Recovering phosphate from sludge-incinerated ash by modified biochar as a soil conditioner and P-fertilizer

Phosphate (P) recovery by directly selective adsorption from the acidic leachate of sludge-incinerated ash (SIA) becomes more attractive due to avoiding heavy metal removal, and biochar from agricultural biomass could be applied for adsorption. However, the surface of biochar is negatively charged and is not effective to adsorb phosphate. Thus, biochar needs to be modified by some metals. With this study, Ca2+ and Mg2+ were used to modify biochar, and then the modified biochar was used to directly adsorb phosphorus from the acidic leachate of SIA. With P-adsorbed biochar used as the slowly-released P-fertilizer, the pot tests were carried out to check the P-fertilizer efficiency and safety. The results indicate that the modified biochar could complete > 80% of adsorption on phosphate, achieving a maximal adsorption capacity, up to 56.20–93.76 mg P/g biochar. The slowly-released effect of the P-adsorbed biochar was quite good, and the release rate was about 50%-60% within 15 d. The pot trials discovered that the P-adsorbed biochar could be directly used as a slowly-released P-fertilizer, which had the obvious effect on the growth of lettuce, and could also improve pH of the acidic soil and promote the absorption and utilization of nutrients by plants.