Ball milling enhanced Cr(VI) removal of zero-valent iron biochar composites: Functional groups response and dominant reduction species

Highlights

• BM-ZVI/BC with well-dispersed iron was successfully prepared by ball milling.

• The ball milling improved the electron transfer ability of ZVI/BC.

• Fe species accounted for 66.1% in the removed Cr(VI).

• Phenolic –OH and Fe (0) enhanced adsorption affinity and electron transfer.

Abstract

Zero-valent iron biochar composites (ZVI/BC) have been widely used to remove Cr(VI) from water. However, the application of ZVI/BC prepared by the carbothermal reduction was limited by the non-uniform dispersion of ZVI on the biochar surface. In this work, ball milling technique was introduced to modify ZVI/BC. Results showed that after ball milling, the maximum Langmuir adsorption capacity for Cr(VI) was 117.7 mg g⁻¹ (298 K) which was 2.08 times higher than ZVI/BC. The initial adsorption rate of the Elovich model increased from 4.57 × 10² mg g⁻¹ min⁻¹ to 3.74 × 10⁹ mg g⁻¹ min⁻¹ after ball milling. Dispersibility of ZVI on biochar surface and contact between ZVI and biochar were improved by the ball milling, thus accelerating the electron transfer. Besides, ball milling increased the content of oxygen-containing functional groups in biochar, contributing to the chemisorption of Cr(VI). The response sequence of oxygen-containing functional groups was analyzed by two-dimensional correlation spectroscopy, indicating that Cr(VI) preferentially complexed with phenolic –OH. Shielding experiments showed that Fe (0) was the dominant reducing species with a contribution of 73.4%, followed by surface-bound Fe(II) (21.3%) and dissolved Fe²⁺ (5.24%). Density functional theory calculations demonstrated that ball milled ZVI/BC improved the adsorption affinity and electron transfer flux towards Cr(VI) by introducing phenolic –OH and Fe (0). Combining all the textural characterization, the Cr(VI) removal mechanism of the ball milled ZVI/BC could be proposed as adsorption, reduction, and precipitation. Eventually, stable Cr–Fe oxides (FeOCr₂O₃ and Cr_1·3Fe_0·7O₃) were formed. This work not only provides a simple method to modify ZVI/BC to remove Cr(VI) in water efficiently and rapidly, but also improves the mechanistic insight into the Cr(VI) removal by iron-carbon composites via the response sequence of functional group analysis and the quantitative analysis of reducing species.

Introduction

Industrial processes (e.g., mineral extraction, electroplating, textile dyeing, leather tanning) produce tons of chromium-containing wastewater, leading to serious environmental issues (Li et al., 2021a). In the natural environment, chromium (Cr) exists in the form of trivalent chromium (Cr(III), Cr(OH)₃ and Cr₂O₃ precipitates) and hexavalent chromium (Cr(VI), CrO₄²⁻/Cr₂O₇²⁻/HCrO₄⁻ oxygenated anions), (Miretzky and Cirelli, 2010). The mobility and toxicity of Cr depend on its form. For instance, Cr(VI) has much higher mobility and 100 times more toxicity than Cr(III) (Ghadikolaei et al., 2019), which has been classified as a priority pollutant in China (Sun et al., 2022). The maximum Cr(VI) concentration in drinking water had been regulated as 0.05 mg L⁻¹ by the World Health Organization (WHO, 2022). However, 30–200 mg L⁻¹ is the typical Cr(VI) concentration in water contaminated with Cr(VI) (Lyu et al., 2017). Various treatment methods (e.g., adsorption, membrane filtration, ion exchange, and electrochemical methods) have been employed to remove Cr(VI) from water (Zheng et al., 2021). Adsorption stands out for its advantages of high efficiency, low cost, and simple operation.

