Relevant surface reactions involving carbonates for heavy metals removal

In this work, marble waste powder resulting from cutting processes from a Sardinian quarry was used as a sorbent for wastewater treatment. For the study, an analytical methodology combining ex situ XPS surface chemical (XPS) and morphological (SEM) characterization and solution analysis (pH, ICP-AES) was developed. The “as received” sorbent has been characterized in depth by both bulk (XRD, ICP-AES, ICP-MS) and surface analytical techniques (XPS). Marble waste was found to be a calcite isomorph containing magnesium (2.8 g kg-3) as well as with strontium (Sr), iron (Fe), aluminum (Al), and manganese (Mn) with concentrations ranging between 50 and 150 mg kg-3. The interaction of marble waste micro-particles was first studied with Pb2+-bearing model solutions via batch experiments in close system. Solutions with lead (Pb2+) concentrations, ranging from 0.02 mg dm-3 to 16,576 mg dm-3, have been exploited before and after interaction with marble waste and commercial calcite micro-particles. Pb2+ removal efficiency was found to be very high for both sorbent materials: it is nearly100% when initial Pb2+ concentrations ranged from 20.7 mg dm-3 to 16,576 mg dm-3. XPS spectra of marble waste or commercial calcite following interactions with Pb2+-bearing model solutions, combined with SEM micrographs, energy dispersive X-ray (EDX) spectra, and XRD patterns spectra showed the presence of (co)precipitated calcium and lead carbonate phases onto calcite surfaces; lead carbonate phases consist of cerussite (PbCO3) and hydrocerussite [(PbCO3)2Pb(OH)2]. In a second step, the interaction of marble waste micro-particles with Mn2+-bearing model solutions was investigated; the effects of both Mn2+ concentration and contact time were examined. Chemical state identification based on XPS signals allowed the identification of the oxidation state as Mn2+ and manganese (co)precipitated phase onto marble waste calcite surfaces. ICP-AES analyses revealed a very high Mn removal efficiency when Mn2+ concentration was up to 5.494 mg dm-3 but II gradually decreased at higher initial Mn2+ concentrations, probably due to hetero-epitaxial growth of MnCO3 onto calcite surface that inhibit further calcite dissolution. The interaction of multi-element (Mn, Zn, Cd, Co, Ni, Pb) model solutions mimicking a potential real wastewater with marble waste micro-particles was investigated in the last part of this work. A manganese inhibition effect was observed relative to the other metal carbonates precipitation and therefore metal removal from the solution. For this reason, Zn2+, Cd2+, and Mn2+ residual concentrations were found to be above legal limit values even if their initial concentration was reduced by about 45%, 90%, and 30% respectively. Using marble waste powder as sorbent for water remediation is in line with the "circular economy action plan", one of the main building blocks of the European “Green Deal” policy. The results show excellent removal efficiency for lead (Pb2+) contaminated wastewater, whereas the simultaneous presence of manganese (Mn2+) decreased the efficiency. Further studies are needed to clarify selective precipitation phenomena and competitive interaction between different toxic elements present.