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Among various plant-based natural biomaterials, pine bark was chosen as an efficient biomaterial capable of removing toxic Cr(VI) from aqueous solution. XPS spectra indicated that Cr(VI) was abiotically reduced to Cr(III) in both liquid and solid phases. The Cr(VI)-reducing capacity of pine bark was determined as 545 (+/-1.3)mg-Cr(VI)g(-1) of it, which was 8.7 times higher than that of a common chemical Cr(VI)-reductant, FeSO4 x 7H2O. Because pine bark could completely reduce toxic Cr(VI) to less toxic or nontoxic Cr(III) even at neutral pH, it was used as an organic reductant to remediate Cr(VI)-contaminated soil in this study. Soil slurry system using a bottle roller was applied to ex situ slurry-phase remediation experiments. In the soil slurry system, pine bark completely reduced Cr(VI) to Cr(III) and adsorbed the reduced-Cr(III) on its surface. Abiotic remediation rate of Cr(VI)-contaminated soil increased with the increase of pine bark dosage and with the decreases of Cr(VI) and water contents. In conclusion, pine bark can be used to remediate Cr(VI)-contaminated soil efficiently and environmentally friendly.

Among useless but abundant agricultural biowastes such as banana skin, green tea waste, oak leaf, walnut shell, peanut shell and rice husk, in this study, banana skin was screened as the most efficient biomaterial to remove toxic Cr(VI) from aqueous solution. X-ray photoelectron spectroscopy (XPS) study revealed that the mechanism of Cr(VI) biosorption by banana skin was its complete reduction into Cr(III) in both aqueous and solid phases and partial binding of the reduced-Cr(III), in the range of pH 1.5-4 tested. One gram of banana skin could reduce 249.6 (+/-4.2)mg of Cr(VI) at initial pH 1.5. Namely, Cr(VI)-reducing capacity of banana skin was four times higher than that of a common chemical Cr(VI)-reductant, FeSO(4).7H(2)O. To diminish undesirable/serious organic leaching from the biomaterial and to enhance removal efficiency of total Cr, its powder was immobilized within Ca-alginate bead. The developed Cr(VI)-biosorbent could completely reduce toxic Cr(VI) to less toxic Cr(III) and could remove almost of the reduced-Cr(III) from aqueous phase. On the basis of removal mechanisms of Cr(VI) and total Cr by the Cr(VI)-biosorbent, a kinetic model was derived and could be successfully used to predict their removal behaviors in aqueous phase. In conclusion, our Cr(VI)-biosorbent must be a potent candidate to substitute for chemical reductants as well as adsorbents for treating Cr(VI)-bearing wastewaters.

Recently, a new and simple kinetic model was derived from a basic concept of the redox reaction between Cr(VI) and biomaterials, and successfully described the removal behavior of Cr(VI) under various Cr(VI) and biomaterial concentrations. However, this model did not consider the effects of pH and temperature on the Cr(VI) removal by biomaterials. In this study, a new efficient biomaterial, pine needle, capable of removing Cr(VI) was used as a model one to study the Cr(VI) removal by biomaterials. Analysis of chromium species in aqueous and solid phases revealed that the removal mechanism of Cr(VI) by pine needle was its reduction into Cr(III). The removal rate of Cr(VI) increased with a decrease in pH or with an increase of temperature. Finally, an advanced kinetic model in the form of -d[Cr(VI)]/dt = Ae(Ea/RT)[H+]n[Cr(VI)][OCs] was derived, and successfully predicted the time-dependent Cr(VI) concentration at various pHs (2-4) and temperatures (10-55 degrees C).

