Fabrication of rice straw nano-biochar by ball milling for efficient adsorption of ammonium nitrogen and reduction of ammonia volatilization: effects and mechanisms

Abstract

The use of nitrogen fertilizers leads to substantial nitrogen losses and subsequent environmental pollution. Biochar (BC) demonstrates considerable potential for enhancing N fixation and reducing emissions, but it frequently induces a liming effect that increases ammonia (NH₃) volatilization. Nano-biochar (NBC) is attracting considerable attention due to its higher surface energy. However, little information is available whether it could enhance nitrogen adsorption and reduce NH₃ volatilization. Therefore, this study utilized a one-step ball milling method to produce NBC, characterized its physicochemical properties, and investigated its effects and mechanisms on NH₄⁺–N adsorption and NH₃ volatilization. Our results showed that the specific surface area, pore volume and acidic functional groups (carboxyl, lactone group and phenolic hydroxyl groups) of NBC were higher than those of bulk BC, while the pore diameter, zeta potential and pH were the opposite, which was more conducive to promoting adsorption. The maximum adsorption amount of NBC for NH₄⁺–N was 6.880 mg g⁻¹, approximately 1.9 times that of bulk BC. The adsorption process conformed to the Langmuir adsorption isotherm model and the pseudo-second-order kinetic equation, indicating that the adsorption was monolayer and chemical. The primary adsorption mechanisms included physical adsorption, ion exchange, electrostatic and π–π interactions. The addition of 0.30–30% of bulk BC and NBC reduced NH₃ volatilization by 6.40–31.50% and 5.00–42.20%, relative to no BC addition, respectively. The main reason for lower NH₃ volatilization observed with NBC was its ability to improve soil mineral nitrogen content, mineralization and nitrification rates, and decrease urease activity and pH. Therefore, NBC is a green and efficient adsorbent for reducing nitrogen emissions.