Effect of pH on hexavalent and total chromium removal from aqueous solutions by avocado shell using batch and continuous systems

Taguchi method for optimization of Cr(VI) removal, isotherm, kinetic and thermodynamic studies

In this study, Taguchi optimization method was applied to determine the optimum operating conditions for batch adsorption of Cr(VI) from aqueous solution. Initial pH of solution, adsorbent dose, initial hexavalent chromium concentration, contact time and adsorbent type were selected as the variables, and the removal efficiency of Cr(VI) was chosen for the designated response. L18(35) orthogonal array, signal-to-noise (S/N) ratio and analysis of variance statistical procedures were applied to identify the effect of each operating parameter on the removal of Cr(VI) from aqueous solution. The signal-to-noise (S/N) ratio results showed that the optimal combination for Cr(VI) removal was at pH 1.0, adsorbent dose of 3.6 g.L-1, Cr(VI) concentration of 30 mg.L-1, contact time of 95 min and olive leaves as adsorbent type. A removal of 95.09% was obtained at these optimum conditions. The analysis of variance of the data revealed that initial pH of solution was the most dominant parameter affecting Cr(VI) removal efficiency, followed by adsorbent type, adsorbent dose, contact time and initial metal concentration. Under optimal conditions, adsorption kinetic of Cr(VI) was studied and modeled using the pseudo first-order, pseudo-second-order and intraparticle diffusion models. It was found that the pseudo-second-order model fitted the adsorption data most with the highest determination coefficient (R2 = 0.996). Freundlich isotherm model, with regression coefficient R2 of 0.953, fit well with the equilibrium isotherm data. The Langmuir maximum adsorption capacity was found to be 62.5 mg.g-1. The experimental values of ΔH°, ΔG° and ΔS° revealed that the adsorption process was spontaneous and endothermic.

Chromium Recovery from Chromium-Loaded Cupressus lusitanica Bark in Two-Stage Desorption Processes

Hexavalent chromium (Cr(VI)) contamination poses serious health and environmental risks. Chromium biosorption has been employed as an effective means of eradicating Cr(VI) contamination. However, research on chromium desorption from chromium-loaded biosorbents is scarce despite its importance in facilitating industrial-scale chromium biosorption. In this study, single- and two-stage chromium desorption from chromium-loaded Cupressus lusitanica bark (CLB) was conducted. Thirty eluent solutions were evaluated first; the highest single-stage chromium desorption efficiencies were achieved when eluent solutions of 0.5 M NaOH, 0.5 M H2SO4, and 0.5 M H2C2O4 were used. Subsequently, two-stage kinetic studies of chromium desorption were performed. The results revealed that using 0.5 M NaOH solution in the first stage and 0.5 M H2C2O4 in the second stage enabled the recovery of almost all the chromium initially bound to CLB (desorption efficiency = 95.9-96.1%) within long (168 h) and short (3 h) desorption periods at each stage. This study clearly demonstrated that the oxidation state of the recovered chromium depends on the chemical nature and concentration of the eluent solution. The results suggest the possible regeneration of chromium-loaded CLB for its subsequent use in other biosorption/desorption cycles.

Agricultural Solid Wastes Based Adsorbent Materials in the Remediation of Heavy Metal Ions from Water and Wastewater by Adsorption: A Review

Adsorption has become the most popular and effective separation technique that is used across the water and wastewater treatment industries. However, the present research direction is focused on the development of various solid waste-based adsorbents as an alternative to costly commercial activated carbon adsorbents, which make the adsorptive separation process more effective, and on popularising the sustainable options for the remediation of pollutants. Therefore, there are a large number of reported results available on the application of raw or treated agricultural biomass-based alternatives as effective adsorbents for aqueous-phase heavy metal ion removal in batch adsorption studies. The goal of this review article was to provide a comprehensive compilation of scattered literature information and an up-to-date overview of the development of the current state of knowledge, based on various batch adsorption research papers that utilised a wide range of raw, modified, and treated agricultural solid waste biomass-based adsorbents for the adsorptive removal of aqueous-phase heavy metal ions. Metal ion pollution and its source, toxicity effects, and treatment technologies, mainly via adsorption, have been reviewed here in detail. Emphasis has been placed on the removal of heavy metal ions using a wide range of agricultural by-product-based adsorbents under various physicochemical process conditions. Information available in the literature on various important influential physicochemical process parameters, such as the metal concentration, agricultural solid waste adsorbent dose, solution pH, and solution temperature, and importantly, the adsorbent characteristics of metal ion removal, have been reviewed and critically analysed here. Finally, from the literature reviewed, future perspectives and conclusions were presented, and a few future research directions have been proposed.

