Mechanochemically ball-milled zerovalent iron and ferrous composite for effective removal of various metal(loid)s from water

Highlights

• Mechanochemically ball-milled ZVI and Fe²⁺ composite (Fe²⁺-ZVI^bm) was prepared.

• Fe²⁺-ZVI^bm was superior to many other enhanced-ZVI techniques for Se(VI) removal.

• Fe²⁺-ZVI^bm could effectively remove a variety of metal(loid)s.

• Fe²⁺-ZVI^bm could hold its reactivity under various storage conditions.

• Fe²⁺-ZVI^bm was effective for treating real wastewater in semi-continuous flow mode.

Abstract

As a clean, fast, and efficient strategy for chemical synthesis, mechanochemical ball-milling approach has been developed promisingly to prepare functional materials for water treatment. Based on our previous findings that both the accelerated corrosion and facilitated electron-transfer are crucial for contaminants removal by zerovalent iron (ZVI), ball-milled FeCl₂ and ZVI (Fe²⁺-ZVI^bm) was synthesized for metal(loid)s sequestration in this study. The results showed the complete removal of Se(VI) was obtained by Fe²⁺-ZVI^bm and there was no residual Fe²⁺ in the solution after Se(VI) removal. Compared to other enhanced-ZVI techniques, Fe²⁺-ZVI^bm showed evident superiorities for Se(VI) removal in many aspects, including removal rate, removal capacity, utilization ratio of reductants, electron efficiency of reductants, specific removal capacity, adaptability of initial pH, and operation cost. The influence of storage condition on the performance of Fe²⁺-ZVI^bm was examined and the performance of Fe²⁺-ZVI^bm for Se(VI) removal could be ensured when it was sealed-stored in air. The rates and capacities of various metal(loid)s (V(V), Cr(VI), Cu(II)-EDTA, As(III), As(V), Se(IV), Sb(III)-tartrate, Sb(V), and U(VI)) removal by Fe²⁺-ZVI^bm were increased by 2.2–32.4 times and 2.8–11.6 times, respectively, compared to those by pristine ZVI. In the semi-continuous flow reactor, Fe²⁺-ZVI^bm synthesized with industrial-grade ZVI powders displayed advantages on the removal of target contaminants (V, P, Se, As) in two practical wastewaters, further revealing its great potential for application.

Introduction

As a reagent for groundwater remediation and wastewater treatment, zerovalent iron (ZVI) is capable of removing various contaminants via multiple mechanisms including reduction, adsorption, coprecipitation, and generation of reactive oxidizing species [1]. Moreover, ZVI is economical, easily-accessible, and environmentally-friendly [2], [3], [4], [5]. Thus, ZVI has attracted considerable attention in the field of water pollution control in the past three decades. However, owing to the passive layer produced during the manufacturing process, the micrometer-sized ZVI generally has low reactivity towards various contaminants [6]. Consequently, a variety of countermeasures, mainly including reductive pretreatment [5], [7], fabrication of bimetals [8], [9], coupling with chemicals [10], [11], [12], and doping with heteroatoms [13], [14], [15], [16], have been developed to modify ZVI for enhancing its reactivity. There are two main approaches to synthesize novel modified-ZVI materials: aqueous-phase synthesis method and mechanochemical synthesis method [17]. The preparation of modified-ZVI material with the aqueous-phase method conventionally requires the use of excessive solvents and causes secondary pollution. Moreover, the aqueous-phase method has several drawbacks including the complex operation and the limited yield.

