Biochar produced from tobacco stalks, eggshells, and Mg for phosphate adsorption from a wide range of pH aqueous solutions

Preparing biochar from tobacco stalks modified with Mg is a feasible way to utilize tobacco stalks as a resource. However, the capacity of phosphate removal of Mg-biochar is affected greatly with increasing pH. In the present work, biochar produced from the tobacco stalks was modified via Mg addition and eggshells during the preparation for improving the phosphate adsorption performance at higher pH. The results revealed that the theoretical maximum adsorption capacity of phosphate for the biochar increased from 110.62 to 272.37 mg P/g when the mass ratio of Mg to tobacco stalk increased from 0.12:1 to 0.48:1 (M-C). The adsorption capacity of the biochar with a mass ratio of 0.3:1 (M-C2) was more than 200 mg P/g in the pH range of 3 to 7 but with further increase of pH from 7 to 12 the capacity decreased from 205 to 135.2 mg P g(-1). Addition of eggshells with the magnesium solution during the dipping process can effectively improve the capacity of the modified biochar for phosphate in the alkaline condition. When the mass ratio of eggshell to tobacco stalk was >= 0.25:1, the phosphate adsorption capacities of the biochar can reach above 200 mg P/g within a wide pH range of 3 to 12, and the maximum adsorption capacity can reach more than 270 mg P g(-1) for the mass ratio of 1:1 (EM-C24). The characterization results demonstrated that Ca and Mg were loaded on the biochar. The high adsorption performance provides reference values for the utilization of tobacco stalks and eggshells.