Arsenic and fluoride contaminated groundwaters: A review of current technologies for contaminants removal

Highlights

• Groundwater contaminated by As and F is a health hazard for millions of people.

• Arsenic and fluoride co-removal with other ions reduces 15–20% removal efficiency.

• Membranes show consistent co-removal and nanofiltration preserves water quality.

• Encapsulation methods must evolve to isolate loaded sorbents for final disposal.

Abstract

Chronic contamination of groundwaters by both arsenic (As) and fluoride (F) is frequently observed around the world, which has severely affected millions of people. Fluoride and As are introduced into groundwaters by several sources such as water–rock interactions, anthropogenic activities, and groundwater recharge. Coexistence of these pollutants can have adverse effects due to synergistic and/or antagonistic mechanisms leading to uncertain and complicated health effects, including cancer. Many developing countries are beset with the problem of F and As laden waters, with no affordable technologies to provide clean water supply. The technologies available for the simultaneous removal are akin to chemical treatment, adsorption and membrane processes. However, the presence of competing ions such as phosphate, silicate, nitrate, chloride, carbonate, and sulfate affect the removal efficiency. Highly efficient, low-cost and sustainable technology which could be used by rural populations is of utmost importance for simultaneous removal of both pollutants. This can be realized by using readily available low cost materials coupled with proper disposal units. Synthesis of inexpensive and highly selective nanoadsorbents or nanofunctionalized membranes is required along with encapsulation units to isolate the toxicant loaded materials to avoid their re-entry in aquifers. A vast number of reviews have been published periodically on removal of As or F alone. However, there is a dearth of literature on the simultaneous removal of both. This review critically analyzes this important issue and considers strategies for their removal and safe disposal.

Introduction

Water is not only an essential component for life but also a basic building block to maintain quality of life. Water scarcity has already revealed adverse effects on all populations in every continent. More recently, UNICEF and WHO reports have confirmed that 748 million people do not have adequate and safe water resource and over 2.5 billion people have access to meagre water supply. The WHO also estimates that 1.8 billion people use faecally contaminated source of drinking water (UNICEF/WHO, 2014). Groundwater is used for potable purposes by over 50% of the global population. Thus, groundwater is sometimes described as the ‘hidden sea’. This is indeed true to a greater extent in countries like India where local supply to ∼80% rural and ∼50% urban dwellings is provided by groundwater sources alone (Ayoob et al., 2008).

Presence of several naturally occurring, anthropogenic and industry generated ions such as fluoride, arsenic, nitrate, sulfate, iron, manganese, chloride, selenium, heavy metals, and radioactive materials may greatly compromise water quality, leading to health problems. The most significant inorganic pollutants in groundwater affecting human health at the global scale, according to the WHO, are arsenic and fluoride (Thompson et al., 2007). In this context, fluoride pollution of drinking water receives much less consideration than arsenic.

Fluoride is the only chemical in potable water that can cause different health effects depending upon its concentration in dissolved form. A very small amount of fluoride is beneficial for bone and teeth development and dental health. However, concentrations higher than 1.5 mg/L are damaging to human health, causing dental or skeletal fluorosis (Miretzky and Cirelli, 2011). Children below 12 years are likely to be most exposed to fluorosis as their body tissues continue to grow during the formative age. Moreover, fluorosis is non-reversible and the disorder has no medical treatment. The WHO permits a fluoride concentration of 0.5–1 mg/L in drinking water (WHO, 2011). Effluent limit of 4 mg/L for F from the wastewater treatment facilities has been set by USEPA (Shen et al., 2003). Fig. 1 depicts the statistics on population exposed to F contamination. Clearly, China and India are the most affected countries where nearly 35 and 26 million, respectively, people are at fluoride risk.

As for arsenic, As(V) (arsenate) and As(III) (arsenite) are the most predominant valence states, which are found in aerobic surface waters and anaerobic groundwaters, respectively. Between pH 4 and 10, major As(III) compound is charge neutral, whereas As(V) species exists as charge negative. The occurrence of As in groundwater poses even a greater danger than F hazards due to its extreme toxicity at low concentration which goes undetected especially in As(III) form (Camacho et al., 2011). Arsenic is well known for its carcinogenicity in kidney, lung, liver, skin, and bladder. At high concentrations, As causes gastrointestinal problems and arsenicosis, which arise mainly via consumption of water containing As and its subsequent accumulation in the body (Sharma and Sohn, 2009, Villaescusa and Bollinger, 2008). Therefore, the WHO recommends As concentration of 10 μg/L as the upper permissible limit in water (WHO, 2011). This limit is applicable in India, Japan Taiwan, USA and Vietnam (Hug et al., 2008, Reddy and Roth, 2013) while other countries like China, Bangladesh, and most of South American nations have permitted a higher concentration of 50 μg/L (Camacho et al., 2011, Chakraborti et al., 2010). A report prepared by UNICEF consultant Ravenscroft (2007) estimates that natural As pollution of groundwater and surface water affects more than 140 million people in at least 70 countries worldwide. A large population of South Asia is exposed to As toxicity (Fig. 2).

When two different types of harmful contaminants are ingested, they may function independently or synergistically or antagonistically to one another (Chouhan and Flora, 2010). While the harmful effects of As and F individually have been widely studied, the exposure to both together has received little attention. Rao and Tiwari (2006) reported that As and F in combination affect integrity of cells genetic material more than the individual exposure. In animal studies for rats, co-exposure of As and F even at low concentrations resulted in decreased comet tail and detrimental effect on liver and kidney (Flora et al., 2009, Mittal and Flora, 2006). Wang et al. (2007) reported that children's growth and intelligence were severely influenced by high concentrations of As or F. Hence, it is important to remove these toxicants from potable water. Despite the extreme seriousness of the issue, very less data exist on the populations facing simultaneous toxicity of As and F.

This article analyzes the genesis of the combined presence of geogenic fluoride and arsenic in groundwater and drinking water as well as the treatment methods for their removal. Also, it reviews the effectiveness of several treatment methods when these two contaminants are present together.