| Biochar is a group of carbon-rich, aromatized solid substances. The aromatic structures of biochars attribute to their high biochemical and thermal stability, which makes them impervious to chemical and microbial degradation in soil. Because of their stability and special physicochemical properties (such as pore structure, surface oxygen-containing functional groups, high ash content and rich inorganic mineral components), biochar has been applied in sequestrating carbon, improving soil quality, reclamation of polluted environment, and increasing crop yield. In this study, twenty four biochars were prepared from agricultural by-product (peanut shells) and wood processing industry by-product (pine chips) at different pyrolysis temperatures (200-500 ℃). The relationship between biomass sources, pyrolysis temperature and physico-chemical properties of biochars was investigated. Heavy metal pollution is widely reported in China (especially in the mining regions), greatly decreased the quality of soil and water resources and posed a grave concern. The adsorption kinetics and capacity of copper on biochars produced from peanut shells and pine chips were determined. Biochar sorption properties are associated with their ash content. Comparing the sorption of copper ions on these biochars before and after ash removal (treated using deionized water or HC1/HF) will provide useful information to reveal the role of ash content in copper sorption. Low molecular weight organic acids are ubiquitous in soil and will affect the performance of heavy metal remediation. The adsorption of Low molecular weight organic acids on biochars in the presence or absence of copper ion and the complexation of Low molecular weight organic acids with copper were investigated. The adsorption mechanisms of Cu2+on biochars were discussed. The results of this study will provide a theoretical basis for the application of biochar as an economic and efficient new material towards reclamation of heavy metal polluted environment. The main results of this research are as follows:(1) The effect of raw materials and pyrolysis temperature on biochar physical and chemical properties:biochars were produced from peanut shells and pine chips within 200-500 ℃, and the physical and chemical properties of these biochars were evaluated. With the increasing pyrolysis temperature, biochar yield continued to decrease. In addition. the surface area, pH and ash content increased with pyrolysis temperature; C contents increased, while H, O and N contents decreased with increased pyrolysis temperature. The pyrolysis resulted in the reduction and disappearance of aliphatic, phenolic hydroxyl and carboxyl functional groups, but increased polycyclic aromatic compounds, heterocyclic and aromatics. Proportion of soluble cations (Ca+, Mg2+) and soluble anion (SO42- and PO43-) decreased at high pyrolysis temperature.(2) Adsorption kinetics of Cu2+on biochars was also studied. The adsorption of Cu2+ on biochars can be divided into in two stages:fast and slow adsorption stages, and the distribution proportion and adsorption rate of these two stages depended on the types and content of oxygen-containing functional group and particle diffusion. The adsorption Cu2+ on biochar could be divided into two processes:Firstly, Cu2+was quickly adsorbed on the active sites (oxygen-containing functional group) of biochars surface; Then, Cu2+diffused though the pore and into the inner adsorption sites of biochars. With increasing pyrolysis temperature, surface oxygen containing functional groups reduced, and inner pore volumes increased. Hence, the apparent adsorption of Cu2+ on biochars was slowed, which resulted in increased t9o%(equilibration time to reach 90%of equilibrium adsorption).(3) The adsorption capacity of Cu2+on Peanut shell biocars was higher than Pine chips biochars. The sorption isotherms were fitted using FM model. The nonlinearity index (n) was within the range of 0.23-0.67 and decreased with increasing pyrolysis temperature. In addition, the adsorption of Cu2+ on Peanut shell biochars and Pine chips biochars decreased as pyrolysis temperature increased, then the adsorption of Cu2+ on Peanut shell biocars sudden increased at 500 ℃, and Pine chips biochars at 400 ℃.(4) Cation exchange was an important mechanism controlling Cu2+ sorption on biochars. Precipitation was not important for most of the biochars. but contributed up to 20% for biochars produced from PS at 500℃. Removal of ash content increased Cu+ sorption on PC biochars, but a significantly positive relationship was observed between ash content and sorption coefficients for Cu2+ for PS biochars. The impact of ash content on Cu2+ sorption was biochar property dependent. For biochars dominated with PO43-and SO42-, ash content played a positive role in Cu2+removal from aqueous phase. However, for biochars dominated with competitive cations, ash content played a negative role.(5) The sorption isotherms were fitted using LM model. The adsorption isotherms of Low molecular weight organic acids on acid treated biochars showed that the adsorption of Low molecular weight organic acids on acid treated Peanut shell biochars were greater than the acid treated Pine chips biochars due to their larger surface area. With increasing pyrolysis temperature, the biochar specific surface area augmented, and the adsorption capacity of Low molecular weight organic acids on acid treated biochars ware increased. The adsorption of oxalic acid on treated biochars (4444±631～6730±802 mg/kg) were higher than citric acid (1209±149～4603±352 mg/kg) due to the smaller molecular weight and molecular molar volume of the former. The planar configuration of the molecule also facilitated its adsorption to biochar pores.(6) In this study, citric acid and oxalic acid were used as model chemicals to investigate the effect of Low molecular weight organic acids on Cu2+adsorption by biochars. The results showed that the sorption of Cu2+in biochar was prompted by citric acid at low concentrations (below 10 mg/L), but inhibited at high concentrations. The adsorption of citric acid on biochars provided more adsorption sites for Cu+, thus promoted the adsorption of Cu2+. But increased citric acid loading on biochars may have blocked the internal pores in biochars, which inhibited Cu2+adsorption. The oxalic acid inhibited the adsorption of Cu2+in biochars at the tested concentrations (0.5-50 mg/L). This result may be explained by the strong complexation of oxalic acid with Cu2+in the aqueous phase, and the competition between oxalic acid and Cu2+for adsorption sites (such as the oxygen-containing functional groups, and the internal pores of biochar) on solid particles. |
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