Adsorptive Characteristics of Phosphorous on Four Biochar Types

ABSTRACT    

Biochar prepared from cocoa pod, sawdust, rice straw and husk wastes may provide new low cost technology for environmental management with emphasis on P removal from waste water to minimize eutrophication and to enhance P availability in tropical soils. To achieve this, the sorption characteristics of the biochar types would have to be understood. In this study, laboratory experiments were conducted to investigate the P adsorptive characteristics on four biochar types derived from cocoa pod, sawdust, rice husk and straw. Batch sorption experiment was conducted to investigate the time for maximum adsorption by shaking at two (2) hours intervals between 2 to 24 hours. X-ray diffractograms of the various biochars were obtained to help elucidate possible mechanisms of adsorption. Effect of pH on adsorption was also studied for initial pH ranging between 2 and 12 and initial P concentrations between 0.4 mM and 1.6 mM. Results from the research revealed that six (6) hours of shaking time was sufficient to achieve maximum adsorption onto the various biochars. Optimum pH for adsorption occurred at equilibrium pH of 5.7 for rice husk, 6.2 for sawdust, 6.7 for cocoa pod and 7.2 for the rice straw biochar. The isotherms indicated that the amount of P adsorbed increased with increasing equilibrium P concentrations. Increases in equilibrium pH above 7.2 led to decreases in adsorption for all the biochar types. Rice husk and sawdust biochar types were found to have the highest affinity for P with estimated maximum adsorption of 7300 mg/kg P. Phosphate adsorption mechanism varied with biochar type. Surface precipitation of P by Ca and Mg was proposed as an important mechanism of P adsorption on the sawdust biochar. Magnesium precipitation of P was also proposed as a mechanism of P removal by the rice husk biochar. Both electrostatic attraction and ligand exchange reactions by periclase (MgO) with P could be the main mechanisms of adsorption on cocoa pod, rice straw and sawdust xiii biochar types. Phosphorus adsorption via ligand exchange and or electrostatic attraction could have accounted for P removal by the rice husk biochar.