However, maximum release of Ni, Zn, and Cu was 28%, 35%, and 90%, respectively. The release of metals was related to the production of chelating acids, but also to the pH-decrease. This illustrates the potential to use fungi exudates in bioremediation of contaminated soil. Nonetheless, the extent of the generation of organic acids is depending on several processes and mechanisms that need to be further investigated.     

Technical and Economical Assessment and Extrapolation of a 200-dm3 Pilot Bioreactor for Reduction of Sulphate and Metals in Acid Mine Waters

Acidity, presence of metals and high-sulphate concentration are typical characteristics found in acid mine waters (AMWs). A 200-dm³ pilot, allowing bacterial conversion of sulphate into sulphide, was designed to treat AMWs. A fixed-bed column was filled with pozzolana, inoculated with a bacterial population containing the sulphate-reducing organism Desulfomicrobium norvegicum and fed with a H₂ and CO₂ gas mixture. The pilot worked in continuous-feeding conditions during 36 days. An actual AMW was sequentially treated by neutralisation of acidity, precipitation of metals using sulphide produced by the bioreactor and bioconversion of sulphate in the bioreactor fed with the sulphide-treated effluent. The residence time in the bioreactor was decreased down to 8.5 h. The sulphate reduction rate, correlated with the temperature between 5 and 17°C, varied between 35 and 95 mg dm^?3 h^?1. On the basis of the technical assessment previously made and after setting up some extrapolation hypotheses and calculations for a 10 m³ h^?1 unit, the treatment cost per cubic meter of AMW was evaluated.     

Selective Determination of Chromium(VI) in Industrial Wastewater Samples by Micro-Electromembrane Extraction Combined with Electrothermal Atomic Absorption Spectrometry

This study describes application of free liquid membrane (FLM) in micro-electromembrane extraction (μ-EME) of Cr(VI) from wastewater samples. Amount of Cr(VI) was quantified by electrothermal atomic absorption spectrometry. The transportation of Cr(VI) across the FLM was explored by electrokinetic migration and ion-exchange process. FLM and acceptor solution types, pH of donor and acceptor solutions, applied electrical potential, as well as FLM thickness were optimized. Presence of an anion exchange carrier (methyl trialkyl-ammonium chloride, Aliquat 336) in FLM facilitated Cr(VI) transportation. The best performance was observed for 1-octanol (containing 5% Aliquat 336) with thickness of 1 mm used as FLM, under applied electrical potential of 75 V, when 0.5 M NaClO₄ and 0.1 M HCl were used as the acceptor and donor phases, respectively; and the extraction time was set to 5 min. Linearity was obtained in the working range of 0.5–14.0 ng mL^?1 Cr(VI) (R²?>?0.98). The calculated limit of detection was below 0.06 ng mL^?1. Application of this method to wastewater samples showed that relative recoveries of the spiked Cr(VI) in the samples were in the range of 73.8–85.1%, based on the standard addition method.     

Toxicity of Lead to Freshwater Invertebrates (Water fleas; Daphnia magna and Cyclop sp) in Fish Ponds in a Tropical Floodplain

Acute toxicity of Pb to the water flea; (Daphnia sp) and Copepod, (Cyclop sp) both important component of zooplankton diet of fish was determined by static assay. A positive relationship between percentage mortality and exposure concentration was found in all tests. Mean 24-h LC50, 48-h LC50 and 96-h LC50 values were 2.51?±?0.0.04 mg l^?1, 1.88?±?0.06 mg l^?1 and 1.65?±?0.19 mg l^?1 for Daphnia spp and 3.11?±?0.03 mg l^?1, 2.97?±?0.05 mg l^?1 and 2.61?±?0.09 mg l^?1 for Cyclop spp, respectively. For all tested species did the LC50 values decrease with time; the decrease was more marked for Daphnia spp. Observed symptoms include spiral movement followed by change of body colour to white and rapid disintegration of the skin. The Daphnia spp. appear to be more sensitive to Pb poison than Cyclop spp. The results showed that concentrations of Lead (Pb) in excess of 0.19 mg l^?1 and 0.30 mg l^?1 can be potentially harmful to Daphnia magna and Cyclop spp respectively.     

