Removal of arsenic from drinking water using modified natural zeolite
Highlights
► Characterization and kinetics of arsenic sorption by modified clinoptilolite. ► Higher efficiency provides at lower equilibrium time as compared to other studies. ► Pseudo-second-order rate equation describes better kinetics. ► Iron concentrations in the clinoptilolite and arsenate concentrations are important.
Introduction
Arsenic is a semi-metallic element, occurs naturally in rocks and soils, and contaminates water that comes in contact with these natural components. Therefore varying amounts of soluble arsenic are present in some water sources [1]. It is mobilized through a combination of natural processes such as weathering reactions, biological activity and volcanic emissions [2], [3] as well as through a range of anthropogenic activities such as gold mining, non-ferrous smelting, petroleum-refining, combustion of fossil fuel in power plants and the use of arsenical pesticides and herbicides [2], [4], [5]. The U.S. Environmental Protection Agency (EPA) decreased the maximum contaminant level (MCL) of arsenic in drinking water from 50 μg L− 1 to 10 μg L− 1 due to fatal toxicity of arsenic on human health [6]. The World Health Organization (WHO), the European Union, and several other countries such as Turkey also lowered their recommended or required arsenic limit to 10 μg L− 1 in drinking water [7]. Natural water sources that contained much higher levels of arsenic (20–3000 μg L− 1) than MCL were determined in western Turkey. The natural enrichment of arsenic in groundwater is related to the borate deposits, and the arsenic complexes present in soils. Arsenic found in soil either naturally occurring or from anthropogenic releases forms insoluble complexes with iron, aluminum, and magnesium oxides found in soil surfaces, and in this form, arsenic is relatively immobile. Arsenic can liberate from these complexes under some circumstances. Since arsenic in soils is highly mobile, once it is liberated, it results in possible groundwater contamination [8], [9], [10], [11], [12]. According to Human Development Report Beyond Scarcity: Power, Poverty and Global Water Crisis by the United Nations Development Programme, arsenic contaminated water creates risks for millions of people in some countries including Turkey in the world [13].
Occurrence of arsenic in natural water depends on the local geology, hydrology and geochemical characteristics of the aquifer materials [14]. The chemistry of arsenic in aquatic systems is complex, and consists of oxidation–reduction, precipitation, adsorption, and ligand exchange [15]. Arsenic can occur in the environment in several oxidation states but in natural waters is mostly found in inorganic form as oxyanions of trivalent arsenite [As(III)] or pentavalent arsenate [As(V)] [3]. The forms of arsenic present are dependent on the type and amounts of sorbents, pH, redox potential, and microbial activity [16]. Under oxidizing conditions (positive Eh) As(V) is the primary form of arsenic, while under reducing conditions (negative Eh) the primary form is As(III). In the common groundwater pH range of 6 to 9, the predominant As(III) species is neutral (H3AsO3), whereas the As(V) species are monovalent (H2AsO4−) and divalent (HAsO42−) [17]. Therefore As(III) is less efficiently removed than As(V) from aqueous solutions by almost all of the arsenic removal technologies and preoxidation of As(III) to As(V) and using some oxidizing chemical agents like chlorine and potassium permanganate is necessary for better removal [18].
The toxicity of an arsenic-containing compound depends on its valence state, its form, and the physical aspects governing its absorption and elimination. In general, inorganic arsenic is more toxic than organic arsenic, and trivalent arsenite is more toxic than pentavalent and zero-valent arsenic [19]. Its toxicity is hard to investigate because of its ability to convert between oxidation states and organometalloidal forms [20]. Two types of toxicity, acute and sub-acute are known for a long time. The major early manifestation due to acute arsenic poisoning includes burning and dryness of the mouth and throat, dysphasia, colicky abnormal pain, projectile vomiting, profuse diarrhea, and hematuria. Sub-acute arsenic toxicity mainly involves the respiratory, gastro-intestinal, cardio-vascular, nervous and hematopoietic systems [14].
