Pyrite enhanced the reactivity of zero‐valent iron for reductive removal of dyes

Abstract

BACKGROUND Zero‐valent iron (ZVI) is an environmentally friendly and cost‐efficient material for the removal of contaminants. However, its application is challenged by problems such as surface passivation and pH increase in the reaction media. As a potential solution for this problem, pyrite, a waste of the mining industry, was used together with ZVI for reductive removal of azo dyes. RESULTS In comparison with ZVI alone, the ZVI/pyrite mixture showed dramatically enhanced efficiency and reusability for dye removal, although pyrite itself can hardly remove dyes. The reaction rate constant kobs for the removal of Orange II at the initial pH (pH0) of 7.0 was enhanced 9.4‐ to 28.0‐fold by using the mixture at a pyrite/ZVI mass ratio ranging from 1 to 4, respectively. The mechanism studies indicate that pyrite not only helped produce more iron(II) by facilitating iron corrosion and the iron(II)/iron(III) cycle in the mixed system, but also activated the ZVI surface by in situ sulfidation. The high reactivity of the adsorbed iron(II) in the mixed system significantly contributed to dye removal, especially at the alkalescent pH. CONCLUSION Pyrite accelerated the reductive removal of dyes by ZVI, through inhibiting its surface passivation and suppressing the pH increase in the reaction media. Therefore, this research proposes a facile method for enhancing the reactivity and applicability of ZVI in environmental remediation. © 2020 Society of Chemical Industry

Understanding the Operating Mode of Fe0/Fe-Sulfide/H2O Systems for Water Treatment

“…Therefore, mixing Fe-sulfides and Fe 0 corresponds to mixing two reducing agents, and should result in even more reducing systems (without considering the pH shift capacity of Fe-sulfides). Yet studies have been published comparing the relative kinetics and extent of contaminant removal in the following three systems: (i) Fe-sulfide alone, (ii) Fe 0 alone and (iii) the Fe 0 /Fe-sulfide mixture [18,47,48,52,53,55,56,82]. For example, Lü et al [18] observed no nitrobenzene removal in the pure FeS 2 system, moderate removal in the pure Fe 0 system and increased removal in the Fe 0 /FeS 2 system.…”

“…Despite its misleading nature, the term "reactivity loss" is partly maintained here, for the sake of clarity. Several tools have been introduced to address "reactivity loss" [15,37,[44][45][46][47][48], including the amendment of granular Fe 0 with granular iron sulfide minerals, which is the focus of the current study. Other tools as summarized by Lü et al [18] include using the following: fabrication of (dispersed) nanoscale Fe 0 , multi-metallic materials, weak magnetic field and the fabrication of several Fe 0 -based composites.…”

“…Lipczynska-Kochany et al [38] was probably the first research group to use iron sulfide minerals to sustain the Fe 0 reactivity, thereby increasing the efficiency of conventional Fe 0 /H 2 O systems for water treatment. Accordingly, during the past 25 years, a large number of scientific publications have reported on the enhanced efficiency of Fe 0 /Fe-sulfide/H 2 O systems relative to their Fe 0 /H 2 O counterparts [18,29,44,[47][48][49][50][51][52][53]. However, the mechanisms for this enhancement are still to be established [18,44,52].…”

Metallic Iron for Environmental Remediation: Starting an Overdue Progress in Knowledge

“…Clearly, reports justifying the reductive efficiency of Fe 0 /H 2 O systems using the semi-conductive nature of FeCPs are mistaken [96]. This assertion encompasses the Fe 0 /pyrite/H 2 O system, whose efficiency is mainly justified by the semi-conductive nature of FeS species [97][98][99].…”

“…Several hybrid systems have been successfully tested for water treatment including Fe 0 /activated carbon [184], Fe 0 /Fe 3 O 4 [185], Fe 0 /MnO 2 [186], Fe 0 /pyrite [99] and Fe 0 /sand [187]. While inert sand alone has clearly improved the efficiency of Fe 0 systems [188], the efficiency of other tested aggregates was attributed to the specific materials.…”