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

Biogas residues (i.e., digestate) are rich in NH that has great agricultural value but environmental risk if not recycled. Biochar can be an effective adsorbent retaining NH from digestate. However, it remains unclear how the unique composition of digestate affects the capacity and mechanisms of NH adsorption on biochar. This study examined the mechanisms and driving factors of NH recovery from digestate containing different molecular-weight organic particles by using wood-derived biochar with or without HO modification. Four solutions were prepared, including pure NH, synthetic NH 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 NH solution, the adsorbed NH was 42% lower in the synthetic NH 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 NH adsorption in pure NH solution despite higher specific surface area than raw biochar, but it increased the adsorption of NH in digestate solutions with high pH (e.g., 4.03 vs. 3.37 mg N g 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 NH adsorption on modified but not raw biochar. The desorption rate of NH 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 NH adsorption, the important role of high pH and organic particles in digestate in facilitating NH adsorption on biochar, and the suitability of the wood-derived biochar in recovering NH from digestate and releasing N for agricultural application.