The reduction of toxic Cr(VI) to the less or nontoxic Cr(III) may be an useful detoxification technique for the treatment of Cr(VI)-contaminated waters. Recently, the protonated biomass of brown seaweed, Ecklonia, was shown to completely reduce Cr(VI) to Cr(III) in the pH range 1-5. The reduction of Cr(VI) to Cr(III) appeared to occur at the surface of the biomass. In this study, abiotic Cr(VI) reduction by the biomass was performed with various contact times, pHs and initial Cr(VI) concentrations, and surface and bulk characteristics of the Cr-laden biomass was then investigated using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The XPS spectra indicated that the Cr(VI) bound to the biomass was completely reduced to Cr(III) at tested various conditions. XANES and EXAFS spectra of the Cr-laden biomass were very similar to those of Cr(III)-acetate, which means that the Cr bound to the biomass during Cr(VI) reduction had an octahedral geometrical arrangement. The bonding distance of the chromium oxygen atoms was approximately 1.97-1.99 A. In conclusion, it was obvious that oxygen containing groups, such as carboxyl and phenolic groups, play a major role in the binding of the Cr(III) resulting from the abiotic reduction of Cr(VI) by the biomass.

For the last few decades, over 200 papers have been published in the Cr(VI) biosorption research field. Most early studies have claimed that Cr(VI) was removed from aqueous phase through an anionic adsorption, but this approach has been lost old original position. It has been newly explained that these findings were misinterpreted due to errors in measuring the concentrations of different chromium species in the aqueous phase, insufficient contact time required for equilibrium and the lack of information about the oxidation state of the chromium bound to biomaterials. Although 'adsorption-coupled reduction' is now widely accepted as the mechanism of Cr(VI) biosorption by natural biomaterials, a number of researchers still believe that Cr(VI) is removed by anionic adsorption onto the biomaterials. Therefore, the objective of this study was to show reliable evidences that the removal mechanism of Cr(VI) by natural biomaterials is 'adsorption-coupled reduction'. Sixteen natural biomaterials were used to study the Cr(VI) biosorption. Not only Cr(VI) but also total Cr in the aqueous phase were analyzed. X-ray photoelectron spectroscope was also used to verify the oxidation state of the chromium bound to the biomaterials. Finally, the removal behavior of Cr(VI) by each biomaterial was described by a kinetic model based on a redox reaction.

The dead biomass of the brown seaweed, Ecklonia sp., is capable of reducing toxic Cr(VI) into less toxic or nontoxic Cr(III). However, little is known about the mechanism of Cr(VI) reduction by the biomass. The objective of this work was to develop a kinetic model for Cr(VI) biosorption, for supporting our mechanism. The reduction rate of Cr(VI) increased with increasing total chromate concentration, [Cr(VI)], and equivalent concentration of organic compounds, [OCs], and decreasing solution pH. It was found that the reduction rate of Cr(VI) was proportional to [Cr(VI)] and [OCs], suggesting the simple kinetic equation -d[Cr(VI)]/dt=k[Cr(VI)][OCs]. When considering the consumption of organic compounds due to the oxidation by Cr(VI), an average rate coefficient of 9.33 (+/-0.65)microM(-1)h(-1) was determined, at pH 2. Although the function of the pH could not be expressed in a mechanistic manner, an empirical model able to describe the pH dependence was obtained. It is expected that the developed rate equation could likely be used for design and performance predictions of biosorption processes for treating chromate wastewaters.

The potential use of the brown seaweed, Ecklonia, biomass as a bioreductant for reducing Cr(VI) was examined in a continuous packed-bed column. The effects of the operating parameters, such as influent Cr(VI) concentration, influent pH, biomass concentration, flow rate and temperature, on the Cr(VI) reduction were investigated. Increases in the influent Cr(VI) concentration and flow rate or a decrease in the biomass concentration inside the column led to a higher breakthrough of the Cr(VI) ions in the effluent. Particularly, the influent pH and temperature most significantly affected on the breakthrough curve of Cr(VI); a decrease in the influent pH or an increase in the temperature enhanced the Cr(VI) reduction in the column. For process application, a non-parametric model using neural network was used to predict the breakthrough curves of the column. Finally, the potential of the column packed with Ecklonia biomass for Cr(VI) detoxification was demonstrated.

In this study, the removal of Cr(VI) was examined in a binary aqueous system containing Ni(II), and the competitive interaction between them was successfully modeled. The removal rate of Cr(VI) was unaffected by the presence of Ni(II). However, in an equilibrium state, the uptake of total Cr or Ni(II) was decreased in the presence of the other metal. The mono- and multi-component Langmuir adsorption models clearly represented the uptake behavior of these metals.

