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An Iron-Anode Enhanced Sand Filter for Arsenic Removal from Tube Well Water Shiwei Xie, Songhu Yuan, Peng Liao, Man Tong, Yiqun Gan, and Yanxin Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04387 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016
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An Iron-Anode Enhanced Sand Filter for Arsenic Removal
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from Tube Well Water
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Shiwei Xie, † Songhu Yuan,*, † Peng Liao,† Man Tong,† , ‡ Yiqun Gan,† , ‡
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Yanxin Wang†, ‡
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†
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of Geosciences, 388 Lumo Road, Wuhan, 430074, P. R. China
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‡
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430074, PR China
State Key Laboratory of Biogeology and Environmental Geology, China University
School of Environmental Studies, China University of Geosciences, Wuhan, Hubei
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RECEIVED DATE (to be automatically inserted after your manuscript is
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accepted if required according to the journal that you are submitting your paper
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to)
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* To whom correspondence should be addressed. E-mail:
[email protected] (S.H. Yuan), Phone: +86-27-67848629, Fax: +86-27-67883456.
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ABSTRACT Sand filters are widely used for well water purification in endemic
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arsenicosis areas, but arsenic (As) removal is difficult at low intrinsic iron
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concentrations. This work developed an enhanced sand filter by electrochemically
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generated Fe(II) from an iron anode. The efficiency of As removal was tested in an
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arsenic burdened region in the Jianghan Plain, central China. By controlling a current
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of 0.6 A and a flow rate of about 12 L/h, the filter removed total As in the tube well
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water from 196-472 µg/L to below 10 µg/L, while the residual As was about 110 µg/L
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without electricity. Adsorption and subsequent oxidation on the surface of Fe(III)
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precipitates are the main processes controlling the removals of As and Fe. During a
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30-day intermittent operation, both effluent As concentration and electrical energy
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consumption decreased progressively. Although filter clogging was observed, it can
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be alleviated by replacing the top layer of sand. Our findings suggest that dosing Fe(II)
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by an iron anode is an effective means to enhance As removal in a sand filter.
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INTRODUCTION
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Despite As is of long legendary toxicity, it is unrevealed until recent decades that
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widespread symptoms of disease are related to drinking groundwater with elevated
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As.1,2 Bengal Delta, one of the most serious area of arsenicosis, has attracted intensive
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attention worldwide.3,4 In China, a population of about 19.6 million was estimated to
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be at risk of exposure to unsafe As levels as endemic areas of arsenicosis have been
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emerging since the 1960s.5 In 2005, six villagers were diagnosed as symptoms of
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chronic arsenic poisoning in the Jianghan Plain in central China, and then further
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investigations revealed that water from 863 wells in 179 villages contained As
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exceeding the World Health Organization (WHO) guideline (10 µg/L).6,7 Involved in
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the severe situations people are making concerted efforts to seek alternative As-free
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water sources and inevitably, to develop simple and efficient methods for As removal.
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Owing to the common co-occurrence of elevated Fe(II) with As in groundwater,
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simple sand filters have been developed and widely employed to remove the “bad
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taste” and muddiness after aeration in the arsenicosis areas.8 Although apparently
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“clean” water is produced from the sand filters, the effluent As concentration often
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exceeds the WHO guideline. Berg et al. surveyed the performances of 43 household
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sand filters in rural areas of the Red River Delta in Vietnam, and concluded that Fe/As
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mass ratios of ≥250 were required for removing As to below 10 µg/L.9 High
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concentrations of phosphate and silicate compete with As for the limited adsorption
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sites on Fe(III) precipitates, increasing the required ratio of Fe/As.10 Therefore, the
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application of sand filters is largely restricted in As-burdened regions with low Fe
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concentrations such as Bangladesh.9,11 In the Jianghan Plain in central China, Fe in
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most of the well water is also insufficient for As removal due to the low average
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Fe/As mass ratio (e.g., 60).6 These regions emphasize the need of Fe addition to
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enhance the performance of sand filters for As removal.
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Various scenarios have been developed for Fe addition in the sand filtration process.
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Direct addition of ferric salts is a simple way, but oxidants such as hypochlorite salts
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are usually required for As(III) oxidation.12,13 Dosing Fe(II) is a better choice as the
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reactive intermediates produced by Fe(II)/O2 reactions could partially oxidize As(III)
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in groundwater.11,14 Moreover, multiple or continuous additions of Fe(II) facilitate
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As(III) removal and reduce Fe(II) consumption compared to one-time addition.11,15 In
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practical applications, Fe(II) can be continuously supplied from elemental iron by acid
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dissolution, corrosion in aerobic water and electrolysis.11 Acid dissolution of iron (e.g.
