Preparation Of Crop Stalk-Based Sorption Materials With High Performances And Their Applications For Pah Removal From Water

As a large agricultural country, the crop stalks are produced in huge amounts in our country, but the most practical application technologies are not perfect for the comprehensive utilization of crop stalks. Most of crop stalks are set on fire or arbitrarily discarded. These disposals result in resource loss and environmental pollution. Therefore, the technical processes of stalk biochars prepared from crop stalks were studied, and the sorption performances of polycyclic aromatic hydrocarbons (PAHs), the strong "three-induced" effect contaminant, in water by them were explored. Results obtained from this work will provide the basis of achieving the resource utilization of crop stalks and the development of cost-effective water pollution control technology.In this paper, the preparation of stalk biochars and the performances of PAHs removal from water were studied. The changes of property indexes and the sorption performances of stalk biochars were explored after surface modification by H3PO4, besides, the effects of coexistent heavy metal Hg on PAHs sorption by stalk biochars were revealed. The main results are as follows:(1) The preparation methods of stalk biochars and the PAHs sorption performances from water were studied. The soybean, sesame and corn stalks were pyrolyzed and charred for 8h at 300-700℃, and then stalk biochars were obtained. The BET specific surface areas, methylene blue, and iodine sorption abilities of the stalk biochars were determined. The sorption efficiencies of a single PAH and mixing PAHs on these materials were investigated. With an increase of treatment temperature, the BET specific surface areas of stalk biochars enlarged, and the sorption abilities of methylene blue and iodine enhanced. The prepared stalk biochars could effectively remove the PAHs from water. For instance, 91.28%,89.01% and 99.66% of naphthalene, acenaphthene, and phenanthrene in 32 mL water were removed by 0.01 g biomaterials prepared by soybean stalk at 700℃, respectively, The removal efficiencies of the mixed PAHs in water by biomaterials were in the order of phenanthrene> naphthalene> acenaphthene. However, the sorption abilities of prepared stalk biochars differed significantly, and followed the order of corn> soybean> sesame for the removal of naphthalene and acenaphthene, and soybean> corn> sesame for phenanthrene removal. Results obtained from this work will provide the basis of achieving the resource utilization of crop stalks and the preparation of cost-effective biomaterials.(2) Surface modification of stalk biochars by H3PO4, and their performances of PAHs removal from water were studied. The BET specific surface areas, methylene blue and iodine sorption abilities of the stalk biochars modified by H3PO4 were determined, and the sorption efficiencies of a single PAH and mixing PAHs on these biomaterials were investigated at two solid-liquid ratios (0.01g/32mL and 0.02g/32mL), respectively. The results showed that with increasing the treatment temperature, the BET specific surface areas of stalk biochars enlarged; the sorption abilities of methylene blue and iodine enhanced; and the property indexes of the modified stalk biochars were better. For example, the BET specific surface area of the stalk biochars which charred at 700℃and modified by H3PO4 was 269.95 m2 g-1, and the sorption abilities of iodine and methylene blue were 433.95 and 150.07 mg g-1, respectively. The modified stalk biochars could effectively remove the PAHs from water. For instance,94.44%,95.47% and 100% of naphthalene, acenaphthene, and phenanthrene in water (solid-liquid ratio was 0.01g/32mL) were removed by the stalk biochars which prepared at 700℃and modified by H3PO4. Under the same conditions, the removal efficiencies were more than those by the stalk biochars without modification.(3) The co-sorption of phenanthrene and Hg2+ in water by the prepared stalk biochars were studied. The results showed that regardless of the presence of Hg2+in the solution or not, the sorption isotherm of phenanthrene on stalk biochars was linear. Under the condition that there was phenanthrene or not in the solution, the adsorption isotherm of Hg2+ by stalk biochars could fit with Langmiur equation. The coexistent Hg2+ in solution reduced the sorption of phenanthrene, and with the increase of Hg2+ concentration, the phenanthrene distribution coefficient decreased. Similarly, phenanthrene also reduced the adsorption of Hg2+ in water by stalk biochars. So there was a competitive adsorption between Hg2+ and phenanthrene. The magnitude of competitive action had a relation to the concentrations of phenanthrene and Hg2+ in the solution. For the same concentration of the phenanthrene, when the initial concentration of Hg2+ was low, the effect of Hg2+ on phenanthrene sorption by stalk biochars was not significant; but when the concentration of Hg2+> 32 ug L-1, the coexistent Hg2+ in water significantly reduced phenanthrene removal efficiency by stalk biochars. At the same concentration of Hg2+, when the initial concentration of phenanthrene was low, phenanthrene had greater impact on Hg2+ removal efficiency; but when the concentration of phenanthrene> 5 mg L-1, the coexistent phenanthrene had no significant effect on Hg2+ removal.