Adsorption characteristics of ammonia nitrogen by modified waste corn straw-based biochar

A mmonia nitrogen contamination poses severe threats to aquatic ecosystems and human health, with its induced eutrophication requiring urgent and efficient remediation. Adsorbents are effective in removing aqueous ammonia nitrogen, but existing adsorbents are costly and inefficient. It is important to develop a low-cost and efficient adsorbent. Corn straw is a kind of cheap and easy-to-obtain material. In this study, biochar prepared from corn straw was used as an adsorbent for ammonia nitrogen. In order to explore biochar with high adsorption ability, sodium carbonate, magnesium chloride, and potassium hydroxide were used as additives. The results show that the magnesium chloride-modified biochar (MBC) demonstrates remarkable advantages: a carbon yield of 51.901%, an increased specific surface area (208.322 m²/g), and a mesoporous structure providing abundant active sites for ammonia nitrogen adsorption. To investigate adsorption ability, ammonia nitrogen adsorption experiments are conducted, revealing favorable adsorption capacity and practical application potential of the biochar. The maximum adsorption capacity of MBC is determined as 12.005 mg/g, achieving a removal efficiency of 92.901% at 45 °C. The positive $\Delta H^0$ value confirms the endothermic nature of the adsorption process. Both pseudo-second-order kinetic (R² 0.992-0.999) and Langmuir isotherm (R² 0.936-0.996) models effectively describe the adsorption kinetics and equilibrium, respectively, indicating predominant monolayer chemisorption on homogeneous surfaces. Mechanistic analyses through characterization demonstrate that ammonia nitrogen removal is governed by pore filling, complexation, cation-π interactions, cation exchange, and electrostatic attraction.