Arsenic removal from groundwater by MnO2-modified natural clinoptilolite zeolite: Effects of pH and initial feed concentration
Introduction
Arsenic (As) is considered a contaminant of major concern due to its high toxicity at small concentrations and its ability to go undetected [1]. It is naturally present in the environment due to geological formations, such as lacustre sediments and volcanic rocks [2]. From these As-rich geological sources, soluble forms of the metal leach into shallow groundwater [3] and transform due to oxidation–reduction, ligands exchange, precipitation and biochemical reactions [2]. Industrial processes and products such as wood preservatives, semi-conductors, and agricultural applications may also introduce arsenic into the environment [4], [5].
Even though arsenic is found in several forms in food and environmental media, it is predominantly encountered in drinking water as inorganic arsenic. In this form it is both highly toxic and readily bioavailable. Chronic ingestion of inorganic arsenic contaminated drinking water can develop into arsenicosis which causes respiratory, renal, and immunologic effects [6]. Evidence that inorganic Arsenic may be diabetogenic has been also reported [7]. The World Health Organization [8] established a maximum contaminant level (MCL) for arsenic in drinking water of 10 μg/L. This limit was also adopted in the United States (US) in 2006 [9].
Treatment methods for arsenic contaminated waters include oxidation, lime softening, coagulation/precipitation, anion-exchange, adsorption, membrane process, and phytoremediation [10], [11]. In the majority of the treatment cases, Arsenic removal was affected by the initial pH of the water which can be between 7 and 12 [12], [13], [14]. One of the disadvantages of working at these pH levels is the sludge generation, which makes the process inefficient from a waste minimization point of view. Adsorption has proven to be an efficient method for treating arsenic contaminated waters [15]. Some of the adsorbents widely used include silica, alumina, granular ferric hydroxides, synthetic resins, and zeolites [16]. Among these materials, natural zeolites, a group of crystalline alumina-silicates, gained increasing attention due to their specific structure and adsorption and ion-exchange capabilities. However, its main disadvantage is the low arsenic removal capacity.
Studies have been conducted using modified natural zeolites for arsenic removal. Some of the reported modifications include iron-modified [17], [18], surfactant-modified [19], [20] and lanthanum-modified zeolites [21]. Chemical pre-treatment and initial pH adjustment were required in most of the cases and no significant improvement in the arsenic removal efficiency was reported [22], [23]. The objective of this work was to investigate the adsorption performance of a MnO2-modified natural clinoptilolite zeolite without chemical pre-treatment, and to compare it with the Arsenic adsorption by the unmodified clinoptilolite zeolite at similar conditions. The effect of pH and initial concentration on the arsenic adsorption capacity were given emphasis in this work. The results obtained will enhance the understanding of adsorption equilibrium of arsenic by clinoptilolite zeolite and its modification, and provide valuable insights on adsorption breakthrough process development and implementation.
Section snippets
Modification of natural clinoptilolite zeolite
The natural clinoptilolite zeolite used in this study is from a zeolite deposit located in Truth or Consequences, New Mexico (St. Cloud Mining Company, USA). Prior to modification, the clinoptilolite zeolite (CZ) was washed with deionized water and dried at room temperature. The modified clinoptilolite zeolite (MCZ) was obtained by pouring a mixture of 50 mL of 2.5 M MnCl2 and 0.5 mL of 10 M NaOH over 50 g of the washed clinoptilolite zeolite in a heat-resistant dish and then heating the mixture in
Physical and chemical properties of natural and modified zeolites
Major and trace element compositions of the CZ sample showed that aside from the main Si and Al components, Ca is the next highest component at 3.18 wt.% followed by K at 2.44 wt.% (Table 1). Silica to alumina ratio (Si/Al) of 5.16 was obtained for the natural clinoptilolite zeolites. The Si/Al ratio provides the negatively charged structure of the zeolite due to the difference between the (AlO4)5− and (SiO4)4− tetrahedral [24]. Positive sites may be available from the alkaline and alkaline-earth
Conclusions
A natural clinoptilolite zeolite from Truth or Consequences, New Mexico was modified with MnO2, without any other chemical pretreatment and applied for arsenic removal. Both the natural and MnO2-modified clinoptilolite zeolites were characterized and evaluated to explore the feasibility of using this inexpensive adsorbent for arsenic removal from drinking water. Modification of the natural zeolite produced the decrease of the BET surface area due to the introduction of manganese and chloride
Acknowledgements
We appreciate the generous donations from Freeport-McMoRan for establishing the Water Quality Laboratory (LC-ICP-MS used in this work) in the College of Engineering at New Mexico State University. Dr. Nancy McMillan from the Geological Sciences Department and Dr. Peter Cook from the Electron Microscope Laboratory at New Mexico State University for their assistance with XRF and SEM/EDS analysis.
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