Cation-π interaction in Mg(OH)2@GO-coated activated carbon fiber cloth for rapid removal and recovery of divalent metal cations by flow-through adsorption

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Abstract

Sorption is considered a promising approach for removing various divalent metal cations, yet the complicated recovery and regeneration strategy restricts the application of powder-like adsorbents. Herein, we report an innovative Mg(OH)2@GO adsorbents loaded on activated carbon fiber cloth (ACFC) using an electrophoretic deposition process (EDP) of Mg(OH)2 and graphene oxide (GO) suspensions to achieve rapid removal of Cd2+, Pb2+, Ni2+, and Cu2+ via flow-through adsorption over an ultra-short period time. FESEM and HRTEM results revealed a cross-linked structure on the ACFC surface derived from the cation-π interaction between Mg2+ and GO under EDP. Importantly, by virtue of oxygen-containing functional groups and π electron region, Mg(OH)2@GO exhibited high adsorption capacities for Pb2+, Cu2+, Cd2+, and Ni2+, which could reach 647.2, 439.5, 297.3, and 219.1 mg g−1 at 293 K, respectively. Furthermore, Mg(OH)2@GO-ACFC possessed a rapid regeneration capability with EDTA by a flow-through system for divalent metal cations desorption and exhibited excellent repeatability in 5-cycle tests. Overall, we propose the novel flow-through system that facilitates the rapid adsorption and recovery of divalent metal cations for wastewater treatment.

Introduction

Heavy metals such as copper, lead, cadmium, and nickel are released into soil and aquatic environments from wastewater discharges in mining, manufacturing and smelting industries, as well as large-scale use of fertilizers and pesticides in agriculture (Li et al., 2022, Jambeck et al., 2006, Fu and Wang, 2011). Considering their high water solubility, non-biodegradable, toxicity and persistence, heavy metal ions pose a serious threat to environmental safety and human health (Mark and Warshawsk, 2002, Vilela et al., 2016). Currently, new regulations are pushing metal effluent limits to parts per billion (ppb) or even lower levels. Furthermore, monitoring results of the Three Gorges Reservoir from 2008 to 2013 showed that the concentrations of Pb and Cd exceeded the WHO suggested level, China's drinking water guidelines and the USEPA drinking water guidelines (Gao et al., 2016, Deng et al., 2010, Li et al., 2022). Therefore, there is an enormous demand for the effective removal of heavy metals, especially those at low concentrations.

Extensive approaches have been employed to treat heavy metal wastewater, including alkaline precipitation, ion exchange, membrane separation, and adsorption (Zhao et al., 2011). Alkaline precipitation has been used for a long time, however, it can only reduce the concentration to parts per million levels, which is attributed to the finite solubility of metal hydroxyl oxides, and thus, the removal of toxicity indicators (Cd2+, Ni2+, etc.) cannot meet the limits. Ion exchange using resins represents another efficient technology (Kurniawan et al., 2006, Li et al., 2020), while the ion-exchange process is simply motivated by electrostatic and nonspecific interactions with metals. Therefore, competing ions coexisting at high concentrations in water reduce the removal efficiency of ion exchange technology, while resin method requires frequent regeneration.

