Mechanism of orthophosphate (PO4-P) adsorption onto different biochars

Abstract

The adsorption mechanisms of phosphate (PO4-P) onto cocoa pod husk (CP), corn cob (CC), rice husk (RH) and palm kernel shell (PK) biochar pyrolyzed at 300 ◦C and 650 ◦C were investigated. A series of batch experiments were undertaken to assess the effects of contact time and pH. Results show that PO4-P adsorption equilibria for the biochar types was within 6–15 h, being rapid in the 300 ◦C-biochar types. The equilibrium pH for maximum PO4-P adsorption varied among biochar types, ranging from of 2.6 to 4.8 and increasing with decreasing PO4-P adsorption. Pseudo-secondorder and Elovich models explained the adsorption data well indicating a chemisorption process on heterogeneous biochar surface. PO4-P adsorption was controlled initially by intraparticle diffusion and subsequently by chemisorption. Per the properties of the biochars (FTIR and elemental composition) and pH (equilibrium pH and ΔpH), PO4–P was adsorbed through electrostatic attraction, surface precipitation and ligand exchange, and the relative importance of these processes differed among the biochar types. Biochar types (PK300, PK650, CP300, CP650, RH650 and CC650) that adsorbed PO4-P through surface precipitation and ligand exchange reactions can be used to remove PO4-P from wastewater since PO4-P is strongly adsorbed, controlling PO4-P enrichment of water bodies.