Ca(OH)2 coated nanoscale zero-valent iron as a persulfate activator for the degradation of sulfamethazine in aqueous solution

Abstract

In this work, a novel composite, Ca(OH)2 coated nanoscale zero-valent iron (denoted as nZVI-Ca(OH)2), was synthesized and used as an activator of persulfate for the degradation of sulfamethazine (SMT). The effects of sonication time during composite synthesis, pH, nZVI-Ca(OH)2 dosage and typical groundwater components were investigated by batch experiments. Sonication time during the synthesis of the composite could affect the thickness of Ca(OH)2 shell, however, it exerted no obvious effect on SMT removal by nZVI-Ca(OH)2 activated persulfate. The initial pH also had insignificant effect on SMT removal in nZVI-Ca(OH)2/persulfate system. There was an optimum dosage of nZVI-Ca(OH)2 composites for the activation of persulfate, and the SMT removal efficiency decreased at both higher and lower dosage. The efficient performance of nZVI-Ca(OH)2 in synthetic groundwater was observed over a wide pH range (3–9). However, with the presence of high concentration of HCO3− or SO42−, SMT removal was inhibited greatly due to the quenching of radicals by HCO3− or SO42− and the buffering effect of HCO3−. In addition, the longevity of nZVI-Ca(OH)2 was also investigated. Even after 30 days of aging in the air, Fe0 could still be detected in nZVI-Ca(OH)2. Despite some loss of Fe0, the composites kept high reactivity for activation of persulfate for SMT removal, which might be attributed to the excellent activation ability of amorphous iron oxides. To sum up, the Ca(OH)2- shell can protect the inner iron core and consequently prolong the lifetime of nZVI.