Organic particles and high pH in food waste anaerobic digestate enhanced NH₄⁺ adsorption on wood-derived biochar

Biogas residues (i.e., digestate) are rich in NH₄⁺ that has great agricultural value but environmental risk if not recycled. Biochar can be an effective adsorbent retaining NH₄⁺ from digestate. However, it remains unclear how the unique composition of digestate affects the capacity and mechanisms of NH₄⁺ adsorption on biochar. This study examined the mechanisms and driving factors of NH₄⁺ recovery from digestate containing different molecular-weight organic particles by using wood-derived biochar with or without H₂O₂ modification. Four solutions were prepared, including pure NH₄⁺, synthetic NH₄⁺ with multiple cations mimicking digestate solution, supernatant of digestate with small organic particles and dissolved organic matter, and digestate mixture containing supernatant and large organic particles. The results showed that compared with pure NH₄⁺ solution, the adsorbed NH₄⁺ was 42% lower in the synthetic NH₄⁺ solution with multiple cations but was 2.2 time higher in the supernatant of digestate on two biochars following 48-h adsorption. Modified biochar did not change NH₄⁺ adsorption in pure NH₄⁺ solution despite higher specific surface area than raw biochar, but it increased the adsorption of NH₄⁺ in digestate solutions with high pH (e.g., 4.03 vs. 3.37 mg N g⁻¹ for modified and raw biochar, respectively, in the supernatant of digestate). Compared with the supernatant, the large organic particles in digestate mixture significantly but slightly decreased NH₄⁺ adsorption on modified but not raw biochar. The desorption rate of NH₄⁺ on the biochar was up to 74%–100%, and it was not supressed by the adsorption of organic particles in digestate. The findings here demonstrate the dominant role of electrostatic attraction in NH₄⁺ adsorption, the important role of high pH and organic particles in digestate in facilitating NH₄⁺ adsorption on biochar, and the suitability of the wood-derived biochar in recovering NH₄⁺ from digestate and releasing N for agricultural application.