In-situ arsenic remediation by aquifer iron coating: Field trial in the Datong basin, China

Highlights

• An in-situ As removal technology based on Fe-coating has been first confirmed in the field.

• The impact factors including water chemistry and redox conditions on in-situ Fe coating for As removal were discussed.

• This technology has promising for the field application to treat natural high As groundwater.

Abstract

An aquifer Fe-coating technology was evaluated for in-situ As remediation. The groundwater in the aimed aquifer has low dissolved Fe(II) concentration and high As(III) concentration, which has a low affinity toward Fe-oxides/hydroxides. To overcome these challenges, dissolved Fe(II) (5.0 mM) and NaClO (2.6 mM) were injected into the studied aquifer to promote the formation of Fe oxides/hydroxides and to oxidize As(III) into As(V), thus removing aqueous As via adsorption and/or co-precipitation. During field experiment, As concentration in groundwater from the pumping well significantly decreased. Fe and As speciation calculations indicate that incorporation of negatively charged As(V) into goethite was the probable mechanism for As removal. Both chemical sequential extraction results and spectroscopic data also support that alternating injection of Fe(II) and NaClO can achieve aquifer Fe coating and immobilize As via adsorption onto Fe oxides/hydroxides. Geochemical modelling further confirms that although competition for sorption sites between As and other dissolved species is expected in the natural groundwater system, high surface area of the Fe oxides/hydroxides can provide sufficient sites for As retention. The ability to effectively decrease As concentration of in-situ aquifer Fe-coating technology indicates that this approach should have extensive applicability to similar high As groundwater occurred worldwide.

Introduction

Arsenic (As)-contaminated groundwater has been documented in many parts of the world, especially in India, Bangladesh, Vietnam, Cambodia and China [1], [2], [3], [4]. In these areas, As contamination in groundwater poses a great risk to human health, especially in the rural areas where local residents are reliant on As-contaminated groundwater as drinking water source. Modified or novel treatment technologies are urgently needed in those areas for As removal from contaminated groundwater to meet drinking water standard (10 μg/L).

Various ex-situ treatment technologies have been developed and applied for As removal [5], [6], [7], [8], [9], [10]. Many of them, however, have shortcomings such as high cost, requirement of As-rich waste disposal, technological complexity or certain limitations when applied to rural areas. In-situ treatment of As-contaminated groundwater is another type of technology that is more promising for supplying safe drinking water in the rural areas [11], [12]. Iron (Fe)-based materials such as Fe(III) oxides/hydroxides have been widely proposed for As removal from groundwater [13], [14], [15]. One recently developed in-situ treatment technology is to coat Fe oxides/hydroxides onto the aquifer sediments via directly delivering Fe salts and associated oxidants into aquifers to achieve As immobilization. In the presence of oxidants, Fe(II) salts can be converted into goethite (FeOOH) or ferrihydrite [Fe(OH)₃] within the aquifers under alkaline pH conditions [16], [17]. Apart from As retention, the precipitation of Fe oxides/hydroxides onto sediment surfaces can prevent further As release from sediments into the groundwater through water-sediment interaction. The As retention mechanism of this approach is mainly based on adsorption and co-precipitation reactions between As and Fe oxides/hydroxides [18]. However, these processes could be impacted by a variety of factors such as groundwater oxidation–reduction potential and the presence of hydrochemical species. For examples, As removal by Fe(III) oxides/hydroxides suffers a challenge under reducing conditions that usually prevail in high As groundwater systems. This because that Fe(III) oxides/hydroxides may reductively dissolve into aqueous Fe(II) with the simultaneous release of As. The study conducted by Roberts et al. [18] indicated that Fe(III) oxides/hydroxides could sustain for more than one year under oxidizing to moderately reducing conditions. Therefore, it is feasible apply Fe(III) oxides/hydroxides to immobilize As in groundwater under oxidizing to moderately reducing conditions. In addition, natural groundwater is a complex multi-components system. It is likely that the occurrence of high concentration of phosphate and bicarbonate, which are usually the dominant anions in groundwater, can result in the desorption of As from Fe(III) oxides/hydroxides [19], [20]. Therefore, to overcome these problems encountered in natural groundwater aquifers, it requires not only to promote the formation of Fe(III) oxides/hydroxides, but also to transfer As(III) into As(V) to improve As adsorption onto Fe(III) oxides/hydroxides. Our previous study has confirm that Fe(III) oxides/hydroxides coating onto quartz sands has achieved As removal from groundwater [21]. However, for the implications of this method in natural aquifers, some concerns remains to be clearly answered. For instance, Fe coating approach through alternating injection Fe(II) salt and oxidants need to be tested within natural groundwater systems under oxidizing to moderately reducing conditions. Moreover, the effects of concurrent competitive sorption and co-precipitation of bicarbonate, phosphate and sulfate toward As(III) and As(V) during the formation of Fe(III) precipitates also need to be explored.

In this study, therefore, we present a field trial that combines hydrochemical monitoring and modelling to: (1) confirm the feasibility of aquifer Fe coating approach under a moderately reducing to oxidizing natural groundwater conditions; (2) discuss main factors affecting the removal capacity of Fe coating for As during field trial; (3) investigate the potential of the in-situ treatment of natural As-contaminated groundwater by aquifer Fe coating technology.