Development of Analytical Procedure for the Determination of Exchangeable Cr(VI) in Soils by Anion-exchange Fast Protein Liquid Chromatography with Electrothermal Atomic Absorption Spectrometry Detection

Analytical procedure for the determination of exchangeable Cr(VI) was developed. In order to optimise the extraction procedure, the efficiency of extraction of exchangeable Cr(VI) in soil samples was investigated in KH₂PO₄–K₂HPO₄ buffer solutions (0.015 up to 0.2 mol l^?1), adjusted to the pH of the soil. Phosphate buffer was used to efficiently desorb Cr(VI) from soil particles. The extraction time (mechanical shaking) ranged from 1 up to 72 h. Cr(VI) in soil extracts was determined by anion-exchange fast protein liquid chromatography with electrothermal atomic absorption detection (FPLC-ETAAS). The study was performed on soil samples from the field treated with the tannery waste for seventeen years. Samples were analysed in the 16 year after the last waste application. It was experimentally proven that the optimal phosphate buffer concentration was 0.1 mol l^?1 and extraction time 16 h. An additional experiment was done to confirm that during the extraction, soluble Cr(III) was not oxidised to Cr(VI) by Mn(IV) oxides present in soil samples. For this purpose soil with the same characteristics, but not treated with tannery waste, was spiked with Cr(III) and the analytical procedure performed. No measurable Cr(VI) concentrations were detected. The repeatability of measurement was 2.5%, while the reproducibility of measurement was 6.9%. The accuracy of the analytical procedure was tested by spiking of soil samples with Cr(VI). The recoveries were better than 95%. The analytical procedure with limit of detection (LOD) 15 ng g^?1 of Cr(VI) was sensitive enough for the determination of exchangeable Cr(VI) in soils. In field soil samples analysed the concentrations of exchangeable Cr(VI) were found to be about 200 ng g^?1.     

Hg, Cu, Pb, Cd, and Zn Accumulation in Macrophytes Growing in Tropical Wetlands

The concentrations of Hg, Cu, Pb, Cd, and Zn accumulated by regional macrophytes were investigated in three tropical wetlands in Colombia. The studied wetlands presented different degrees of metal contamination. Cu and Zn presented the highest concentrations in sediment. Metal accumulation by plants differed among species, sites, and tissues. Metals accumulated in macrophytes were mostly accumulated in root tissues, suggesting an exclusion strategy for metal tolerance. An exception was Hg, which was accumulated mainly in leaves. The ranges of mean metal concentrations were 0.035?C0.953 mg g^?1 Hg, 6.5?C250.3 mg g^?1 Cu, 0.059?C0.245 mg g^?1 Pb, 0.004?C0.066 mg g^?1 Cd, and 31.8?363.1 mg g^?1 Zn in roots and 0.033?C0.888 mg g^?1 Hg, 2.2?C70.7 mg g^?1 Cu, 0.005?C0.086 mg g^?1 Pb, 0.001?C0.03 mg g^?1 Cd, and 12.6?C140.4 mg g^?1 Zn in leaves. The scarce correlations registered between metal concentration in sediment and plant tissues indicate that metal concentrations in plants depend on several factors rather than on sediment concentration only. However, when Cu and Zn sediment concentrations increased, these metal concentrations in tissues also increased in Eichhornia crassipes, Ludwigia helminthorriza, and Polygonum punctatum. These species could be proposed as Cu and Zn phytoremediators. Even though macrophytes are important metal accumulators in wetlands, sediment is the main metal compartment due to the fact that its total mass is greater than the corresponding plant biomass in a given area.     

Characterization of Reactive Red-120 Decolorizing Bacterial Strain Acinetobacter junii FA10 Capable of Simultaneous Removal of Azo Dyes and Hexavalent Chromium

