The sorption and desorption of phosphate-P, ammonium-N and nitrate-N in cacao shell and corn cob biochars

The sorption of PO4-P, NH4-N and NO3-N to cacao shell and corn cob biochars produced at 300-350 degrees C was quantified. The biochars were used; (i) as received (unwashed), (ii) after rinsing with Millipore water and (iii) following leaching with Millipore water. In addition to sorption, desorption of PO4-P from the unwashed biochars was quantified. There was no sorption of PO4-P to either washed or rinsed biochars, but following leaching, both biochars adsorbed PO4-P and distribution coefficients (K-d L kg(-1)) were very similar for both materials (10(1.1 +/- 0.5) for cacao shell biochar and 10(1.0 +/- 0.2) for corn cob biochar). The BET surface area and micropore volume increased 80% and 60% for the cacao shell and corn cob biochars following leaching. After 60 d, 1483 +/- 45 mg kg(-1) and 172 +/- 1 mg kg PO4-P was released from the cacao shell and corn cob biochars. NH4-N was sorbed by both unwashed biochars, albeit weakly with K-d values around 10(2) L kg(-1). We speculate that NH4-N could bind via an electrostatic exchange with other cationic species on the surface of the biochar. There was no significant release or sorption of NO3-N from or to either of the biochars. (C) 2013 Elsevier Ltd. All rights reserved.