Preparation and evaluation of thiol-functionalized activated alumina for arsenite removal from water

Abstract

Arsenic species such as arsenite [As(III)] and arsenate [As(V)] are known human carcinogens. Though lots of metal oxide adsorbents have been developed for removal of As(V), they are much less effective for As(III) adsorption. In this study, various inorganic–organic hybrid adsorbents bearing thiol groups have been prepared by modifying activated alumina (AA) with mercaptopropyl-functionalized silica under different experiment conditions. Raman spectra demonstrated the successful functionalization of AA and verified the formation of As–S complexes after As(III) adsorption. Batch experiments were applied to evaluate the As(III) adsorption performance of the hybrid adsorbents. Compare with AA, the hybrid adsorbents exhibited enhanced adsorption abilities for As(III) due to the introduction of thiol groups, and as the thiol loading increased, the uptake of As(III) increased. Experimental results indicated that the hybrid adsorbents still maintained the merit of the AA for As(V) adsorption. Based on the results, one hybrid adsorbent referred to as BL(AA)₃₀(MPTS)_3.3 has been selected by consideration of not only the adsorption capacity but also its environmentally friendly and cost-effective production. The investigation has indicated that the hybrid adsorbents are very promising for As(III) removal from water.

Introduction

Arsenic (As) is one of the most toxic contaminants found in the environment. Arsenic contamination has been receiving increased attention and emerged as a major concern on a global scale in recent years [1], [2]. Chronic intake of arsenic has been associated with increased risk of cancers, diabetes, developmental and reproductive problems, and cardiovascular disease [2], [3], [4], [5]. To minimize these risks, the World Health Organization (WHO) has set a provisional guideline limit of 0.01 mg L⁻¹ for arsenic as the drinking water standard [6]. The United States Environmental Protection Agency (USEPA) announced its ruling in 2001 to lower the maximum contaminant level (MCL) from 50 to 10 ppb, and this new regulation has become effective from January 2006 [7], [8].

The more stringent drinking water standard has a significant impact on the management of arsenic contaminated sites and has motivated researchers and water treatment industries to develop innovative arsenic removal technologies. The current water treatment technologies, such as coagulation/precipitation, ion exchange, lime softening, and metal oxide adsorption, are effective for removing arsenate [As(V)], but much less for arsenite [As(III)] [9], [10], [11], [12] mainly because As(III) is a hydrophilic, neutral species below pH 9 and is not accessible to the major removal mechanisms of anion sorption and anion exchange [11]. Compared with As(V), As(III), which is present predominantly in anoxic groundwater, is more toxic and more mobile [13], [14], [15], [16]. To enhance As(III) removal, a pre-oxidization of As(III) to As(V) is usually involved using oxidizing agents or photocatalytic oxidation on TiO₂ [9], [11], [17], [18], [19], [20], [21], [22]. However, the application of a pre-oxidation step causes operational complexity, increases cost of water treatment, and especially diminishes the overall viability of the fixed-bed process [23]. These problems have highlighted the urgent necessity for the development of direct and cost-effective As(III) remediation technologies without the need for additional pre-oxidization.

It is well known that As(III) has an especially high affinity for tissue proteins by strongly binding to mercaptan (thiol) groups existing in biomolecules such as amino acids, peptides and proteins (including some enzymes), which explains the higher toxicity of this species and the metabolism of arsenic in mammals [5], [24], [25], [26]. Based on the above understanding, some thiol-based adsorbents including polymer resins and silica gels have been developed for As(III) removal [27], [28], [29], [30], and some of them exhibited the selective removal of As(III) and had a high adsorption capacity. Recently, McKimmy et al. [31] synthesized thiol-functionalized mesostructured silica using direct assembly methods for As(III) trapping, and the As(III) concentration was reduced by up to 98% under batch equilibrium conditions. More interestingly, a biomass (treated waste chicken feathers) with a cysteine-rich protein has been used for selective As(III) adsorption up to 270 μmol g⁻¹ of biomass [26]. These results suggested that materials bearing thiol functional groups as As(III) adsorbents are very promising, and prompted us to develop new thiol-functionalized adsorbents for As(III).

Activated alumina (AA), as a low-cost material, has a high surface area and a distribution of both macro- and micropores, and has been extensively studied and used as arsenic adsorbent [2], [32], [33]. The AA adsorption has been classified among the best available technologies (BAT) for arsenic removal from water [34]. However, the conventional commercially available AA has ill-defined pore structures, low adsorption capacities and exhibits slow kinetics [35]. Moreover, the uptake of As(III) by AA is much less than that of As(V) in most pH conditions [32]. To improve its performance, some modified types of AA such as Fe(OH)₃-coated AA, iron oxide-impregnated AA, manganese-amended AA and biopolymer chitosan-coated AA, have been developed and proved to be more effective for As(III) and As(V) removal than virgin AA [36], [37], [38], [39], [40], [41]. However, to our knowledge, there is no report on the use of thiol-functionalized AA for arsenic removal up to present although thiol-modified alumina materials have been used to adsorb certain heavy metals like Hg, Pb, Cd, etc. [42], [43]. The thiol-functionalized AA is expected to take advantage of the strength of the two materials.

In this study, various novel hybrid adsorbents bearing thiol groups were prepared by introducing mercaptopropyl-functionalized silica onto the surface of the AA under different conditions. The hybrid adsorbents were characterized by scanning electron microscopy (SEM), Raman spectroscopy and Brunauer–Emmett–Teller (BET) surface area analysis. Their adsorption properties for As(III) were investigated and the effects of the various factors on As(III) removal are discussed. It is also expected that the hybrid adsorbents will still be effective for As(V) removal due to the presence of aluminol active sites remaining uncovered. The broad objective of this investigation was to develop a cost-effective and environmentally friendly hybrid adsorbent for remedying groundwater and drinking water contaminated by As(III), or both As(III) and As(V), which is also viable for a fixed-bed process without pre-oxidation of As(III).