Influence of sulfamethazine (SMT) on the adsorption of antimony by the black soil: Implication for the complexation between SMT and antimony

This paper reported when sulfamethazine (SMT) and antimony (Sb(V)) coexisted in aqueous solution at pH of 3.0, 5.0 and 7.0, the complexation between SMT and Sb(V) occurred. Such a complexation impeded the adsorption of Sb(V) on the black soil. The higher the solution pH value was, the more the amount of Sb(V) was prevented from adsorbing on the black soil. The maximum adsorption capacity (q_m) of Sb(V) at the presence of SMT under pH of 3.0, 5.0 and 7.0 was 5.28, 3.45 and 1.95 mg/g, respectively. -NH₂, N–H, S[dbnd]O and C–N of pyrimidine ring carried by SMT acted as the complexation sites with Sb(V). The complexation constant K were − 3.15, −3.26 and − 3.48 at pH of 7.0, 5.0 and 3.0, respectively, indicating that the complexation strength between SMT and Sb(V) followed the order of pH 7.0 > pH 5.0 > pH 3.0. The binding energy between Sb(V) and the C–N group of pyrimidine ring was the highest (1.42 eV), and then followed by the groups of -NH (1.37 eV), S[dbnd]O (0.66 eV) and -NH₂ (0.39 eV). Besides S[dbnd]O and C–N, Sb(V) tends to complex with N–H via coordination bond at pH of 7.0 while -NH₂ via cation-π interaction at pH 3.0 and 5.0. Compared to pH of 5.0, the strength of cation-π interaction at pH of 3.0 weakened according to the molecular electrostatic potential map. These results demonstrated that different from the situation where Sb(V) exists in aqueous solution alone, the coexistence of SMT with Sb(V) affected the adsorption behavior of Sb(V) in soil and solution pH was also an influence factor. These findings in this paper would be helpful for further understanding the mobility, bioavailability and other environmental behavior of Sb(V) in soil when Sb(V) coexists with antibiotics even other organic compounds.