Functional Group Analysis for Screening Biochars for Nitrogen Recovery from Human Urine

Applying biochar enriched with total ammonia nitrogen (TAN) recovered from human urine to soils offers a promising approach to improve global sanitation, agricultural productivity, and close the nitrogen cycle. Despite its potential, biochar's capacity to sorb nitrogen varies significantly, with few studies conducted considering the complex solutions and high total TAN concentrations in urine. We provide evidence for a direct relationship between TAN sorption capacity and biochar functional groups, assessed via bulk oxygen-to-carbon ratio (O:C), surface O:C, and proton density (δσH). These measures serve as screening tools to identify effective biochars for TAN recovery. Biochars with bulk O:C of 0.13–0.31, surface O:C of 0.15–0.35, or δσH of 0.01–0.25 mmol c/g exhibited TAN sorption capacities of 0.2–2.5 mmol TAN/g. We observed an unanticipated behavior; as equilibrium TAN activity approached full-strength synthetic urine, the TAN sorption capacity increased dramatically to previously unreported values, warranting further investigation. To contextualize our findings, we estimate around 3% of TAN from human urine across African countries could be recovered by select biochars. This work strengthens evidence for biochar's TAN sorption potential and presents screening methods to identify optimal biochars for low-resource settings.