Highly efficient removal of hexavalent chromium from aqueous solution by calcined Mg/Al-layered double hydroxides/polyaniline composites

Highlights

• The organic–inorganic hybrid materials were fabricated by a simple in situ oxidative polymerization.

• The maximum adsorption capacity of Cr(VI) on PANI/LDOs was 409.77 mg/g at pH 2.

• The reaction mechanism for the removal of Cr(VI) was proposed.

• PANI/LDOs was a promising material for Cr(VI)-containing pollution remediation.

Abstract

In this study, the organic–inorganic hybrid materials of polyaniline/Mg-Al layered double oxides (PANI/LDOs) were synthesized by a simple in situ oxidative polymerization. PANI/LDOs have been researched via various characterization technologies and used to remove hexavalent chromium (Cr(VI)) from aqueous solutions. The influences of adsorption kinetics, isotherms, pH, coexisting ion types and strength on Cr(VI) removal were systematically studied. The results indicated that the existence of LDOs reduced the accumulation density of PANI and enhanced the specific surface area, which is beneficial to absorb Cr(VI) by PANI/LDOs. Kinetic studies reveal that the Cr(VI) adsorption onto PANI/LDOs was well fitted by the pseudo-second-order model. The isotherms were well simulated by Langmuir model, and the maximum adsorption capacity of Cr(VI) on PANI/LDOs reached 409.77 mg/g at pH ~ 2.0, which was superior to those of conventional adsorbents. Thermodynamic data revealed that the process of removing Cr(VI) was spontaneous and endothermic. The interaction mechanisms between PANI/LDOs and Cr(VI) were studied, indicating that Cr(VI) (HCrO₄^- and Cr₂O₇^2- were the main species when the aqueous solution pH 2) were reduced to Cr(III) through –NH-/=N- groups after uptaking to the protonated active sites of PANI/LDOs via electrostatic attraction. Furthermore, the PANI/LDOs composites can be regenerated and used repeatedly more than five cycles without significantly reducing in its adsorption capacity. All these results demonstrated that the fabricated PANI/LDOs possess great potentiality for Cr(VI)-containing wastewater treatment.

Introduction

With the development of many industries such as electroplating, mining industry, battery manufacturing and metallurgical processes, various heavy metal ions (e.g., Cr(VI), Cd(II), Pb(II), Cu(II)) are discharged into the surroundings [1], [2]. These heavy metal ions are usually enriched by organisms through food chains, and it will cause a serious harm to the ecosystem even at low concentrations [3], [4]. Among them, Cr(VI) is a recognized hazardous heavy metal, which is high mobility, toxicity, carcinogenicity and mutagenicity to organisms [5], [6], [7], [8] posing a severe threat to human health and ecological environment [9]. The World Health Organization has stipulated that the maximum allowable concentration of Cr(VI) in water is 0.05 mg/L [10]. Therefore, many technologies have been carried out to treat Cr(VI) from sewage through a number of chemical and physical treatment processes [11], such as adsorption [12], [13], oxidation–reduction [14], chemical precipitation [15], photocatalysis [16], [17] and membrane filtration [10]. Among them, adsorption is considered being one of the most prospective technologies owing to its high-efficiency, economy and environmental protection for removing Cr(VI) [18]. Presently, various organic and inorganic materials are used for adsorption Cr(VI) [19], [20], [21]. However, their restricted specific surface area and fewer active sites make it hard to achieve large adsorption capacities. Accordingly, one strategy is to exploit organic/inorganic hybrid materials to increase their overall property in capturing Cr(VI) from solution [22], [23].

Layered double hydroxides (LDHs) are a class of hydrotalcite-like layered materials, its chemical composition can be expressed as [MII 1-xMIII x(OH)₂]^x[(A^n-)_x/n·mH₂O], where M^II and M^III are divalent(Mg²⁺, Ni²⁺, Cu²⁺, Ca²⁺,etc) and trivalent(Al³⁺, Fe³⁺, Mn³⁺,etc) metal ions, respectively [24]. LDH-based nanomaterials have attracted extensive attention in wastewater treatment [25], energy storage [26], sensors [27] and catalysis [28] due to their tunable composition and flexible ion exchange capacity. Calcination treatment as an effective improvement means was generally used to strengthen the surface defects of derivatives, which was favorable for internal reactions [29]. LDHs calcined (layered double oxides, expressed as LDOs) at a certain temperature(<600 °C) may keep the “memory effect” in aqueous solution [29]. Studies have shown that the LDOs had larger specific surface area and more stable chemical properties than original LDHs [30]. Thence, the controlled synthesis of LDOs are conducive to increasing the surface properties and chemical stability, which is beneficial to fine-tuning the binding site and removal rate [31]. But, to a certain extent, the adsorption capacity of LDOs is still limited by the small number of functional groups on its surface. If the high thermal stability of LDOs is combined with a large number of active sites in organic materials, better adsorption performance may be anticipated.

Polyaniline (PANI) is an organic polymer compound rich in amine and imine groups[10]. Due to the simple synthesis route, low preparation cost, good electrical conductivity and environmental stability [32], [33] it has attracted extensive attention from researchers in various fields. Including fuel cells [34], sensors [35], supercapacitors [36], absorbing materials [37] and so on. PANI excels in adsorbing pollutants like heavy metals and organic dyes [38], [39]. Amine and imine groups onto PANI offer possible reaction sites for these contaminations that can be adsorbed through electrostatic interaction or hydrogen bonding [40], [41]. Das et al. [42] reported that folic acid-polyaniline hybrid hydrogel has a high removal capacity toward anionic dye and Cr(VI) from water. Lyu et al. [43] synthesized magnetic polyaniline, which can effectually adsorb Cr(VI) in wastewater. Incorporating organic PANI and inorganic materials may retain or even improve the main characteristics of each phase, and new performances may produce from the synergistic effect of both components [44]. Wang et al. [45] synthesized PANI/α-zr phosphate hybrid materials to adsorb organic pollutants from water. Gao et al. [16] prepared PANI@MoS₂ composite for Cr(VI) adsorption. Thus, it is hoped to combine PANI and LDOs to prepare the organic–inorganic PANI/LDOs hybrid materials to remove anions is feasible.

Herein, the PANI/LDOs composites were fabricated via hydrothermal-calcination and in-situ oxidative polymerization. The influences of diverse solution conditions such as initial pH, temperature, reaction time and coexisting anionic type were researched via a batch of adsorption experiments. Furthermore, the interaction mechanism of Cr(VI) on PANI/LDOs was discussed through a series of characterization techniques. The results showed a tremendous potential of PANI/LDOs in Cr-containing pollution remediation.