Utilization of nanostructured iron sulfides to remove metals and nutrients from wastewater
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Wastewater containing metals and nutrients is a global environmental issue, threatening human life and natural ecosystems. Iron sulfide minerals are abundant on Earth and are commonly discarded as mine wastes, causing acid mine drainage (AMD). Thus, economically beneficial use of iron sulfides is of great significance, and one option is to use iron sulfide minerals to develop functional environmental materials, which can be applied into environmental pollution control. Nanostructured pyrrhotite (NPyr) was manufactured by calcination of pyrite mineral under N2 atmosphere. This Ph.D. research was aimed at the assessment of efficiencies and mechanisms of NPyr into metal and nutrient removals from wastewater using laboratory-scale column reactors (diameter, 10 mm; height, 50 cm). NPyr was added to Fixed–bed columns and used to test for the removal and recovery of Cu, Pb, Cd, and Zn from the single–metal and Cu–Pb–Cd–Zn multi–metal solutions. Results showed that the removal capacities of Cu and Pb were 77.42 mg∙g-1 and 73.68 mg∙g-1 NPyr from single–metal solutions, and were 30.79 mg∙g-1 and 10.86 mg∙g-1 NPyr from the Cu–Pb–Cd–Zn multi–metal solution. The Cu and Pb contents in the used NPyr particles were up to 17.4% and 15.4% in the single sorption column, and 6.8% and 2.5% in the multi–metal sorption column, respectively. The contents of Cu and Pb were high enough, so it would be economically feasible to extract Cu and Pb from the used NPyr particles by means of direct extractive metallurgy. The sequential extraction of the metals, X–Ray diffraction (XRD), and transmission electron microscopy (TEM) analyses showed that the major mechanisms for Cu and Pb removal by NPyr were precipitation and dissolution reactions via the formation of covellite (CuS) and galena (PbS). Longterm Cu removal from real acid mine drainage (AMD) was investigated using a two-column reactor system consisting of Column A (added with limestone as neutralizer) and Column B (added with NPyr). The breakthrough capacity was 21.93 mg Cu∙g-1 NPyr, and the maximum Cu content was up to 9.2% in the used NPyr in Column B. The mechanisms and efficiencies of NPyr-based autotrophic denitrification for simultaneous nitrogen (N) and phosphorus (P) removal from secondary treated wastewater was investigated using two identical biofilters. The hydraulic retention time (HRT) of the nanostructured pyrrhotite autotrophic denitrification biofilters (PADBs) was gradually reduced from 7.2 to 0.6 h over a 536-day trial. Average concentrations of N of 0.05 mg∙L-1 and P of 0.03 mg∙L-1 in the treated effluent were achieved at a HRT of 1.2 h when treating real secondary effluent which contained N of 13.81 mg∙L-1 and P of 2.44 mg∙L-1. The low concentrations of N and P achieved in the nanostructured PADB effluent at very short HRTs indicate the potential of this technology for tertiary wastewater treatment and in meeting strict discharge standards. High-throughput sequencing of 16S rRNA genes showed that Thiobacillus was the most dominant genus (up to 87% relative abundance) in the PADBs. TEM analysis of the used NPyr indicated that P was mainly removed by the precipitation of FePO4(s). A significant SO42- reduction with 32.50–58.01 mg∙L-1 was observed in the nanostructured PADBs treating real secondary effluent. This observation highlights the sustainability of the nanostructured PADB technology. This Ph.D. study shows the potential of synthesized NPyr as i) a novel, cost-effective sorbent for metal removal and recovery, in particular, Cu, from real AMD, and ii) biofilm substratum for autotrophic denitrification in nanostructured PADB technology as tertiary treatment for wastewater. It is suggested that the application of NPyr into wastewater treatment should be demonstrated in a large scale reactor.