Phosphates removal from water by raw and modified biochar from food processing wastes

Wastewater and drainage waters that discharge in the water bodies contain phosphates that increase eutrophication. In order to remove phosphates ions different type of sorbent materials have been examined such as biochar (BC). The aim of our study was to investigate the effectiveness of biochar produced from easily available food processing waste materials, the effect of pyrolysis temperature, and the modification of biochar for the removal of phosphates from water. Eggshells (EGS), rice husk (RH) and Coffee materials were pyrolyzed at 400 and 800^oC. The sorption efficiency of each raw material (eggshells, spent coffee grounds and rice husks) was also examined after modification with MgCl₂⋅6H₂O.

Generally, biochars pyrolyzed at 800^oC had higher sorption capacity compared to biochars pyrolyzed at 400^oC. Kinetic experiments demonstrated that magnesium modified EGS pyrolyzed at 800^oC were superior for the removal of phosphates (27.5 mg P/g). The EGS and RH biochars and the corresponding magnesium modified biochars pyrolyzed in 800^oC were further examined in isotherm studies. The highest sorption capacity (q_max) was observed with EGS pyrolyzed at 800^oC and was 11.4 mg PO₄^3--P/g. Modified EGS biochars pyrolyzed at 800^oC had almost the half sorption capacity compared to unmodified materials. Modification of RH biochar pyrolyzed at 800^oC resulted in higher sorption capacity by 34% for phosphates, compared to the RH biochar. The specific surface area values of the biochars examined is not a decisive factor for nutrient sorption. Reaction between magnesium and calcium (for the eggshell samples) ions with phosphates is responsible for the higher sorption efficiency.