Highly efficient As(III) removal in water using millimeter-sized porous granular MgO-biochar with high adsorption capacity

Highlights

• Porous granular MgO-embedded biochar are prepared by simple gelation-calcination method.

• g-MgO-Bc exhibits exceptional adsorption capacity for As(III) (249.1 mg/g).

• As(III) (300 μg/L) could be lowered far below 10 μg/L using only 0.3 g/L g-MgO-Bc-2.5.

• Operation across a broad range of geochemical conditions.

• Fixed-bed column exhibits high treatment volume and good reusability.

Abstract

Biochar adsorbents for removing As(III) suffer from the problems of low adsorption capacity and ineffective removal. Herein, a granular MgO-embedded biochar (g-MgO-Bc) adsorbent is fabricated in the form of millimeter-sized particles through a simple gelation-calcination method using chitosan as biochar sources. High-density MgO nanoparticles are evenly dispersed throughout the biochar matrix and can be fully exposed to As(III) through the rich pores in g-MgO-Bc. These features endow the adsorbent with a high adsorption capacity of 249.1 mg/g for As(III). The g-MgO-Bc can efficiently remove As(III) over a wide pH of 3–10. The coexisting carbonate, nitrate, sulfate, silicate, and humic acid exert a negligible influence on As(III) removal. 300 μg/L of As(III) can be purified to far below 10 μg/L using only 0.3 g/L g-MgO-Bc. The spent g-MgO-Bc could be well regenerated by simple calcination. In fixed-bed column experiments, the effective treatment volume of As(III)-spiked groundwater achieves 1500 BV (30 L) (3 g of adsorbent, solution flow rate of 2.0 mL/min, C₀ = 50 μg/L). The Mg(OH)₂ generated in situ in g-MgO-Bc is responsible for the adsorption of As(III) through the inner-sphere complex mechanism. The work would extend the potential applicability of biochar adsorbent for As(III) removal to a great extent.

Introduction

Arsenic (As) pollution of groundwater has been a severe environmental issue for decades (Nordstrom, 2002, Argos et al., 2010). Around 150 million people in more than 70 countries are chronically exposed to a toxic level of arsenic due to the consumption of arsenic-contaminated water (Brammer and Ravenscroft, 2009). As a confirmed Class I carcinogenic element, long-term drinking of water with excessive arsenic can cause serious health hazards to humans, such as cancers, neurological disorders (Brown and Ross, 2002). Soluble arsenic in aqueous environment mainly existed in the form of arsenite (As(III)) and arsenate (As(V)) (Cullen and Reimer, 1989). The predominant species of arsenic in groundwater is usually As(III) (Sorg et al., 2014, Chang et al., 2010). Unlike As(V) that exists as anions H₂AsO₄^- and HAsO₄^2-, As(III) exists as the neutral H₃AsO₃ below pH 9.2, which is more mobile and difficult to be removed due to its low affinity to most adsorbents (Mohan and Pittman, 2007). Furthermore, As(III) is more toxic than As(V) (Jain and Ali, 2000). Hence, the development of efficient, stable, and simple techniques for As(III) removal is urgently needed.

To tackle this need, various techniques have been developed and investigated for As(III) removal, such as ion exchange (Miller et al., 2000), membrane filtration (Weng et al., 2005), reverse osmosis (Akin et al., 2011), electro-coagulation (Bandaru et al., 2020) and adsorption (Zhang et al., 2013, Wei et al., 2019). Among them, adsorption is very promising due to its simplicity and low cost (Wei et al., 2019, Li et al., 2016). Accordingly, various adsorbents have been developed for this purpose. Among them, metal oxides modified biochars are highly preferred for As removal (Li et al., 2018). It combines the advantages of metal oxides and biochar. Metal oxides possess a specific affinity toward arsenic (Hristovski and Markovski, 2017). The biochar is stable, environmentally friendly, widespread availability, and low cost, and can be easily employed in fixed-bed columns due to its granular characteristics (Hu et al., 2015). Despite these advances, iron-modified biochar are always under the spotlight (Li et al., 2018). Because these adsorbents are prepared by pyrolyzing metal ions-impregnated biomass or directly pyrolyzing metal salt onto biochar, most of the produced metal oxides are inaccessible to As due to its unreasonable distribution and pore blockage (Wei et al., 2019, Ling et al., 2017). Furthermore, the affinity of As(III) to most metal oxides is weak (Mohan and Pittman, 2007, Dixit and Hering, 2003). As a result, the currently reported metal oxides modified biochars suffer from the fundamental problem of low adsorption capacity (Li et al., 2018).

To tackle this problem, a metal oxide with high affinity toward As(III) is first needed. Among various metal oxides that examined for As(III) removal from aqueous solution, magnesium oxide (MgO) exhibited extraordinary high adsorption capacity for As(III), which reached a staggering 500–900 mg/g (Purwajanti et al., 2016, Tresintsi et al., 2014, Liu et al., 2011, Yang et al., 2016, Jia et al., 2013). Secondly, a rational distribution of MgO in suitable biochar is also important. The biomass and metal salt should be dissolved in a specific solvent to form a homogeneous colloid and subsequent carbonization may make the generated metal oxides uniformly dispersed in the biochar. Chitosan, an abundant biomass, can be easily dissolved in a dilute acid solution, and the resulting colloid will be converted into a hydrogel when added to an alkaline solution (He et al., 2016). Freeze-drying is a highly efficient technology to prepare porous biochar from hydrogel (Wei et al., 2019, Liu et al., 2019). These features make it an ideal biochar precursor for the preparation of MgO-loaded biochar.

In this study, the macroporous magnesium-impregnated chitosan beads were first prepared by gel-bead method and freeze-drying, and then carbonized to obtain macroporous granular MgO-embedded biochar (g-MgO-Bc). The generated MgO was evenly dispersed in the entire biochar, and the abundant macroporous of the g-MgO-Bc made these MgO fully exposed to As(III), resulting in high adsorption capacity. Due to its granular characteristics, the g-MgO-Bc also possesses the advantage of convenient operation for fixed-bed column operation. Batch experiments were performed to investigate its adsorption capacity, kinetics, the influences of the solution chemistry, and regeneration. The underlying removal mechanisms were analyzed. Fixed-bed column experiment with As(III)-spiked natural groundwater was employed to evaluate its potential for practical application.