Various technologies are available for the removal of arsenic from contaminated water including chemical precipitation or coagulation, adsorption, lime softening, ion exchange, and membrane separation [21], [22], [23], [24], [25], [26], [27], [28], [29]. Although precipitation–coprecipitation with ferric and aluminum salts is one of the conventional methods for arsenic removal, handling and disposal of the waste sludge is a significant problem of this process [15]. Adsorption has emerged as an alternative to these traditional methods with advantage of being technically easy, and has the potential for regeneration and sludge free operation [30]. At the same time adsorption will provide an attractive technology if the adsorbent is cheap and ready for use [7]. So far, many kinds of adsorbents such as Zr(IV) loaded orange waste gel [2], iron modified red mud [7], agricultural residue rice polish [30], iron modified calcined bauxite [31], zero valent iron [32], mesoporous alumina [33], and acid modified carbon black [34] have been developed for the removal of arsenic. Arsenic removal efficiency was achieved more than 85% by most of them. However, the necessary time to reach the equilibrium was found as 24 h for Zr(IV) loaded orange waste gel and red mud. For other types of adsorbent, the required time for adsorption was between 1 and 3 h. In the past years, natural zeolite has been explored as effective adsorbent for removal of various heavy metals and other environmental pollutants because of their selectivity, ion exchange capacity, and low cost [35], [36]. However, these zeolitic materials do not remove anionic or organic pollutants and for this reason it is necessary to treat the zeolitic material to change its surface characteristics and improve the adsorption of this kind of water pollutant [1]. Few investigations have been reported on arsenate removal using iron modified natural zeolite and arsenic adsorption mechanisms [1], [37], [38], [39], [40]. In general, very little information is available on the characterization and kinetics of adsorption of arsenic onto natural and iron modified zeolites. In order to gain an understanding of the adsorption process kinetics, a detailed study was conducted in a controlled batch system. Therefore, the major objective of this study is to modify the adsorption characteristics of natural zeolite using ferric chloride for investigation of removal efficiency of arsenate from tap water by adsorption. The present paper reports the synthesis and characterization of natural and iron modified zeolites. Furthermore, the adsorption capacities and adsorption kinetics of clinoptilolite were measured to understand the adsorption process mechanism and kinetics.
Section snippets
Materials
The chemical composition and some properties of the tap water used in this study are listed in Table 1. Analytical-reagent grade chemicals were used for the preparation of all solutions without further purification. Arsenate stock solution (2 mg L− 1) was prepared by dissolving Na2HAsO4.7H2O (Sigma, USA) in the tap water. In the experimental studies, arsenic working solutions were freshly made by diluting this stock arsenic solution until desire concentration. The natural zeolite (clinoptilolite)
The zeolite characterization
The chemical composition of the raw zeolite and modified zeolites is shown in Table 2. The major constituents of the raw zeolite are silicate and alumina. According to the concentration of Na, Mg, K and Ca, GC is a K–Ca zeolite type. The iron found in the natural zeolite GC was less than 1.6 wt.%.
The raw zeolite GC was first pretreated with NaCl solution to improve the ion-exchange capacity of zeolite. Pretreatment of natural zeolite containing several multivalent exchangeable cations by only
Conclusions
The sorption performance of natural clinoptilolite, sodium conditioning clinoptilolite and iron modified clinoptilolite types for the arsenate removal was evaluated. The surface morphologies, chemical composition, physical properties, and specific surface areas of unmodified and modified zeolites were determined for adsorbent characterization. After FeCl3 treatment, the reduction in sodium concentration and the increasing in iron concentration were more significant in the Fe1-GC than Fe2-GC
Acknowledgments
This study was supported by the Izmir Environmental Protection Foundation and the Scientific Research Projects of the Dokuz Eylul University, Izmir, Turkey under grant number 2005.KB.FEN.003.
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