As a reawakening and environmentally-friendly approach of chemical synthesis, mechanochemical method, especially ball-milling, has been advancing rapidly over the past decade [18], [19], [20]. Mechanochemical ball-milling is a solvent-free process which eliminates the use of bulk solvent and wastewater generation, offering opportunities for material synthesis in a simpler and cleaner way than conventional methods [21], [22]. Emerging as an alternative to aqueous chemical synthesizing methods, mechanochemical strategies are not only fast, fertile, and efficient in synthesizing various target products [23], but also create novel materials previously hard or impossible to synthesize in solution [24], [25]. Recently, mechanochemical techniques have been introduced to fabricate iron-based materials for contaminants removal in water. For instance, ZVI was directly ball-milled to a smaller size for the removal of chromate [26], azo dyes [27], and dichlorodiphenyltrichloroethane [28]. Ball-milling ZVI with carbon-based materials (e.g., activated carbon, biochar, and carbon fiber) [29], [30], [31], [32], [33], sulfur-based materials (e.g., sulfur powder, FeS, and FeS₂) [34], [35], [36], and nitrogen-based materials (e.g., melamine) [37] have been employed to prepare carbon-modified ZVI, sulfidated ZVI (S-ZVI^bm), and N-doped ZVI, respectively, for reductive removal of various contaminants. In our previous study, we systematically compared the performance of some promising enhanced-ZVI techniques for selenite (Se(VI)) removal [38]. Among them, sulfidation pretreatment by ball-milling could obviously accelerate Se(VI) removal by ZVI, while dosing Fe²⁺ could evidently improve the capacity of ZVI for Se(VI) reduction [38]. Subsequently, we found that Se(VI) could be reduced rapidly and completely by ball-milled S-ZVI^bm coupled with dosing Fe²⁺ (S-ZVI^bm/Fe²⁺) [39]. It was revealed that sulfidation accelerated ZVI corrosion under aerobic condition through the introduction of sulfur impurity while Fe²⁺ dosing facilitated electron transfer by promoting the in-situ generation of Fe₃O₄ [39].

Nevertheless, the introduction of additive elements may bring problems of the secondary pollution. For example, a large amount of SO₄²⁻ may be left in the solution after contaminants sequestration by S-ZVI^bm [40], while the application of N-doped ZVI could cause ammonia pollution [41]. Moreover, for inflammable and explosive materials, such as elemental sulfur, ball-milling procedure under high temperature may bring the risk of safety accidents. Currently, ball-milling ZVI with iron-based materials, particularly Fe²⁺-containing chemicals (e.g. Fe₃O₄ and FeC₂O₄) and single-atom Fe materials, has inspired great curiosity [37], [42], [43], [44]. In particular, a previous study showed that simply ball-milling FeCl₂, Fe₃O₄, and ZVI together could greatly improve ZVI’s reactivity towards nitrobenzene reduction under anaerobic condition [44]. Based on the research progress above, we believed that the strategy of ball-milling iron-based compounds with high reducing reactivity (such as FeCl₂) and ZVI directly to synthesize modified-ZVI material is promising for reductive removal of contaminants. Our preliminary experiment results have verified that the performance of ball-milled FeCl₂ and ZVI (Fe²⁺-ZVI^bm) for Se(VI) sequestration was notably superior to that of S-ZVI^bm/Fe²⁺. Compared to the enhanced-ZVI systems like N-doped ZVI, S-ZVI^bm/Fe²⁺, and ball-milled FeCl₂/Fe₃O₄/ZVI, Fe²⁺-ZVI^bm has obvious advantages including reduced dose of chemicals, avoided secondary pollution, saved cost, and simplified operation.

To address this issue, taking Se(VI) as a probe contaminant, we extensively investigated the performance of ball-milled Fe²⁺-ZVI^bm for Se(VI) removal under various reaction conditions in this study. It should be specified that the rationale of selecting Se(VI) as the target contaminant relies on the fact that Se(VI) could be reduced by ZVI but was hard to be removed by conventional methods such as adsorption, coagulation, and lime softening [45], [46], [47]. The change of the properties of a material during storage is a critical factor for applying this material in real practice but was rarely investigated in the literature. Hence, the performance of Fe²⁺-ZVI^bm stored under different conditions was investigated in this study. In order to investigate the broad spectrum of fabricated Fe²⁺-ZVI^bm, the performance of Fe²⁺-ZVI^bm for removal of ten metal(loid)s was evaluated in this study. The performance of Fe²⁺-ZVI^bm for treating two real wastewater samples was also inspected by employing a semi-continuous flow reactor to examine the feasibility of applying Fe²⁺-ZVI^bm in real practice.