Biochar (BC) is a low-cost and environment-friendly material in the remediation of Cr(VI) wastewater, and has abundant oxygen-containing functional groups and a peculiar conjugated graphitic structure (Li et al., 2018), which can provide appreciable adsorption sites for Cr(VI) and act as an electron shuttle to enhance Cr(VI) reduction, respectively (Chen et al., 2021). However, the primitive biochar had a low Cr(VI) adsorption capacity (3.2 mg g⁻¹) (Li et al., 2020) due to its poor pore structure and low specific surface area (Li et al., 2017; Liu et al., 2020), which limited its practical applications in wastewater treatment. Meanwhile, zero-valent iron (ZVI) has considerable promise for Cr(VI) removal due to its strong reduction ability, but its reactivity is limited by the property of easy aggregation (Yang et al., 2021). The loading of ZVI on biochar can prevent the aggregation of ZVI (Sun et al., 2022). NaBH₄ reduction and carbothermal reduction are two common methods for the preparation of ZVI loaded biochar. The carbothermal reduction method reduces iron compounds to ZVI with biochar as a reducing agent via heating at high temperatures without oxygen (Zhang et al., 2020). Compared with NaBH₄ reduction, carbothermal reduction has attracted more attention because it does not require highly toxic chemicals (Li et al., 2019). However, carbothermal reduction was a heterogeneous reaction with nitrogen and iron-dipping biomass, resulting in the non-uniform dispersion of iron atoms and the formation of larger iron clusters on the surface of biochar. This can shield the graphitization structures and weaken the electron transfer capacity (Feng et al., 2021); thus the adsorption capacity for Cr(VI) was relatively low (6.67 mg g⁻¹) (Li et al., 2019). Therefore, further modification of ZVI/BC to increase the exposure of ZVI and the electron transfer capacity of ZVI/BC is necessary.

As a well-known mechanochemical technology, high energy ball milling is a convenient and environment-friendly modification method to improve the physicochemical properties of materials (Kumar et al., 2020; Amusat et al., 2021; Yin et al., 2022). Ball milling method was used to modify iron-loaded biochar, which increased the specific surface area by 0.93-fold, provided more active adsorption sites and accelerated the adsorption kinetics of reactive red (Feng et al., 2021). In addition, ball milling method has been proved to enhance the dispersibility of iron oxide on biochar surfaces (Zou et al., 2021). The shear force can fracture the choke point of the iron oxide layer and expose new active edge sites on ZVI (Tang et al., 2021; Yu et al., 2022). Therefore, ball milling could be used to promote the adsorption and reduction capacity of ZVI/BC. Iron species Fe (0) and Fe(II) play critical roles in the reduction of Cr(VI) by ZVI/BC. Fe(II) exists in dissolved and surface-bound forms (Hu et al., 2019), of which surface-bound Fe(II) contributed more (85%) removing Cr(VI) (Hu et al., 2020). Furthermore, ball milling method could change the type and content of functional groups on the surface of biochar. The oxygen-containing functional groups could facilitate the adsorption and reduction of Cr(VI) (Zhu et al., 2022). Carboxyl and hydroxyl groups could adsorb Cr(VI) through complexation, whereas phenolic –OH could provide electrons to reduce Cr(VI) to Cr(III) directly (Chen et al., 2021). In a recent study, the order of the main functional groups (-OH > CO > -COOH) during the adsorption of Cr(VI) on ZnCl₂ modified biochar was proved by two-dimensional correlation spectroscopy (Guo et al., 2021). However, the contribution of various Fe species and the response sequence of oxygen-containing functional groups during Cr(VI) removal by BM-ZVI/BC are still unclear.

Herein, our research objectives were to (I) prepare BM-ZVI/BC modified by ball milling to remove Cr(VI) from water efficiently and rapidly, (II) quantify the contribution of dominant reducing species by shielding experiments, (III) determine the response sequence of functional groups using two-dimensional correlation spectroscopy analysis, and (IV) reveal the removal mechanism at the molecular scale by density functional theory calculation.