The biomass of the brown seaweed, Ecklonia sp., is capable of reducing Cr(VI) to Cr(III). However, very little is known about the mechanism of Cr(VI) reduction by the biomass. The aims of the present investigation were to enhance the Cr(VI)-reducing capacity of the biomass using various chemical treatments and to elucidate the mechanisms governing Cr(VI) reduction. Among the various chemical treatments, acid-treatment showed the best performance with regards the improvement of Cr(VI) removal from the aqueous phase, while organic solvent-treatment significantly improved the removal efficiency of total Cr in the equilibrium state. Based on FTIR study, the biomass was subjected to chemical modification of its amino and carboxyl groups, to examine their roles in the Cr(VI) removal from the aqueous phase. Methylation of the amino group significantly decreased the Cr(VI) removal rate, but amination of the carboxyl group significantly increased the Cr(VI) removal rate. Meanwhile, esterification of the carboxyl group and carboxylation of the amino group decreased the Cr(VI) removal rate, but the former showed a more negative effect than the latter. These findings indicated that the amino and carboxyl groups take part in the Cr(VI) removal from the aqueous phase. In conclusion, mechanisms for direct and indirect Cr(VI) removal are proposed, and some aspects for the application of this biomass to Cr(VI) detoxification are discussed.

When synthetic wastewater containing Cr(VI) was placed in contact with the dead fungal biomass of Aspergillus niger, the Cr(VI) was completely removed from aqueous solution, whereas Cr(III), which was not initially present, appeared in aqueous solution. Desorption and X-ray photoelectron spectroscopy (XPS) studies showed that most of the Cr bound on the biomass was in trivalent form. These results indicated that the main mechanism of Cr(VI) removal was a redox reaction between Cr(VI) and the dead fungal biomass, which is quite different from previously reported mechanisms. The influences of contact time, pH, Cr(VI) concentration, biomass concentration and temperature on Cr(VI) removal were also evaluated. The Cr(VI) removal rate increased with a decrease in pH and with increases in Cr(VI) concentration, biomass concentration and temperature. Although removal kinetics was dependent on the experimental conditions, Cr(VI) was completely removed in the aqueous solution. In conclusion, a new mechanism of Cr(VI) removal by the dead fungal biomass has been proposed. From a practical viewpoint, this abundant and inexpensive dead fungal biomass has potential application in the conversion of toxic Cr(VI) into less toxic or nontoxic Cr(III).

A new type of biomass, protonated brown seaweed Ecklonia sp., was used for the removal of Cr(VI). When synthetic wastewater containing Cr(VI) was placed in contact with the biomass, the Cr(VI) was completely reduced to Cr(III). The converted Cr(III) appeared in the solution phase or was partly bound to the biomass. The Cr(VI) removal efficiency was always 100% in the pH range of this study (pH 1 to approximately 5). Furthermore, the Cr(VI) reduction was independent of the Cr(III) concentration, the reaction product, suggesting that the reaction was an irreversible process under our conditions. Proton ions were consumed in the ratio of 1.15 +/- 0.02 mol of protons/mol of Cr(VI), and the rate of Cr(VI) reduction increased with decreasing the pH. An optimum pH existed for the removal efficiency of total chromium (Cr(VI) plus Cr(III)), but this increased with contact time, eventually reaching approximately pH 4 when the reaction was complete. The electrons required for the Cr(VI) reduction also caused the oxidation of the organic compounds in the biomass. One gram of the biomass could reduce 4.49 +/- 0.12 mmol of Cr(VI). From a practical viewpoint, the abundant and inexpensive Ecklonia biomass could be used for the conversion of toxic Cr(VI) into less toxic or nontoxic Cr(III).

Investigates the biosorption of trivalent chromium with protonated brown alga Ecklonia biomass. Characterization of chromium binding site; Use of chemical precipitation for the treatment of chromium-bearing effluents; Mechanisms of metal binding.