Preparation of a slow-release fertilizer from MgO-modified biochar and struvite and its capacity for cadmium passivation

This study investigates the development of a slow-release coated fertilizer using magnesium oxide-modified coconut shell biochar (MgO-BC), struvite, and urea. The performance of this coated fertilizer in terms nitrogen and phosphorus release and cadmium (Cd) passivation in Cd-contaminated soil was evaluated through soil column leaching tests and pot experiments. The coated fertilizer, prepared with a mass 6: 2: 1 ratio of MgO-BC: urea: struvite, achieved a compressive strength of 38.6 N/grain. The nitrogen and phosphorus release rates from Cd-contaminated soil treated with the coated fertilizer were 887.391 mu g/d and 2.884 mu g/d, respectively, representing decreases of 5.0 % and 32.8 % compared to soil treated with struvite and urea. The coated fertilizer demonstrated a good Cd fixation effect in Cd-contaminated soil. The coated fertilizer effectively reduced Cd mobility, with the total accumulated Cd leached from the soil decreasing by 73.2 % after ten leaching cycles. Furthermore, the coated fertilizer significantly inhibited Cd uptake by peas ( Pisum sativum L. ), with Cd content in the above-ground and under-ground parts of the plants decreasing by 82.78 % and 68.04 %, respectively, compared to the control. The transport coefficient (TFS/R) of Cd for peas decreased by 46.12 %. The Cd passivation efficiency of the coated fertilizer was 37.76 %, attributed to the transformation of acid extractable/ exchangeable Cd to residual Cd. These results demonstrate that the coated fertilizer, prepared from MgOmodified biochar and struvite, effectively combines slow releasing nutrient properties with Cd passivation capabilities.