Biosorptive ascendency of plant based biosorbents in removing hexavalent chromium from aqueous solutions - Insights into isotherm and kinetic studies

Chromium is a toxic heavy metal prevalent in higher levels in aqueous matrices owing to industrial applications. Whilst being a key player in industries, the environmental issues caused by Cr(VI) are highly deleterious. Adsorptive remediation is found to be an effective method adopted by researchers in the past decades for Cr(VI) removal from water streams in which variety of naturally available biosorbents have been explored for handling Cr(VI). This review article briefly sketches up the biosorptive potential of plant-based biosorbents used in raw and chemically modified form for the optimum exclusion of Cr(VI) from aqueous sources. Mechanisms and kinetic behavior of the removal process are also discussed. pH of the solution and initial Cr(VI) concentration were found to be the key parameters in Cr removal. The mechanism of Cr removal from aqueous systems was elucidated to be either adsorption or adsorption-coupled-reduction. After precise discussion on various plant-based biosorbents with their maximum adsorption capacities, desorption and regeneration potential, it is perceived that plant-based biosorbents are superior options for Cr(VI) elimination from aqueous streams.

A review of avocado waste-derived adsorbents: Characterizations, adsorption characteristics, and surface mechanism

Avocado is one of the most important fruits with a high nutritional content; this fruit is consumed and cultivated worldwide. It is originally grown in Central America and the West Indies islands. But it is now cultivated in the tropical and subtropical regions of the world. Avocado waste is an abundantly available raw material that can be converted into adsorbents to remove different pollutants from aqueous solutions. This review article explores the utilization of avocado waste as raw material to develop an efficient adsorbent and its use against various toxicants. Many research papers have been published on the use of avocado waste-derived adsorbents in the recent past. The factors that affect the adsorption processes are examined in light of published references. Some critical adsorption parameters, such as equilibrium (isotherms), kinetics, and thermodynamics, have been reported in the published literature; these parameters and their data are critically discussed. The characterization, mechanism, and surface chemistry of avocado waste-derived adsorbents are also discussed. To date, no review article on avocado waste-derived adsorbents is available, where researchers can get an overview of the preparation, characterization, and adsorption attributes of avocado waste adsorbents against various pollutants. Recent literature demonstrates the effective utilization of avocado waste as a cleaner and sustainable raw material for the production of adsorbents.

A systematic review on adsorptive removal of hexavalent chromium from aqueous solutions: Recent advances

The contamination of natural resources by hexavalent chromium (Cr(VI)) originating from natural and anthropogenic activities is a serious environmental concern. Although many articles on chromium remediation have been published, a comprehensive understanding of the mechanisms involved in remediation with different sorbents is not yet available. In this systematic review, the performance and applicability of several adsorptive materials for Cr(VI) removal from aqueous media are discussed, along with a detailed analysis of the mechanisms involved. Statistical analysis is applied to compare the efficacies of different adsorbents, while a similar approach is used to determine the effects of sorbent properties and experimental conditions on the adsorption capacity. A detailed analysis of the factors involved in fixed-bed column studies is also presented. A suitable desorption approach to the regeneration of the spent adsorbent and its adsorption performance in reuse is also examined. Among the different sorbents, nanoparticles and mineral-doped biochar were found to be the most effective sorbents, while the adsorption was higher at low pH (~4.0) than that at intermediate pH (6-8). Contrary to our expectation, adsorption was high for sorbents with low specific surface areas, suggesting that the adsorption of Cr(VI) is largely influenced by the chemical properties of the sorbents. The optimum adsorption in fixed-bed column systems is obtained at a lower Cr(VI) ion concentration, a lower influent flow rate, and a higher bed height. Since most of the studies reviewed herein were merely experimental and utilized ideal conditions with the presence of a single contaminant, i.e. Cr(VI) in water, further studies on adsorption dynamics with the presence of other interfering ions are suggested. This review is promising for the further development of Cr(VI) removal strategies and closes the research gaps pertaining to their challenges.

