Modeling of phosphate removal by Mg-Al layered double hydroxide functionalized biochar and hydrochar from aqueous solutions

Layered double hydroxide functionalized biochar and hydrochar composites are environmentally friendly and low-cost adsorbents for the removal of phosphate from aqueous solutions. In the present study, Mg-Al layered double hydroxide functionalized apple wood biochar and hydrochar were prepared and their phosphate adsorption characteristics were examined through batch experiments. Moreover, important factors affecting adsorption including initial phosphorus concentration (25-200 mg/L), contact time (5-120 min), ionic strength (deionized water, and 0.001, 0.01, and 0.1 mol/L KCl), pH (3-10), and adsorbent dosage (1, 2, 3, and 4 g/L) were investigated. Based on the results, the phosphate adsorption by Mg-Al layered double hydroxide modified biochar and hydrochar were comparable with Mg-Al layered double hydroxide and were greater than biochar and hydrochar. As expected, phosphate adsorption was decreased by increasing solution pH and ionic strength. The highest phosphate removal was attained at pH 4, adsorbent dosage of 4 g/L, and in the presence of deionized water as a background solution. Determination of adsorption characteristics of the adsorbents revealed that the phosphate adsorption mechanism involved a combination of electrostatic attraction, interlayer anion exchange, and formation of surface complexes. The Mg-Al layered double hydroxide modified biochar and hydrochar composites as cost-effective and efficient adsorbents suggest alternative biochar-and hydrochar-based composites for the phosphate removal from contaminated waters that could be used as P-fertilizers.