Biochar and ash derived from silicon-rich rice husk decrease inorganic arsenic species in rice grain

Abstract

Exposure to arsenic (As) through rice consumption potentially threatens millions of people worldwide. Understanding is still lacking the recycling impacts of rice residues on As phytoavailability in paddy soils and is of indisputable importance in providing a sustainable and effective measure to decrease As accumulation in rice grain. Herein, we examined the effects of rice husk biochar (RHB) and rice husk ash (RHA) on As grain speciation, and As dynamics in the soil porewater and solid-phase fractions. The results corroborated that both the RHB and RHA (0.64% w/w) treatments significantly (p < 0.05) decreased inorganic As accumulation in rice grain to 0.27–0.29 mg kg−1, which was below the maximum inorganic As level in husked rice (0.35 mg kg−1) established by the Codex. The residual phase (F6 = 90% of total soil As) as quantified by the sequential extraction was the dominant As pool; the fractions were subsequently transformed into several As pools associated with soluble and exchangeable (F1), organically bound (F2), Mn oxides (F3), poorly crystalline (F4) and crystalline (F5) Fe oxides during the rice growing periods. The Si-rich amendments enhanced the residual phase formation upon soil flooding, which decreased the As availability to rice plant. The inorganic grain-As concentrations were well explained by the soil-extractable As concentrations in the F2, F3, F5, and F6 fractions. The pore-water analysis indicated that Mn oxides were important sources and sinks for As released to the soil solution. Our findings shed light on the beneficial role of RHB and RHA in alleviating inorganic As uptake in paddy rice.