Study on leaching kinetics of hexavalent chromium from aged calcium-free chromium slag

To provide a scientific basis for the resource utilization of chromium slag, this article studies the release law of hexavalent chromium in the aged calcium-free chromium slag. XRD (X-ray diffractometer) and MLA (Mineral Liberation Analyzer) were used to analyze the composition of the chromium slag; using sulfuric acid-nitric acid as the leaching solution, the release law of hexavalent chromium in chromium slag and the leaching kinetics were studied. The results show that main components of the chromium slag are magnesioferrite, chromite, hematite, hydrargillite, and spinel; chromium is mainly present in chromite and magnesioferrite; the leaching rate of hexavalent chromium increases with the increase of temperature or the decrease of pH. The analysis of leaching kinetics shows the leaching rate is controlled by the internal diffusion reaction, and the apparent activation energy is 11.93 kJ·mol–1. The chromium slag is aged in high temperature seasons, which is conducive to the precipitation of hexavalent chromium in the chromium slag, can increase the yield of chromate in the roasting kiln, and is conducive to resource utilization; chromium slag should be stored in order to prevent acid rain erosion which leads to environmental pollution risk (e.g. drinking water).

Continuous biosorption of acid red 27 azo dye by Eichhornia crassipes leaves in a packed-bed column

In this work, the biosorption behavior of acid red 27 (AR27) dye using Eichhornia crassipes leaves (LECs) in a packed-bed column was investigated by varying relevant operational parameters and assessment of mathematical models. Results showed that the zero-charge point of LECs was 2.37 and that optima pH and volumetric flux of the influent solution for AR27 biosorption were 2.0 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$56.5\ \hbox {L}/\hbox {m}^{2}\cdot \hbox {h}$$\end{document}56.5L/m2·h, respectively. The maximum specific and volumetric biosorption capacities were observed at influent AR27 concentrations and with LEC bed heights ranging between 50 and 400 mg/L and 2 and 8 cm, respectively. It was also found that if LEC bed height was increased and volumetric flux and AR27 concentration of the influent solution decreased, service and saturation time increased. Modeling results revealed that the Thomas, bed depth service time, Yoon–Nelson, dose-response, and logistic models accurately described the dynamic performance of the packed-bed column in terms of pH, AR27 concentration, and volumetric flux of influent AR27 solution, as well as that of LEC bed height. The findings revealed that LECs exhibited remarkable potential for the biosorption of AR27 from aqueous solutions in a packed-bed column and could potentially be useful for the treatment of AR27-laden wastewater.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

A green rust-coated expanded perlite particle electrode-based adsorption coupling with the three-dimensional electrokinetics that enhances hexavalent chromium removal

A green rust-coated expanded perlite (GR-coated Exp-p) microelectrode was synthesized and incorporated into a column-mode three-dimensional electrokinetic (3D-EK) platform to effectively pursue a continuous Cr(VI) removal from the aqueous solution. Brucite-like layers of GR were decorated onto the Exp-p material. The molar ratio of Fe(II) to Fe(III) played a most vital role among the three synthesis factors in influencing the performance of the particle electrode. For the equilibrium adsorption experiments, the target maximum adsorption capacity of 122 mg/g was predicted by a target optimizer and desirability function at the conditions following the pH of 4.7, the initial concentration of 172.4 mg/L, the dosage of 0.28 g/L, and the temperature of 28.96 °C, respectively. SO₄^2-, Cl^-, and NO₃^- fiercely competed with Cr(VI) anions in the acidic conditions for the locally positive sites. A low concentration and a slow flow were favored in the column-mode 3D-EK platform. The pseudo-first-order and Langmuir models were suitable for describing the kinetics and isotherms of the adsorption process, respectively. Cr(VI) anions were electrostatically attracted to the silanol groups and GR surface of the adsorbent, subsequently reduced in both heterogeneity and homogeneity, and finally immobilized by coordinating with silanediol groups and silanetriol groups.

