Adsorption of organic arsenic acids from water over functionalized metal-organic frameworks

Highlights

• Functionalized MOFs (with OH) were applied for adsorptive purification of water.

• Organic arsenic acids such as phenylarsonic acid were adsorbed most effectively.

• The main adsorption mechanism (H-bond) could be confirmed.

• The direction of H-bond could be defined (adsorbates: H-acceptor; MOFs: H-donor).

• MIL-101(OH)₃ was suggested as highly efficient and recyclable adsorbents.

Abstract

Organic arsenic acids (OAAs) are regarded as water pollutants because of their toxicity and considerable solubility in water. Adsorption of OAAs such as phenylarsonic acid (PAA) and p-arsanilic acid (ASA) from water was investigated over functionalized (with OH groups) metal-organic framework (MOF, MIL-101), as well as over pristine MIL-101 and commercial activated carbon. The highly porous MIL-101 bearing three hydroxyl groups (MIL-101(OH)₃) exhibited remarkable PAA and ASA adsorption capacities. Based on the effects of pH on PAA and ASA adsorption, hydrogen bonding was suggested as a plausible mechanism of OAA adsorption. Importantly, OAAs and MIL-101(OH)₃ can be viewed as hydrogen-bond acceptors and donors, respectively. Moreover, MIL-101(OH)₃ could be regenerated by acidic ethanol treatment, being a promising adsorbent for the removal of PAA and ASA from water.

Introduction

Organic arsenic acids (OAAs) such as phenylarsonic acid (PAA) and its derivatives (e.g., p-arsanilic acid (ASA)) are widely used as broiler feeds to improve feed efficiency, promote rapid growth, control intestinal parasites, and prevent dysentery [1], [2]. PAA is also a degradation product of chemical warfare agents used as sternutatory gases in World War II [3]. OAAs are excreted with manure in unchanged form [4] and accumulate in litter [5], with an arsenic level of 30.6 mg per kg of poultry litter [6]. Since poultry litter and manure are commonly used as fertilizers [7], the OAAs present therein can be biotransformed into highly toxic inorganic arsenic compounds [8], [9], which contaminate the environment. For example, approximately 900 t of (4-hydroxy-3-nitrophenyl)arsonic acid (roxarsone or ROX, a derivative of PAA) are annually released into the environment by the US poultry industry [10]. Moreover, these organoarsenic compounds leach into ground water [11], accumulating in plants as well as entering the human food chain [12]. In 2002, the PAA concentration in well water in Japan was determined as 0.072 mg L⁻¹ (0.027 mg L⁻¹ As) [13]. Importantly, arsenic is considered to be a major water pollutant due to its toxicity, being mainly responsible for human cancer, DNA hypomethylation, and arsenicosis [14]. According to the US Environmental Protection Agency, the permissible level of arsenic in drinking water is less than 10 ppb [15]. Therefore, it is important to remove organoarsenicals from water before they are converted into highly toxic inorganic arsenic compounds.

To date, in contrast to the removal of inorganic arsenic compounds, the corresponding procedures for organoarsenic compounds are scarce, being limited to methods such as the advanced oxidation process (involving H₂O₂ and photooxidation) [16], [17], photocatalytic degradation [18], and adsorption [19], [20], [21], [22]. Among them, adsorption is the most promising technique because of its operation simplicity and cost effectiveness. So far, a limited number of adsorbents such as TiO₂ [23] and soil [24] were used for PAA adsorption from water. Similarly, TiO₂ [1], goethite, Al₂O₃ [2], and an Fe-Mn binary oxide [25] were used for ASA adsorption from water.

Metal-organic frameworks (MOFs) [26], [27], [28] are an emerging class of porous materials [29], [30], [31], [32] attracting much attention because of their simple synthesis, high porosity, and facile functionalization. MOFs have many potential applications, including adsorption, particularly liquid-phase adsorption for water and fuel purification [33], [34], [35], [36]. Post-synthetic modification of MOFs can be performed by the functionalization of interesting materials on coordinatively unsaturated sites (CUSs) of metals or organic linkers present in MOFs [37]. Very recently, hydroxyl-functionalized MOFs, MIL-101(OH) and MIL-101(OH)₂ [38], were synthesized and used for the adsorption of pharmaceuticals and personal care products (PPCPs) from water. Adsorption and removal of OAAs were also carried out with MOFs such as MIL-100(Fe) [39], ZIF-8 [40], and UiO-66 [41]; however, the main investigation was limited to the effect of central metal ion, mesoporosity, and defect of MOFs and the studied OAAs were ROX or ASA. Therefore, functionalized MOFs, especially the ones bearing OH groups, exhibit promising applications in aqueous-phase adsorption (especially for OAAs) that require further research.

Herein, hydroxyl-functionalized MIL-101s such as MIL-101(OH)₃ and MIL-101(OH) were prepared by grafting aminoalcohols on virgin MIL-101. The adsorption of PAA and ASA (whose structures and physical properties are in Table 1) from water over functionalized MOFs was compared with that over commercial activated carbon (AC) and pristine MIL-101. MIL-101(OH)₃ showed the highest PAA adsorption capacity and good ASA adsorption capacity, although its porosity was lower than that of pristine MIL-101. Plausible mechanisms of OAA adsorption were suggested based on the effect of pH on adsorbed amounts, expected OAA species, and adsorbent surface charge. The reusability of MIL-101(OH)₃ was confirmed by adsorption following its regeneration by solvent treatment. The adsorption and facile desorption of OAAs were also confirmed by Fourier transform infrared (FTIR) spectroscopy.