Effect of biochar from peanut shell on speciation and availability of lead and zinc in an acidic paddy soil

Biochar has been used to reduce the mobility and availability of heavy metals in contaminated paddy soils. A pot experiment was carried out to analyze the effects of peanut shell biochar (PBC) on the speciation and phytoavailability of Pb and Zn in contaminated acidic paddy soil using rice (Oryza saliva L.) as an indicator plant. Peanut shell biochar was applied to an acidic paddy soil contaminated with Pb and Zn at four rates (0%, 1%, 2%, and 5% w/w), and rice plants were grown in this soil. The soil pH, cation exchange capacity (CEC), water-soluble SO42-, dissolved organic carbon (DOC), CaCl2-extractable heavy metals, and speciation of heavy metals were determined. Additionally, biomass and concentrations of heavy metals in rice tissues were determined. The application of PBC significantly increased the pH, CEC, water-soluble SO42-, and DOC in the paddy soil, but decreased the content of CaCl2-extractable Pb and Zn. The CaCl2-extractable Pb and Zn showed significant negative correlations with the pH, CEC, water-soluble SO42-, and DOC (p < 0.05). Following the application of biochar to the contaminated paddy soil, the Pb and Zn concentrations in the CaCl2 extracts were reduced by 41.04-98.66% and 17.78-96.87% (p < 0.05), respectively. Sequential chemical extractions showed a reduction in the acid-soluble Pb and Zn fraction and an increase in the reducible fraction following the addition of biochars. PBC obviously inhibited the uptake and accumulation of Pb and Zn in the rice plants. The Pb concentrations in the rice grain were significantly reduced by 60.32%, with the addition of 5% PBC. Neither of the biochars significantly changed the Zn concentrations in the rice grain. The influence of biochar on Pb and Zn phytoavailability varied not only with the application rate of biochar, but also with the kind of metals. Overall, the use of peanut shell biochar at a high application rate is more effective in immobilizing Pb and Zn in the acidic paddy soil contaminated with heavy metals, especially in reducing the phytoavailability of Pb to the rice plants.