Cadmium removal by MgCl2 modified biochar derived from crayfish shell waste: Batch adsorption, response surface analysis and fixed bed filtration

Cadmium (Cd) pollution is regarded as a disturbing environmental problem due to its serious risks to the water body and human health. The removal of cadmium from wastewater is thus crucial to avoid its harmful effects on the ecosystem. This study comprehensively investigated Cd(II) adsorption onto MgCl₂ modified biochar (MgC600) and results showed that the adsorption capacity of MgC600 was more than twice of that of pristine biochar due to its enhanced ion exchange ability. Response surface analysis revealed that reaction time played a crucial role in the Cd(II) adsorption, followed by initial concentration and solution pH. Moreover, the optimal adsorption conditions and capacity were precisely given by the quadratic regression model and thus proved that the model can be applied to predict the operation conditions of Cd(II) adsorption. Finally, a new model defined as BJP model [Formula: see text] was proposed and proved to be more suitable for the fixed bed filtration process. Overall, our findings provide a promising material in treatment of Cd(II)-rich wastewater and give a clear picture of its application. More importantly, the newly developed BJP model can accurately describe the fixed bed filtration process and further promote its application in wastewater treatment.

Enhanced arsenate removal by Fe-impregnated canola straw: assessment of XANES solid-phase speciation, impacts of solution properties, sorption mechanisms, and evolutionary polynomial regression (EPR) models

The impact of arsenic (As) contamination of water is an ongoing concern worldwide with As released from anthropogenic activities including mining and agriculture. Biosorption is a promising As treatment methodology used currently for arsenate (As(V)) sorption from water. The biosorbent was developed by a simple and inexpensive treatment of coating of canola straw particles with iron hydroxides. The modification procedure was optimized with consideration of the concentration of iron solution, pH of modification process, and sonication time. A higher concentration of iron and lower pH led to an improved sorption capacity of the iron-loaded canola straw (ICS), while impacts of sonication time were not conclusive. Pareto analyses indicated that the magnitude of the effect of the pH was higher than that of the iron concentration. Overall, the maximum As(V) sorption capacity of the ICS was 5.5 mg/g for an 0.25 M FeCl₃ solution concentration at pH 3. Analysis of kinetic data showed that the sorption processes of As(V) followed pseudo-second order and Elovich mechanisms, while sorption isotherm data were best represented by Freundlich and Temkin isotherm models. Studying the effect of ionic strength using NaCl suggested that the inner-sphere complex was the probable sorption mechanism. The thermodynamic parameters including ΔS°, ΔH°, and ΔG° showed that the As(V) sorption was thermodynamically favorable and spontaneous. Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that no reaction to As(III) occurred during the sorption of As(V) using the optimum ICS biosorbent. The evolutionary polynomial regression (EPR) approach was able to closely match predicted vs. experimental sorption capacities (R² = 0.95). Overall, the improved understanding of the biosorbent's capability for removal of As(V) will be beneficial for assessment of its use for treatment of various water and wastewater matrices. In addition, knowledge gained from this research can assist in the understanding of sorption capacities of a variety of other biosorbents.

Green rust functionalized geopolymer of composite cementitious materials and its application on treating chromate in a holistic system

Green rust functionalized geopolymer of composite cementitious materials (GR-CCM) was synthesized to improve the adsorption and subsequent stabilization/solidification of chromate in a holistic operating system. The initial pH in solution exhibited the most significant effect on the chromate removal by GR-CCM among three adsorption factors. The maximum monolayer adsorption capacity and theoretical saturation capacity of GR-CCM for Cr(VI) in the acidic condition were 55.01 mg/g and 41.70 mg/g, respectively. Amorphousness brought by loading GR weakened the crystallinity of composite cementitious materials (CCM), which enhanced the adsorption capacity of CCM and boosted the solidification process. The mixed-valent iron species in the GR-CCM not only directly engaged in the adsorption and reduction of chromate also positively strengthened the solidification of Cr species during the whole treatment. This study facilitates the application of GRs on the geopolymer materials and demonstrates the combination of adsorption and immobilization for the treatment of other potential heavy metal contamination.

