Removal Of Cr(Ⅵ)/Cr(Ⅲ) By Biochars From Simulated Acidic Wastewater

Metal smelting and mining, excessive discharge of chromium-containing wastewater and waste residues of electroplating and tanning industries have led to accumulation of heavy metals in waters and serious pollution of water, which is harmful to human health due to the transmission of heavy metals through the food chain. On the other hand, There are large quantities of annual crop residues such as crop straws produced from agricultural practices in China. Although part of them has been used as fuels, feed or straw returning, there is still about 2×108 Mt of them has been burned in field, which is not only a waste of resources but also causing environmental problems, resulting a range of social problems. In the partial or total absence of oxygen, thermal decomposition of plant-derived biomass (oxygen-limited pyrolysis) is manipulated to yield a solid carbon-rich residue generally referred to as biochar. Biochar carries large amount of negative surface charge and has ample oxygen-containing functional groups on its surface. Therefore, biochar can absorb heavy mentals from aqueous solutions. However, the mechanisms and capacities of biochar on the adsorption of heavy metals are not understood well.The four crop straws of peanut, soybean, canola and rice were chosen to prepare biochars, the properties of the biochars, and their effects on the removal of the heavy metals were investigated in this study. The major results obtained were summarized as follows:(1) Cr(Ⅲ) adsorption by biochars generated from peanut, soybean, canola and rice straws is investigated with batch methods. Adsorption of Cr(Ⅲ) increased as pH rose from 2.5 to 5.0. Adsorption of Cr(Ⅲ) led to peak position shifts in the FTIR-PAS spectra of the biochars and made zeta potential values less negative, suggesting the formation of surface complexes between Cr3+ and functional groups on the biochars. The adsorption capacity of Cr(Ⅲ) followed the order:peanut straw biochar>soybean straw biochar>canola straw biochar>rice straw biochar, which was consistent with the content of acidic functional groups on the biochars. The increase in Cr3+ hydrolysis as the pH rose was one of the main reasons for the increased adsorption of Cr(Ⅲ) by the biochars at higher pH values. Cr(Ⅲ) can be adsorbed by the biochars through electrostatic attraction between negative surfaces and Cr3+, but the relative contribution of electrostatic adsorption was less than 5%. Therefore, Cr(Ⅲ) was mainly adsorbed by the biochars through specific adsorption. The Langumir and Freundlich equations fitted the adsorption isotherms well and can therefore be used to describe the adsorption behavior of Cr(Ⅲ) by the crop straw biochars. The crop straw biochars have great adsorption capacities for Cr(Ⅲ) under acidic conditions and can be used as adsorbents to remove Cr(Ⅲ) from acidic wastewaters.(2) Cr(Ⅵ) was first reduced by Na2SO3 or FeSO4 and then biochar generated from peanut straw at 500℃ was used to remove the Cr(Ⅲ). Results indicated that the reduction of Cr(Ⅵ) by Na2SO3 must be conducted under strongly acidic conditions within a narrow pH range of 2.0-2.4, whereas the reduction of Cr(Ⅵ) by FeSO4 can be conducted under acidic, neutral and weak alkaline conditions because protons are generated from the hydrolysis of Fe3+ via Fe2+ oxidation. When the initial concentration of Cr(Ⅵ) was no more than 1.5 mmol L-1, and after Cr(Ⅵ) had been reduced by Na2SO3 at pH 2.0 or FeSO4 at pH 7.6,4 g L-1, peanut straw biochar was able to neutralize solution acidity and remove Cr from the aqueous solution. The optimal reaction time for biochar in the Cr-containing solutions was 6 h. The precipitation of Cr(OH)3 and the formation of Cr3+ surface complexes with the functional groups on the biochar were the main mechanisms for Cr(Ⅲ) removal by biochar. These results suggested that the combination of reductants (Na2SO3/FeSO4) and biochar generated from peanut straw can be used to efficiently remove Cr(Ⅵ) from aqueous solutions.(3) The batch experiments combined with zeta potential measurement, FTIR and XRD were used to investigate the adsorption of Cr(Ⅵ) by Fe(Ⅲ)-modified crop-straw biochars and its mechanisms to evaluate the effect of Fe(Ⅲ) modification on adsorption ability of the biochars for Cr(Ⅵ). Results showed that Fe(Ⅲ) modification increased the positive charge and decreased the negative charge on the biochars and thus enhanced the adsorption of Cr(Ⅵ) by the biochars. Complexation of Fe(Ⅲ) with functional groups and physical coverage due to precipitation of iron hydroxide on the biochars were responsible for the changes of surface charge on the biochars. The promoting effect of Fe(Ⅲ) modification on Cr(Ⅵ) adsorption by peanut straw biochar was greater than that by rice straw biochar. At pH5.0, the maximum Cr(Ⅵ) adsorption by peanut straw-biochar and rice straw biochar was increased by 79% and 29%, respectively, after the biochars were modified by Fe(Ⅲ). In pH 4.0-6.5, the Cr(Ⅵ) adsorption by biochars and Fe(Ⅲ)-modified biochars decreased with the rising pH. The promoting effect of Fe(Ⅲ) modification on Cr(Ⅵ) adsorption by the biochars showed a similar trend. The increase in negative charge on the biochars was the main reason for the decrease in Cr(Ⅵ) adsorption with the rising pH. Both electrostatic adsorption and specific adsorption contributed to Cr(Ⅵ) adsorption on Fe(Ⅲ)-modified biochars. The specific adsorption of Cr(Ⅵ) also decreased with the rising pH. Therefore, Fe(Ⅲ)-modified crop straw biochars have high adsorption capacity for Cr(Ⅵ) under acidic conditions and can be used as absorbents to remove Cr(Ⅵ) from acidic wastewater.