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by sulfuric acid) needs pH adjustment and introduces undesirable anions. Household
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sand filters based on the corrosion of zero-valent iron (ZVI) have been used in
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Bangladesh.16-18 However, production of Fe(II) is limited by the influent dissolved
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oxygen (DO) and the precipitates coated on ZVI.16 Fe(II) addition by a sacrificial iron
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anode is an expedient means, which has been widely used in electrocoagulation for As
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treatment.15,19-25 Most of the studies dealt with either salt solution or synthetic
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groundwater with elevated As concentrations, and only limited studies tested real
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groundwater.20,26,27 In a community scale Electro-Chemical Arsenic Remediation
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(ECAR) reactor developed by Amrose et al., clean water was separated through
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gravitational settling aided by 6‒15 mg/L aluminum.27 Compared to the hours needed
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for separation in the ECAR system,27 the time can be reduced to minutes by the sand
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filtration.
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In the rural areas of Jianghan Plain, both household (Figure S1 in the Supporting
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Information (SI)) and community-scale sand filtration are being used to purify the
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groundwater, but the water quality still cannot satisfy the local people. In this study,
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we adopted electrochemical addition of Fe(II) from an Fe anode into a household sand
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filter and tested the system performance for As removal from the local tube well water
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in the Jianghan Plain. The objectives are to (1) verify the enhancement of an iron
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anode for As removal from the local groundwater, (2) assess the performance of the
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new filter in the field, and (3) decipher the mechanisms of As and Fe removal.
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Knowledge generated from this study could provide a new strategy for enhancing As
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removal in sand filters.
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EXPERIMENTAL METHODS
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Field Site. The field site (30.176817°N, 113.680651°E) is located in the Nanhong
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Village in the central part of Jianghan Plain in Hubei Province, central China. This
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site was chosen because the first arsenicosis case was found there and the highest As
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concentration (up to 1072 µg/L) was measured in the well water.6 The sediment
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lithology in the area is generally sandy silts and clays in the top layer of 0-18 m, and
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fine to coarse sand in the layer of 18-50 m.28 The residential wells mainly extract the
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confined groundwater at about 10‒45 m depth, where elevated concentrations of As
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primarily occur.6,28 The high concentrations of dissolved organic carbon (DOC, mean
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6 mg/L) and easily reduced Fe oxides/hydroxides favor the release of As in the
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aquifer.6,28 Two wells were drilled into the depths of 26 and 23 m, respectively, on
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May, 2015. Polyvinyl chloride (PVC) tubes (5-cm inner diameter) were then installed
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with the bottom 2.4 m perforated and screened. The main compositions of the tube
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well water are presented in Table 1. All the experiments were excuted in an residential
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house near the tube wells from June to September in 2015.
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Design and Operation of the Sand Filter with Iron Electrodes. Referring to the
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experience of SONO filters,17 we also adopted two buckets for a two-stage treatment
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(Figure 1). The food-grade polypropylene (PP) buckets (17 L) were packed with
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13-kg coarse yellow sand and 1.5-kg brick chips, yielding a total pore volume of
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2.5-2.9 L in each bucket. The yellow sand was taken from the bank of Hanjiang River.
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The coarse fraction with the grain size of >0.4 mm was separated and washed by the
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groundwater from tube well 2 prior to use. Iron from the groundwater could be
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adsorbed and precipitated on the sand surface during the washing (98% Fe, 0.42-0.50% C, 0.17-0.37% Si and
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0.50-0.80% Mn, supplied by Wuhan Steel Processing Co., Ltd.) were used as the
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electrodes in each bucket (Figure S2). They were placed in parallel above the sand
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with an interval of 1 cm. The electrode, which has a diameter of 27 cm and a weight
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of 0.8 kg, was evenly distributed with 241 holes (6-mm diameter) for groundwater
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flow. A constant current was applied on the electrodes by a direct-current (DC) supply
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(GPC-3060D, Taiwan Goodwill). To alleviate the passivation of Fe anode, the
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electrode polarity was reversed when the voltage increased drastically.29 Additionally,
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the electrodes were brushed once a week to remove the sand and precipitates loading
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on them. Assuming 100% current efficiency for Fe(II) production from the Fe anode,
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the concentration of Fe(II) (CFe(II), mg/L) produced was a function of the operating
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current (I, A) and the flow rate (Q, L/h) in the filter (eq 1):
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CFe ( II ) = 3.6 ×106
I M ( ) Q zF
(1)
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where z is the number of electrons involved (= 2), F is Faraday’s constant (96500
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C/mol), and M is the molecular weight of Fe (56 g/mol).
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The operation details were presented in Section S1. Briefly, groundwater from the
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two tube wells was fed to the filter by a peristaltic pump, or alternatively, was stored
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in a tank and munully transferred to the filter using a bailer. A flow rate of 12 L/h was
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set referring to the operation of SONO filters in Bangladesh.18 The filter was drained
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periodically for aeration. Fresh sand was used in each test. During the tests, aqueous
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samples were taken from Ports R1, R2, G1, G2 and the well. The samples for As and
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Fe analysis were collected in pre-acidified (pH