Continual discharge of textile wastewaters loaded with a variety of synthetic dyes and metals is considered as a huge threat to surrounding ecosystems. In order to treat these undesirable pollutants, microbial bioremediation is considered as an efficient and economical technique. This study was conducted to evaluate the use of bacterial strains for simultaneous removal of azo dyes and hexavalent chromium [Cr(VI)]. Fifty-eight bacterial strains were isolated from Paharang drain wastewater and tested for their potential to decolorize reactive red-120 (RR-120) in the presence of 25 mg L^?1 of Cr(VI). Among the tested isolates, FA10 decolorized the RR-120 most efficiently and was identified as Acinetobacter junii strain FA10. Based on quadratic polynomial equation and response surfaces given by the response surface methodology (RSM), Cr concentration and pH were found to be the main factors governing the RR-120 decolorization by FA10. The strain FA10 also exhibited a substantial salt resistance since it showed a considerable decolorization of RR-120 even in the presence of 150 g L^?1 of NaCl. Moreover, the strain FA10 also showed the potential to simultaneously remove the Cr(VI) and the selected azo dyes in the same medium. More than 80 % of the initially added Cr(VI) was removed over 72 h of incubation along with the appreciable decolorization efficiency. The strain FA10 also exhibited good tolerance to considerable levels of different heavy metals. The findings of this study suggest that the strain FA10 might serve as an efficient bioresource to develop the biotechnological approaches for simultaneous removal of different azo dyes and heavy metals including Cr(VI).     

Phosphorus Entrainment Due to Resuspension in a Lowland River, Spree, NE Germany – A Laboratory Microcosm Study

Resuspension of benthic phosphorus (P) often constitutes a high percentage of the annual P flux in lowland rivers. To study P entrainment at controlled shear velocity (u*) sediment from lowland River Spree of slower flowing (0.1–0.3 m s^?1) stretch Kossenblatt (KOB) and of faster flowing (0.5–0.7 m s^?1) stretch Freienbrink (FRB) was incubated in a microcosm at incrementally enhanced u* (0.34–1.9 cm s^?1). Particle and P entrainment rates as well as the number of particle-associated bacteria of fine-grained mud-like KOB sediment were much higher (16.7 g m^?2 h^?1, 104.9 mg P m^?2 h^?1, 15.47 10⁶ cells ml^?1) than those (4.3 g m^?2 h^?1, 2.1 mg P m^?2 h^?1, 3.06 10⁶ cells ml^?1) of coarser sandy FRB sediment. The microcosm used so far in marine research is suited to compare riverine resuspension suggesting the lower u* the more particles are deposited and the more P can be retained (KOB ? FRB). Conversely, correspondingly more and easier particulate P and bacteria can again be remobilised (KOB ? FRB) if u* increases. The general relationship found for u* and the entrainment of particulate P and bacteria as well as their decelerated and selective deposition where bacteria may stay longer in the water implies a temporarily enhanced P bioavailability, turnover and subsequent P transformations.     

Chromium-resistant microorganisms isolated from evaporation ponds of a metal processing plant

Chromium occurs naturally at trace levels in most soils and water, but disposal of industrial waste and sewage sludge containing chromium compounds has created a number of contaminated sites, which could pose a major environmental threat. This study was conducted to enumerate and isolate chromium-resistant microorganisms from sediments of evaporation ponds of a metal processing plant and determine their tolerance to other metals, metalloids and antibiotics. Enumeration of the microbiota of Cr-contaminated sediments and a clean background sample was conducted by means of the dilution-plate count method using media spiked with Cr(VI) at concentrations ranging from 10 to 1000 mg L^?1. Twenty Cr(VI) tolerant bacterial isolates were selected and their resistance to other metals and metalloids, and to antibiotics was assessed using a plate diffusion technique. The number of colony-forming units (cfu) of the contaminated sediments declined with increasing concentrations from 10 to 100 mg L^?1 Cr(VI), and more severely from 100 to 1000 mg L^?1 Cr(VI). The background sample behaved similarly to 100 mg L^?1 Cr(VI), but the cfu declined more rapidly thereafter, and no cfu were observed at 1000 mg L^?1 Cr(VI). Metals and metalloids that inhibited growth (from the most to least inhibitory) were: Hg > Cd > Ag > Mo = As(III) at 50 μg mL^?1. All 20 isolates were resistant to Co, Cu, Fe, Ni, Se(IV), Se(VI), Zn, Sn, As(V), Te and Sb at 50 μg mL^?1 and Pb at 100 μg mL^?1. Eighty-five percent of the isolates had multiple antibiotic resistance. In general, the more metal-tolerant bacteria were among the more resistant to antibiotics. It appears that the Cr-contaminated sediments may have enriched for bacterial strains with increased Cr(VI) tolerance.     

