Dec 20, 2016 - ABSTRACT: Sand filters are widely used for well water purification in endemic arsenicosis areas, but arsenic (As) removal is difficult at low ...
Article pubs.acs.org/est
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†,‡ †
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Wuhan 430074, P. R. China ‡ School of Environmental Studies, China University of Geosciences, Wuhan, Hubei 430074, P. R. China S Supporting Information *
ABSTRACT: Sand filters are widely used for well water purification in endemic arsenicosis areas, but arsenic (As) removal is difficult at low intrinsic iron concentrations. This work developed an enhanced sand filter by electrochemically generated Fe(II) from an iron anode. The efficiency of As removal was tested in an arsenic burdened region in the Jianghan Plain, central China. By controlling a current of 0.6 A and a flow rate of about 12 L/h, the filter removed total As in the tube well water from 196 to 472 μg/L to below 10 μg/L, whereas the residual As was about 110 μg/L without electricity. Adsorption and subsequent oxidation on the surface of Fe(III) precipitates are the main processes controlling the removals of As and Fe. During a 30-day intermittent operation, both effluent As concentration and electrical energy consumption decreased progressively. Although filter clogging was observed, it can be alleviated by replacing the top layer of sand. Our findings suggest that dosing Fe(II) by an iron anode is an effective means to enhance As removal in a sand filter.
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precipitates, increasing the required ratio of Fe/As.10 Therefore, the application of sand filters is largely restricted in As-burdened regions with low Fe concentrations such as Bangladesh.9,11 In the Jianghan Plain in central China, Fe in most of the well water is also insufficient for As removal due to the low average Fe/As mass ratio (e.g., 60).6 These regions emphasize the need of Fe addition to enhance the performance of sand filters for As removal. Various scenarios have been developed for Fe addition in the sand filtration process. Direct addition of ferric salts is a simple way, but oxidants such as hypochlorite salts are usually required for As(III) oxidation.12,13 Dosing Fe(II) is a better choice, as the reactive intermediates produced by Fe(II)/O2 reactions could partially oxidize As(III) in groundwater.11,14 Moreover, multiple or continuous additions of Fe(II) facilitate As(III) removal and reduce Fe(II) consumption compared to one-time addition.11,15 In practical applications, Fe(II) can be continuously supplied from elemental iron by acid dissolution, corrosion in aerobic water and electrolysis.11 Acid dissolution of iron (e.g., by sulfuric acid) needs pH adjustment and introduces undesirable anions. Household sand filters based on the corrosion of zerovalent iron (ZVI) have been used in Bangladesh.16−18 However, production
INTRODUCTION Despite As is of long legendary toxicity, it has been unrevealed until recent decades that widespread symptoms of disease are related to drinking groundwater with elevated As.1,2 Bengal Delta, one of the most serious areas of arsenicosis, has attracted intensive attention worldwide.3,4 In China, a population of about 19.6 million was estimated to be at risk of exposure to unsafe As levels as endemic areas of arsenicosis have been emerging since the 1960s.5 In 2005, six villagers were diagnosed with symptoms of chronic arsenic poisoning in the Jianghan Plain in central China; further investigations revealed that water from 863 wells in 179 villages contained As exceeding the World Health Organization (WHO) guideline (10 μg/L).6,7 Involved in the severe situations people are making concerted efforts to seek alternative As-free water sources and inevitably, to develop simple and efficient methods for As removal. Owing to the common co-occurrence of elevated Fe(II) with As in groundwater, simple sand filters have been developed and widely employed to remove the “bad taste” and muddiness after aeration in the arsenicosis areas.8 Although apparently “clean” water is produced from the sand filters, the effluent As concentration often exceeds the WHO guideline. Berg et al. surveyed the performances of 43 household sand filters in rural areas of the Red River Delta in Vietnam and concluded that Fe/ As mass ratios of ≥250 were required for removing As to below 10 μg/L.9 High concentrations of phosphate and silicate compete with As for the limited adsorption sites on Fe(III) © 2016 American Chemical Society
Received: Revised: Accepted: Published: 889
August 29, 2016 December 14, 2016 December 20, 2016 December 20, 2016 DOI: 10.1021/acs.est.6b04387 Environ. Sci. Technol. 2017, 51, 889−896
Article
Environmental Science & Technology of Fe(II) is limited by the influent dissolved oxygen (DO) and the precipitates coated on ZVI.16 Fe(II) addition by a sacrificial iron anode is an expedient means, which has been widely used in electrocoagulation for As treatment.15,19−25 Most of the studies dealt with either salt solution or synthetic groundwater with elevated As concentrations, and only limited studies tested real groundwater.20,26,27 In a community scale Electrochemical Arsenic Remediation (ECAR) reactor developed by Amrose et al., clean water was separated through gravitational settling aided by 6−15 mg/L aluminum.27 Compared to the hours needed for separation in the ECAR system,27 the time can be reduced to minutes by the sand filtration. In the rural areas of Jianghan Plain, both household (Figure S1 in the Supporting Information (SI)) and community-scale sand filtration are being used to purify the groundwater, but the water quality still cannot satisfy the local people. In this study, we adopted electrochemical addition of Fe(II) from an Fe anode into a household sand filter and tested the system performance for As removal from the local tube well water in the Jianghan Plain. The objectives are to (1) verify the enhancement of an iron anode for As removal from the local groundwater, (2) assess the performance of the new filter in the field, and (3) decipher the mechanisms of As and Fe removal. Knowledge generated from this study could provide a new strategy for enhancing As removal in sand filters.
Table 1. Properties of the Tube Well Water and Effluent Water in the Iron-Anode Enhanced Sand Filter Effluentb a
a
Properties
Tube well 1
Tube well 2
R2
G2
pH Eh (mV)c DO (mg/L) EC (μS/cm) HCO3− (mg/L) Ca (mg/L) Mg (mg/L) Si (mg/L) P (mg/L) As (μg/L) Fe (mg/L) Mn (mg/L)
7.18 41 0.2 950 689 157 33 45 0.9 272 4.7 3.23
7.34 −3 0.2 963 664 146 32 34 1.1 413 9.8 3.75
7.26 88 1.2 740 463 107 31 26 0.02 24.4 0.5 3.16
7.47 121 2.4 702 463 98 29 25
