Efficient recovery of phosphate from urine using magnesite modified corn straw biochar and its potential application as fertilizer

Conventional phosphate fertilization is costly, carbon intensive, induces water pollution, and depletes natural phosphate ores, calling for alternative circular sources such as recycling phosphate from urine. However, actual phosphate adsorbents are moderately efficient and generate additional waste. Magnesite-modified biochar was synthesized by co-pyrolysis of corn straw and magnesite, which was used to adsorb phosphate in simulated urine. We compared the properties with other magnesium-based biochar materials and conducted pot experiments to assess feasibility of the saturated magnesite-modified biochar using as an alternative phosphate fertilizer. The results showed that the maximum adsorption capacity of magnesite-modified biochar for phosphate in urine was 176.16 mg/g, with the primary adsorption mechanisms involving electrostatic attraction and ligand exchange. Pot experiments showed that after 21 days, the germination rate and plant height of plants were 5.0–5.8 and 9.4–10.0 times respectively higher than those of plants in soil without biochar application. Overall, the prepared magnesite-modified biochar promises as an adsorbent with dual benefits in water treatment and phosphorus recovery. The saturated magnesite-modified biochar is proved as a good alternative phosphate fertilizer with carbon fixation, emission reduction and phosphorus cycling functions.