Avocado-Derived Biomass as a Source of Bioenergy and Bioproducts

The avocado (Persea americana Mill.) is a tree native to Mexico and Guatemala. Avocado consumption, fresh or in the form of processed products, is growing everywhere and it has caused a large number of countries to invest heavily in avocado production. The industrialization of avocado gives as a result a huge amount of waste, not only the peel and stone but also that waste generated by the pruning practices and oil extraction. These biomasses could be converted into raw materials to obtain different types of co-products, but this implies changes in the use of these resources, the design of efficient production systems, and integration to take full advantage of them, e.g., by developing biorefinery models. Therefore, this review firstly gives a snapshot of those residues generated in the avocado industry and provides their chemical composition. Secondly, this review presents updated information about the valorization ways of avocado-derived biomass to obtain bioenergy, biofuels, and other marketable products (starch, protein, phenolic compounds, and biosorbents, among others) using a single process or integrated processes within a biorefinery context. Green technologies to obtain these products are also covered, e.g., based on the application of microwaves, ultrasound, supercritical fluids, etc. As a conclusion, there is a variety of ways to valorize avocado waste in single processes, but it would be promising to develop biorefinery schemes. This would enable the avocado sector to move towards the zero-waste principle.

Effect of Ionic Strength and Coexisting Ions on the Biosorption of Divalent Nickel by the Acorn Shell of the Oak Quercus crassipes Humb. & Bonpl

This study investigated the effect of ionic strength and background electrolytes on the biosorption of Ni2+ from aqueous solutions by the acorn shell of Quercus crassipes Humb. & Bonpl. (QCS). A NaCl ionic strength of 0.2 mM was established to have no effect on the Ni2+ biosorption and the biosorption capacity of the heavy metal decreased as the ionic strength increased from 2 to 2000 mM. The background electrolytes (KCl, NaNO3, Na2SO4, CaCl2, MgSO4, and MgCl2) had no adverse effects on the biosorption of Ni2+ at a concentration of 0.2 mM. However, at background electrolyte concentrations of 2 and 20 mM, divalent cations (Ca2+ and Mg2+) had greater negative effects on the biosorption of Ni2+ compared to the monovalent cations (Na+ and K+). Additionally, the SO42− and Cl− anions affected the biosorption of Ni2+. The fractional power, Elovich, and pseudo-second order models represented the kinetic processes of the biosorption of Ni2+ adequately. The results show that QCS can be a promising and low-cost biosorbent for removing Ni2+ ions from aqueous solutions containing various types of impurities with different concentrations.

Single and Binary Equilibrium Studies for Ni2+ and Zn2+ Biosorption onto Lemna gibba from Aqueous Solutions

The biosorption ability of Lemna gibba for removing Ni2+ and Zn2+ ions in aqueous batch systems, both individually and simultaneously, was examined. The influences of solution pH and initial single and binary metal concentrations on equilibrium Ni2+ and Zn2+ biosorption was explored. The optimal solution pH for Ni2+ and Zn2+ biosorption was 6.0, for both the single and binary metal systems. Ni2+ and Zn2+ biosorption capacities increased with increasing initial metal concentrations. The presence of Zn2+ ions more adversely affected the biosorption of Ni2+ ions in the binary metal systems than vice versa. The single and binary biosorption isotherms of Ni2+ and Zn2+ revealed that L. gibba’s affinity for Zn2+ ions was higher than that for Ni2+ ions. The Redlich–Peterson and Freundlich isotherm models fit well to the experimental equilibrium data of Ni2+ ions, whereas Redlich–Peterson and Langmuir models better described the equilibrium data of Zn2+ ions in single metal systems. The modified Sips isotherm model best fit the competitive biosorption data of Ni2+-Zn2+ on L. gibba. FTIR analyses suggest the involvement of hemicellulose and cellulose in the biosorption of Ni2+ and Zn2+. The presence of Ni2+ and Zn2+ on the L.gibba surface was validated by SEM-EDX.