Reduction of Hexavalent Chromium in Soil and Ground Water Using Zero-Valent Iron Under Batch and Semi-Batch Conditions

Chemical remediation of soil and groundwater containing hexavalent chromium (Cr(VI)) was carried out under batch and semi-batch conditions using different iron species: (Fe(II) (sulphate solution); Fe⁰ _G (granulated elemental iron); ZVIne (non-stabilized zerovalent iron) and ZVIcol (colloidal zerovalent iron). ZVIcol was synthesized using different experimental conditions with carboxymethyl cellulose (CMC) and ultra-sound. Chemical analysis revealed that the contaminated soil (frank clay sandy texture) presented an average Cr(VI) concentration of 456?±?35 mg kg^?1. Remediation studies carried out under batch conditions indicated that 1.00 g of ZVIcol leads to a chemical reduction of ～280 mg of Cr(VI). Considering the fractions of Cr(VI) present in soil (labile, exchangeable and insoluble), it was noted that after treatment with ZVIcol (semi-batch conditions and pH 5) only 2.5% of these species were not reduced. A comparative study using iron species was carried out in order to evaluate the reduction potentialities exhibited by ZVIcol. Results obtained under batch and semi-batch conditions indicate that application of ZVIcol for the “in situ” remediation of soil and groundwater containing Cr(VI) constitutes a promising technology.     

Emerging resistant microbiota from an acidic soil exposed to toxicity of Cr, Cd and Pb is mainly influenced by the bioavailability of these metals

Purpose

The effect of pollutants in soil microorganisms is an important issue in order to understand their toxic effects in the environment, as well as for developing adequate bioremediation strategies. In this sense, the main objective of this study was to assess the involvement of the indigenous microbiota of an acidic forest Mediterranean soil by artificial pollution with heavy metals, and to detect and isolate resistant microorganisms that could be useful for bioremediation.

Materials and methods

Samples from a previously unpolluted acidic forest soil were amended with Cr(VI), Cd(II) or Pb(II) at total amounts ranging from 0.1 to 5,000 mg?kg^?1. These soil microcosms were incubated under controlled laboratory conditions for 28 days. Soluble fractions of metals were determined from aqueous extracts. Both activity and composition of the microbial community were assessed, respectively, by respirometric assays and molecular analysis (polymerase chain reaction denaturing gradient gel electrophoresis). The isolation of metal-resistant microorganisms was attempted by culture plating from microcosms incubated with high concentrations of metals. Isolated strains were tested in cultures with minimal medium to check for their metal resistance and their capacity to reduce the presence of toxic Cr(VI).

Results and discussion

A decrease in the soil respirometric activity and changes in the microbial community composition were detected from 10/100 mg?kg^?1 Cr and 1,000 mg?kg^?1 Cd and Pb. Presumably resistant bacterial and fungal populations developed in most of these polluted microcosms; however, the microbiota was severely impaired at the highest additions of Cr. Even though Cr was the most damaging metal in soil microcosms, if the soluble fractions of metals are considered instead of their total added amounts, the comparison among their toxic effects suggests a similar potential toxicity of Cr and Pb. Isolated multiresistant microorganisms were related mainly to Actinobacteria, Firmicutes and Ascomycota. Some of them showed the capacity to reduce Cr(VI) concentrations between 54 % and 70 % of the initial value. These strains were affiliated to several species of Streptomyces and Bacillus.

Conclusions

The combination of respirometric assays with molecular methods has been useful to assess the effect of metals on the soil microbial community, which can greatly be explained by their differential bioavailability. Cultivation-dependent and -independent approaches have proved the presence and development of multiresistant microorganisms in a previously unpolluted soil. Due to their properties, some of the isolated strains are potentially useful for soil bioremediation.     

Comparison of Several Amendments for In-Site Remediating Chromium-Contaminated Farmland Soil

