Dynamic response of pH to NO₂^- and S⁰ directional accumulation during sulfur autotrophic denitrification

Double short-cut sulfur autotrophic denitrification (DSSADN) is capable of reconverting NO₃^--N produced by partial nitrification -Anammox (PN/Anammox) into NO₂^--N, while simultaneously reducing SO₄^2- byproduct pollution. Understanding the nitrogen and sulfur transformation processes as well as the functional enzyme activity changes in DSSADN under varying pH conditions is crucial for achieving synchronous accumulation of S⁰ and NO₂^--N. This study investigated the directional accumulation characteristics of S⁰ and NO₂^--N in sulfur autotrophic denitrification (SADN) under different pH conditions by directionally adjusting the pH within the SADN system. The results showed that within the pH range of 7.5-8.5, the NO₃^--N removal rate (NaRR) and S^2- removal rate (S^2-RR) increases with the increase of the growth of microorganisms. When the pH exceeded 8.5, NaRR and S^2-RR were reduced, and the inhibitory effect intensified with increasing pH, this resulted in a decrease in NaRR and S^2-RR from 1.76 kg/(m³·d) and 4.47 kg/(m³·d) (pH 8.5) to 0.60 kg/(m³·d) and 1.36 kg/(m³·d), respectively. As pH increased, the gene copy numbers of nirk and soxB gradually decreased, while the discrepancy between NaRR, S^2-RR, and the removal rates of NO₂^--N and S⁰ widened, resulting in gradual increases in NO₂^--N and S⁰ accumulation efficiencies, which peaked at 96 % and 91 %, respectively. The accumulation rates of NO₂^--N and S⁰ increased initially and then declined as pH rose from 7.5 to 10, reaching maximum values of 1.29 kg/(m³·d) and 3.31 kg/(m³·d) at pH 8.5. Meanwhile, increasing pH also induced significant changes in microbial community structure, the functional microflora shifted from the full SADN bacterial genus Sulfurimonas (pH 7.5-8.5) to the DSSADN common bacterial genus Thiobacillus (pH 9-10).