Sorption proves to be the most effective and widely used method due to its relatively low cost, ease of operation, and fewer harmful secondary products, and especially for nano-materials, which have attracted widely attention at present (Zhang et al., 2014, Hua et al., 2012). Owing to its large specific surface area and abundant surface oxygen-containing functional groups, graphene oxide (GO) exhibits high surface adsorption activity (Tofighy and Mohammadi, 2011). In addition, nanoscale magnesium hydroxide possesses a high reactivity for separating and removing heavy metal ions in wastewater because of its strong adsorption capacity, corrosion resistance, and cost effectiveness (Hu et al., 2011). Nevertheless, nanomaterials have some intrinsic potential problems. On the one hand, the nanomaterials tend to aggregate because of the decrease in electrostatic repulsion between nanomaterials after adsorption, leading to a sharp decline in adsorption efficiency (Sitko et al., 2013). On the other hand, complete separation of nanomaterials from water after adsorption remains difficult, and residual materials are most likely to represent a risk to human health and ecosystems (Zhao et al., 2011). For example, researches have demonstrated severe dose-dependent toxicity of graphene and graphene oxide. Furthermore, when nanomaterials are used in fixed-bed columns or other flow-through systems, the pressure drops excessively, which considerably limits the large-scale application of nano-scale adsorbents (Xia et al., 2011). To overcome these challenges, a new class of hybrid sorbents has recently been developed by incorporating nanomaterials onto porous supported bases to improve their dispersion properties, mechanical stability, and permeability in flow-through systems.

In this study, a novel Mg(OH)2@GO adsorbent with a cross-linked structure was coated on an activated carbon fiber cloth surface (Mg(OH)2@GO-ACFC) via one-step electrophoretic deposition (EPD). A flow-through adsorption system composed of multilayered Mg(OH)2@GO-ACFC was used for adsorption and recovery of divalent metal cations (M2+ = Cd2+, Pb2+, Ni2+, and Cu2+). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were performed to study the chemical interactions between the metal cations and Mg(OH)2@GO, including coordination, electrostatic, and cation-π interactions. Our research provides new insights into the role of the π electron regions and oxygen-containing functional groups in GO structure for rapid adsorption and recovery of M2+ from wastewater.

Section snippets

Materials

Potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), magnesium nitrate hexahydrate (Mg(NO3)2•6H2O), aluminum nitrate nonahydrate (Al(NO3)3•9H2O), Iron(III) nitrate nonahydrate (Fe(NO3)3•9H2O), copper nitrate trihydrate (Cu(NO3)2•3H2O), cadmium nitrate tetrahydrate (Cd(NO3)2•4H2O), lead nitrate (Pb(NO3)2), nickel nitrate hexahydrate (Ni(NO3)2•6H2O), hydrochloric acid (HCl, 36∼38%), sulfuric acid (H2SO4), sodium

EPD Process

Essentially, the EPD process involves two steps: movement of charged colloidal particles under an electric field and electrochemical reduction. Experimentally, the negatively charged GO suspensions were barely deposited on the ACFC cathode even after 30 min when the direct current voltage was 5V (Figure S2a). In contrast, GO bonded with Mg2+ by cation-π interactions in the presence of Mg(NO3)2, thus, allowing the GO-Mg2+ suspensions to migrate more easily towards the ACFC cathode and deposit on

Conclusion

A porous cross-linked graphene oxide (GO) combined with magnesium hydroxide (Mg(OH)2) adsorbent loading on activated carbon fiber cloth (Mg(OH)2@GO-ACFC) was successfully fabricated for high-effective removal of divalent metal cations by electrophoretic deposition of GO-Mg2+ suspensions. The maximum adsorption capacities for M2+ (Pb2+, Cu2+, Cd2+, and Ni2+) reached 647.2, 439.5, 297.3, and 219.1 mg g−1 at 293 K, respectively. The XPS and FTIR results revealed that M2+ adsorption was dominantly

CRediT authorship contribution statement

Xiao Wang: Visualization, Writing – original draft, Data curation. Jingfeng Li: Conceptualization, Methodology, Funding acquisition. Qiang Guo: Conceptualization, Investigation. Menglan Xu: Software, Data curation. Xiaohan Zhang: Data curation, Investigation. Tong Li: Writing – review & editing, Conceptualization, Methodology, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by Science and Technology Project of China Energy Investment Corporation (Grant No. GJNY-20-198 and GJNY-21-129), Open Fund of State Key Laboratory of Water Resource Protection and Utilization in Coal Mining (Grant No. GJNY-21-41-11), and the National Natural Science Foundation of China (Grant No. 21906034).

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