Characteristics of natural biopolymers and their derivative as sorbents for chromium adsorption: a review

Abstract

Chromium is widely used in industry, and improper disposal of wastewater and industrial residues containing excessive chromium can contaminate water and soil, endangering both environmental and human health. Natural biopolymers and their derivatives have been investigated for removal of chromium (Cr) from wastewater. Cellulose, lignin, tannin, chitin, chitosan, and polypeptides are abundant in nature, and have high potential as adsorbents due to their easy access, low cost, and the recyclability of the captured heavy metals. In order to improve their mechanical strength, recyclability, specific surface area, binding site number, and adsorption rate as adsorbents, native materials have also been modified. This review discusses the source of chromium contamination and the main species of interest, as well as their toxicity. The structures of the aforementioned biopolymers were analyzed, and the adsorption mechanism of chromium and the main influencing factors on this process are discussed. The modification methods of various adsorbents and their adsorption effects on chromium are also detailed, and the developmental direction of research on the use of biopolymer adsorption remediation to control chromium contamination is discussed.

Graphical abstract

Reusability of brilliant green dye contaminated wastewater using corncob biochar and Brevibacillus parabrevis: hybrid treatment and kinetic studies

This work highlights the potential of corncob biochar (CCBC) and Brevibacillus parabrevis for the decolorization of brilliant green (BG) dye from synthetically prepared contaminated wastewater. The CCBC was characterized by proximate, Fourier-transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and Brunauer-Emmett-Teller analysis, respectively. Different parameters affecting the adsorption process were evaluated. The experimental results were analyzed by the Langmuir and Freundlich isotherm models. Kinetic results were examined by different models; pseudo-second-order model has shown the best fit to the experimental data. Anew positive values of ΔH^o (172.58 kJ/mol) and ΔS^o (569.97 J/K/mol) in the temperature range of 303-318 K revealed that the adsorption process was spontaneous and endothermic. The present investigation showed that the bacteria immobilized with CCBC showed better BG dye degradation. The kinetic parameters, μ_max, Ks, and μ _max, were found to be 0.5 per day, 39.4 mg/day, and 0.012 L/mg/day using Monod model, respectively. The adsorbent with bacteria showed good potential for the removal of cationic BG dye and can be considered for the remediation of industrial effluent.

Impact of an in-situ Cr(VI)-contaminated site remediation on the groundwater

This study presents the latest results of the groundwater monitoring of a research project, which tested an innovative pump and treat method in combination with an in-situ remediation. This technique was assessed on an abandoned site in Austria, where two hot spots of hexavalent chromium (Cr(VI)) were located. For the in-situ remediation, a strong reducing agent (sodium dithionite) was injected into the underground to reduce Cr(VI) to Cr(III) by using different injection strategies. Throughout this treatment, part of the Cr(VI) is mobilized and not instantly reduced. To prevent a further spreading of the mobilized Cr(VI), the pump and treat method, which uses zero-valent iron to clean the groundwater, was installed downgradient of the hot spots. Based on the groundwater sample analyses, it was possible to distinguish different remediation phases, characterized by excess chromate and excess sulfite. During the excess sulfite conditions, Cr(VI) was successfully removed from the system, but after terminating the sodium dithionite injection, the Cr(VI) rebounded.

Isolation and Identification of Chromium Reducing Bacillus Cereus Species from Chromium-Contaminated Soil for the Biological Detoxification of Chromium

Chromium contamination has been an increasing threat to the environment and to human health. Cr(VI) and Cr(III) are the most common states of chromium. However, compared with Cr(III), Cr(VI) is more toxic and more easily absorbed, therefore, it is more harmful to human beings. Thus, the conversion of toxic Cr(VI) into Cr(III) is an accepted strategy for chromium detoxification. Here, we isolated two Bacillus cereus strains with a high chromium tolerance and reduction ability, named B. cereus D and 332, respectively. Both strains demonstrated a strong pH and temperature adaptability and survival under 8 mM Cr(VI). B. cereus D achieved 87.8% Cr(VI) removal in 24 h with an initial 2 mM Cr(VI). Cu(II) was found to increase the removal rate of Cr(VI) significantly. With the addition of 0.4 mM Cu(II), 99.9% of Cr(VI) in the culture was removed by B. cereus 332 in 24 h. This is the highest removal efficiency in the literature that we have seen to date. The immobilization experiments found that sodium alginate with diatomite was the better method for immobilization and B. cereus 332 was more efficient in immobilized cells. Our research provided valuable information and new, highly effective strains for the bioremediation of chromium pollution.

