Surface-modified apricot pits as biochar feedstock and phosphate sorbent

Pits from fruit like apricots, peaches and cherries are an under-utilized resource. If there is any use at all, they may be extracted for special vegetable oils. Mostly the pits are combusted or left to rot. However, they are also an appropriate feedstock for pyrolytic carbonization. This study investigated the biochar produced from apricot pits for its potential to sorb phosphate from liquid media and from artificial wastewater.

Shredded apricot pits were pyrolyzed at 450 °C in a lab-scale screw reactor (Pyreka 3.0). Additionally, the impregnation of the feedstock with Mg(OH)₂ before pyrolysis was studied to test the hypothesis that phosphate sorption to biochar takes advantage of metal bridges on the biochar surface.

The results of isotherm sorption experiments showed that the pre-pyrolysis Mg-surface modification of the pits improved the sorption capacity of the biochar up to 42 mg PO₄-P/g whereas the unmodified biochar adsorbed only about one tenth. When KH₂PO₄ was used as the only sorbate, EDX-mapping showed the formation of K-struvite-crystals in the pores of the biochar. Desorption experiments showed a major release of the adsorbed phosphate within a few hours. Sorption competition experiments with phosphate and nitrate showed no negative effect of nitrate on phosphate sorption. Feedstock impregnation with Ca(OH)₂ resulted in more variable sorption dynamics.

The results could be confirmed by deploying the surface-modified apricot pit biochar for the reduction of the phosphate load in artificial wastewater.