Effects of Temperature on Physicochemical Properties of Rice Straw Biochar and Its Passivation Ability to Cu2+ in Soil

Biochar produced from the rice straw as a remediation material of heavy metal-contaminated soil has been extensively investigated in environmental science and technology. Biochar prepared at different temperatures exhibits extensive difference in physical and chemical properties. But there is no consensus on passivation ability of metal ions in soil by biochar, and the release potential and plant availability of metal ions retained by biochar. Two types of straw biochar were prepared by oxygen limited pyrolysis at 400 and 600 °C, respectively. And their microstructure, functional groups, and basic physical and chemical properties were determined by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and modified pH drift method, respectively. Furthermore, the adsorption isotherm of Cu2+ was analyzed via an adsorption experiment. The biological effects of two kinds of temperatures on the passivation effect of Cu2+ in soil were evaluated by a pot experiment. SEM images showed that the smooth surfaces of BC400 were evolved to be rougher for BC600, and both of them could provide active sorption sites for Cu2+. FTIR suggested that BC600 was endowed with more condensed aromatic carbon structures and available polar functional groups. Langmuir isotherm model predicted that the maximum Cu2+ sorption capacities were 40.06 and 28.90 mg/g for BC600 and BC400, respectively. In BC400, Cu2+ sorption data was better fitted (R2 = 0.991) by the Langmuir model, suggesting that the relatively smooth surfaces favor monolayer dispersion of Cu2+. Soil incubation experiment demonstrated that biochar had a strong passivation effect on the adsorption of Cu2+. And the adsorbed Cu2+ was not largely released into soil. The addition of biochar increased the content of soil available phosphorus, potassium, and organic matter. After application of heavy metal laden biochar, the total nitrogen, available potassium, and phosphorus changed in soil, but the changes in other nutrients were not significant. It could be concluded that the changes of heavy metals were related to nitrogen, available phosphorus, and potassium. Biochar derived from the rice straw has a strong passivation on the Cu2+, and also prevents the re-release of Cu2+ adsorbed by itself. The existence of them can reduce the bioavailability of Cu2+ in soil. Furthermore, the ability to adsorb Cu2+ based on biochar prepared at two temperatures has obvious difference. And the passivation and release inhibition of 600 °C biochar are stronger, which may be due to more aromatic structures, polar groups, and higher pH in high temperature.