Accumulation and bioavailability of heavy metals in an acid soil and their uptake by paddy rice under continuous application of chicken and swine manure

Heavy metal contamination of agricultural soils is a global concern, as it can cause the accumulation of heavy metals in food. In this study, a field experiment was carried out to investigate the effect of the continuous application of chicken or swine manure on the Pb, Cd, Cr and As bioavailability, fractionation, and accumulation in soil and uptake by rice plants. Results showed that chicken or swine manure significantly reduced the Cd and Pb contents in rice grain by 7.8-79.3% and 7.2-59.4%, respectively, with increasing application rates and number of years; the exchangeable Cd and Pb fractions, and the diethylenetriaminepentaacetic acid (DTPA)-extractable Cd and Pb in the soil were also decreased. Furthermore, the application of chicken or swine manure substantially increased the DTPA-extractable As and exchangeable As fractions in the soil but had limited effect on As accumulation in rice grain. No significant differences in the bioavailability in soil nor accumulation in the rice grain were found for Cr between the treatments. Therefore, livestock manure can be used as soil amendments to decrease Cd and Pb accumulation in rice grains, nevertheless, the potential risk of metal accumulation in soils caused by livestock manure application should be considered.

Hexavalent chromium removal and total chromium biosorption from aqueous solution by Quercus crassipes acorn shell in a continuous up-flow fixed-bed column: Influencing parameters, kinetics, and mechanism

Continuous fixed-bed column studies were carried out, utilizing acorn shell from Quercus crassipes Humb. & Bonpl. (QCS), in order to remove total chromium and Cr(VI) from aqueous solution. Effects of various fixed-bed column parameters such as influent solution pH, influent flow rate, QCS bed height, and influent Cr(VI) concentration were investigated. Results from the fixed-bed column experiments demonstrate that total chromium biosorption and Cr(VI) removal by QCS depend strongly on the pH of influent solution. The highest capacities for Cr(VI) removal and total chromium biosorption are about 181.56 and 110.35 mg g-1 and are achieved at influent solution pH of 1.0 and 2.0, respectively. Besides this, total chromium biosorption capacities increased from 104.25 to 116.14 mg g-1, 109.07 to 117.44 mg g-1, and 85.02 to 129.87 mg g-1, as bed height, inlet flow rate, and influent Cr(VI) concentration increased from 1.7 to 6.5 cm, 0.25 to 1 mL min-1, and 50 to 400 mg L-1, respectively. The dose-response model defines the entire breakthrough curve for total chromium biosorption onto QCS, under all experimental conditions. X-ray photoelectron spectroscopy (XPS) and biosorption kinetic studies revealed that QCS is able to remove toxic Cr(VI) from acidic liquid solution by means of a complex mechanism that involves the binding of Cr(VI) oxyanions to positively charged groups present at the QCS surface, after which the Cr(VI) species are reduced to Cr(III) by adjacent electron donor groups, and the generated Cr(III) ions then become partially bound to the QCS biomass and partially released into the liquid phase. Results show that QCS can be employed as an easily accessible, abundant, eco-friendly, and inexpensive biosorbent for the removal of total chromium and Cr(VI) from Cr(VI) solutions, in continuous operation.

Corncob as an effective, eco-friendly, and economic biosorbent for removing the azo dye Direct Yellow 27 from aqueous solutions

The corncob is an agricultural waste generated in huge quantities during corn processing. In this paper, we tested the capacity of corncob particles for water purification by removing the azo dye Direct Yellow 27 (DY27) via biosorption. The biosorption process was investigated in terms of the kinetics, equilibria, and thermodynamics. Batch biosorption studies showed that the biosorption performance has strong inverse correlations to the solution pH and the corncob particle size, and it increases quickly with increasing contact time and initial dye concentration. The pseudo-second-order kinetic model provides the best fit to the experimental data, whereas the Redlich-Peterson isotherm model is most suitable for describing the observed equilibrium biosorption. The biosorption process is exothermic, spontaneous, and physisorption in character. Fourier transform infrared (FTIR) spectroscopy and confocal scanning laser microscopy (CSLM) studies suggest that lignocellulose and proteins play key roles in the biosorption of DY27 from aqueous solutions by corncob. Furthermore, after biosorption onto the corncob, the dye can be effectively desorbed using 0.1 M NaOH solution. Therefore, the corncob can be used as a promising biosorbent to remediate DY27-contaminated water and wastewater.