Hexavalent chromium (Cr(VI)), which has been classified as a Group A human carcinogens list by the United States Environmental Protection Agency, possesses stronger biological toxicity, and its discharge into farmland has become a pressing environmental problems. To screen the cost-efficient Cr(VI)-contaminated soil in situ amended materials, the effects of ordinary zero-valent iron (ZVI), nanoscale zero-valent iron (nZVI), biochar (B), biochar/zero-valent iron (BZVI), and biochar/nanoscale zero-valent iron (BnZVI) on the immobilization of Cr(VI) in spiked soil (Cr(VI) = 325 mg kg^?1, Cr_total = 640 mg kg^?1) were compared in this paper. After 15 days remediation by those materials, toxicity characteristic leaching procedure and physiological-based extraction test showed that the Cr(VI) leachability and bioaccessibility were reduced by 14–92% and 4.3–92% respectively, and the order of immobilization was found to be nZVI > BnZVI > BZVI > ZVI > B. Moreover, sequential extraction procedure indicated that all materials can increase the proportion of the residual Cr, and nZVI had the most significant effect. Plant seedling growth test proved that the nanoscale zero-valent iron was able to reduce the toxicity of chromium in plants greatly in a short time, while BnZVI treatment is more favorable to the growth of plants. To sum up, the nano zero-valent iron and biochar combined treatment not only removed Cr(VI) and immobilized total chromium efficiently but also enabled plant growth in relative high chromium-containing soil.     

Treatment of Cr(VI)-spiked soils using sulfur-based amendments

The objective of this research was to assess the hexavalent chromium (Cr(VI)) reducing efficiency of sulfur-based inorganic agents including calcium polysulfide (CPS), iron sulfide (FeS), pyrite (FeS₂) and sodium sulfide (Na₂S) in three soils. An alkaline soil (soil 1), a neutral soil (soil 2) and a slightly acid soil (soil 3) constituted the investigated soils. The soils were spiked with two levels of Cr(VI) (100 and 500 mg Cr(VI) kg^?1 soil) and incubated at field capacity (FC) for one month. Then, CPS, FeS, FeS₂ and Na₂S were added at 0, 5 and 10 g kg^?1 and the concentrations of exchangeable Cr(VI) were measured after 0.5, 4, 48 and 168 h in a batch experiment. The pH and organic carbon content of the soils played predominant role in Cr(VI) self-reduction by the soil itself. Complete self-reduction of Cr(VI) from soils 1, 2 and 3 was achieved at maximum Cr(VI) levels of 1, 50 and 500 mg kg^?1, respectively. Therefore, the concentration of Cr(VI) should not exceed the given levels in order to ensure that Cr(VI) is not released into the environment from contaminated sites. Moreover, decreasing pH in the alkaline soil caused significant increase of Cr(VI) reducing efficiency. Na₂S, CPS and FeS, in contrast to FeS₂, were efficient Cr(VI) reducing agents in all three soils. For all added amendments the following order of Cr(VI) reducing capacity was observed: Na₂S > CPS > FeS > FeS₂ in soil 1, Na₂S ? CPS ~ FeS > FeS₂ in soil 2 and Na₂S ? FeS > CPS ~ FeS₂ in soil 3.     

Synthesis and Characterisation of Novel-Activated Carbon from Waste Biomass Pine Cone and Its Application in the Removal of Congo Red Dye from Aqueous Solution by Adsorption

The present study deals with the synthesis and subsequent application of Fe₃O₄@n-SiO₂ nanoparticles for the removal of Cr(VI) from aqueous solutions. Rice husk, an agrowaste material, was used as a precursor for the synthesis of nanoparticles of silica. Synthesized nanoparticles were characterized by XRD and SEM to investigate their specific characteristics. Fe₃O₄@n-SiO₂ nanoparticles were used as adsorbent for the removal of Cr(VI) from their aqueous solutions. The effects of various important parameters, such as initial Cr(VI) concentration, adsorbent dose, temperature, and pH, on the removal of Cr(VI) were analyzed and studied. A pH of 2.0 was found to be optimum for the higher removal of Cr(VI) ions. It was observed that removal (%) decreased by increasing initial Cr(VI) concentration from 1.36?×?10^-2 to 2.4?×?10^-2 M. The process of removal was found to be endothermic, and the removal increased with the rise in temperature from 25 to 45 °C. The kinetic data was better fitted in pseudo-second-order model in comparison to pseudo-first-order model. Langmuir and Freundlich adsorption capacities were determined and found to be 3.78 and 1.89 mg/g, respectively, at optimum conditions. The values of ΔG ⁰ were found to be negative at all temperatures, which confirm the feasibility of the process, while a positive value of ΔH ⁰ indicates the endothermic nature of the adsorption process. The present study revealed that Fe₃O₄@n-SiO₂ nanoparticles can be used as an alternate for the costly adsorbents, and the outcome of this study may be helpful in designing treatment plants for treatment of Cr(VI)-rich effluents.