Effects of biochar and organic fertilizer on the sorption and dissipation of dinotefuran in paddy soil

Dinotefuran (DIN), which is a third-generation neonicotinoid, is the main insecticide against soil-dwelling insect pests in the paddy field. DIN is a potentially persistent, toxic, and bioaccumulative compound with food chain concentration and transfer effects, which is increasingly harmful to the environment and biology. Thus, it has practical significance to seek appropriate soil remediation technologies for reducing DIN mobility and alleviating its ecological risk. The immobilization technique is considered one of the most efficient soil remediation technologies for making pesticides less mobile, bioavailable, and bioaccessible through the addition of remediation agents. Biochar and organic fertilizer are cost-effective and recycled soil remediation agents with strong sorption capacity, sufficient carbon sources, and nutrients. However, few studies assessed the effect of biochar and organic fertilizer application on DIN fate, which is not propitious for the accurate manipulation of their separate or combined use on mitigating DIN adverse effects in soils. In this study, biochars were derived from peanut shells prepared at 400 ℃ and 900 ℃ pyrolysis temperatures (named PS400 and PS900). Composted chicken manure (CM) was produced and selected as a typically organic fertilizer. The sorption, dissipation, and fraction transformation mechanisms of DIN in amended soils were investigated under the effects of biochars and CM. Results showed that the sorption kinetics of DIN were categorized into rapid and slow sorption phases in unamended and amended soils. The sorption processes reached equilibrium after 36 hours. The pseudo-first-order model (PFO) better described the DIN sorption in unamended soil, whereas the Elovich model was the best fit in biochar-amended soils. It indicated that the sorption behaviors of DIN on unamended and CM-amended soils conformed to the first-order reaction kinetics and the adsorption rate is controlled by the diffusion process, and the DIN sorption behaviors on biochar-amended soils involved multi-mechanisms, e.g., heterogeneous diffusion, chemical sorption, intra-particle diffusion, etc. Both biochars and CM were effective in improving soil sorption capacity, and sorption capacities followed the order PS400>PS900>CM. The Freundlich sorption coefficient (K_f) increased from 0.31 in unamended soil to 2.80 (PS400-CM amended soil). The sorption affinity and mechanism of DIN were mainly π-π electron donor-acceptor for PS400, pore filling for PS900, and H-bonding, van der Waals forces sorption for CM, respectively. Meanwhile, both biochar and organic fertilizer accelerated the DIN dissipation rates in soils. Compared to unamended soil (half-life (T_1/2) = 100.3 d), the combined use of PS400 and CM shortened the T_1/2 of DIN to 38.1 d. This might be due to the fact that biochar and organic fertilizer provided favorable habitats and rich nutrients to microorganisms, which enhanced the microorganism’s metabolism and DIN degradation. A 3-step sequential extraction method was developed to describe three fractions of neonicotinoids in soils, i.e., weak-absorbed, moderate-absorbed, and strong-absorbed fractions. Biochar addition resulted in DIN main fraction transformation from weak-absorbed fraction to moderate-absorbed fraction. The proportion of moderate-absorbed fraction in the unamended soil was 13%±3%, while the proportion increased to 46%±7% in biochar-amended soils. In addition, the weak-absorbed fraction gradually transformed into the moderate-absorbed and strong-absorbed fraction due to the action of soil aging. As aforementioned, the findings improved our understanding of DIN fate in the amended soils, and the combined use of biochar and organic fertilizer was a promising soil remediation technology for reducing the DIN ecological risk.