Alternative use of rice straw ash as natural fertilizer to reduce phosphorus pollution in protected wetland ecosystems

Purpose The uncontrolled discharge of phosphorus into aquatic environment leads to the deterioration of the water bodies. Additionally, the agricultural crops present inside the La Albufera de Valencia Natural Park, rice fields mainly, have a high social and environmental value. However, there is a conflict between private interests (farmers) and public interest in the management of agro-waste produced by them. Nowadays, the option used by the farmers is the uncontrolled burning in the own field. The ashes generated during the combustion process could be used to remove phosphorus loading in water bodies of the Natural Park, contributing to its recovery. Methods Adsorption experiments were carried out in batch mode by using different concentrations (5-100 mgP L-1) of sodium phosphate dibasic (Na2HPO4) placed in 100 mL stoppered conical flask with 50 mL of synthetic wastewater and different amounts of adsorbent, during the selected time (5 days). Adsorption studies were performed with doses varying from 5 to 24 g L-1. After finishing the adsorption experiments, the solution was filtered through glass microfiber filter (1.2 µm). Results The adsorption capacity varies for rice straw ash from 31.91% up to 97.48% and rice straw ash with HCl from 17.49% up to 89.04%. An increase in temperature or dosage had a positive effect in the removal capacity, increasing its adsorption. Removal process of phosphorus was endothermic. Conclusion The use of rice straw ash could be a solution to reduce the phosphate in water bodies, providing an advantage to the proposed alternative of agro-waste management.