Mechanochemically incorporating magnesium sulfate into antigorite to provide active nucleation sites for efficient precipitation of cadmium ions from weak acidic solution

Abstract

Utilization of natural clay minerals for the treatment of heavy metal cadmium contamination is appealing as the affordable and readily accessible raw materials. However, the low reactivity of natural serpentine limits its practical application for Cd removal. In the present study, mechanochemical activation of antigorite-type serpentine (SP) as example was introduced to enhance its removal capacity for heavy metal of cadmium high enough for practical use. It was found ball-milling at 600 rpm for 60 min for antigorite resulted in the increased release of hydroxyl group to facilitate the precipitation of Cd2+, giving a capacity of 27.4 mg/g for the treatment of 100 mg/L Cd2+ for 120 min at room temperature, which was 10 times higher than that of the pristine antigorite (2.5 mg/g). More significantly, magnesium sulfate (MgSO4, MS) was introduced for the first time to process antigorite, thus to form MgSO4-incorporated antigorite. As a result, the removal capacity for Cd2+ was dramatically increased to 239.7 mg/g with the equal antigorite dosage (the molar ratio of SP/MS = 1:0.5), which is also much higher than the other reported clay minerals. Results showed that, MgSO4 incorporation promoted the reactivity of antigorite and provided numerous SO42− active sites, which allowed the heterogeneous nucleation of basic cadmium sulfate (CdSO4·3.5 Cd(OH)2·xH2O) precipitate on antigorite, therefore not requiring high alkalinity support as the conventional formation of cadmium hydroxide does. Correspondingly, under the new mechanism, the Cd precipitation could take place in a wide pH range, even from pH 3.0, which was a rarely reported phenomenon happening on natural minerals. Based on these findings, this study demonstrated the effectiveness of mechanochemical incorporation of sulfate for enhancing the Cd2+ removal capacity of serpentine, as well as the high efficiency of new pathway for Cd2+ precipitation. Moreover, the potential of low-cost serpentine as alternative stabilizers for the highly-effective remediation of heavy metal contamination may be expected.