Navigating the arsenic dilemma

Abstract

The debate of white rice vs brown rice variety has received a lot of attention in the recent past. People in general have, for some reason, labeled brown rice as a morally superior variety. Arguments put forward are fibre content and “more nutritious” qualities compared to white varieties.

People on the white rice side argue that white rice has lower arsenic content and hence is a better and safer choice. In this review, based on a realistic assessment of the relevant scientific literature, we present here our view on the debate. The current wave of concern over arsenic contents in rice, while sometimes justified, masks a much starker reality. However, while simple precautions could considerably attenuate the consumption problem, one notable exception remains that of irrigation water, particularly in heavily contaminated areas.

 

Introduction

Arsenic, one of the toxic compounds which pose a high risk to large human populations, is a metalloid element generally present in natural deposits and reaches groundwater through weathering, erosion of rocks/soils, and volcanic emissions.

Arsenic has a complex chemistry, represented by more than 300 mineral forms in soils (Shrivastava Aet al. 2015), and there are approximately 568 known minerals that contain arsenic as a critical component (The new International Mineralogical Association list of minerals. A work in progress.International Mineralogical Society, 2014).

A large number of industrial and agricultural activities, such as mining wastes, coal fly ash, glass manufacturing, wastewater sludge, pharmaceutical waste, metal smelting, phosphate fertilizers, and arsenic-based herbicides, pesticides and livestock antibiotics, contribute significant additional sources of arsenic contamination in groundwater (PunshonetT et al. 2017). Needless to say, we all live with the above activities around us.

Arsenic in the environment exists in both organic and inorganic forms and two major chemical states: arsenate (AsV) and arsenite (AsIII). The inorganic components, which mainly include the mineral forms, are more prevalent than the organic forms which are mostly produced by living organisms due to arsenic consumption. Arsenite (AsV) is less toxic and mostly present in immobile mineral forms, whereas the As(III) form is more toxic and gets mobilized into the water and enters living cells. Thus, the lethal dose (LD50) of As(III) is much lower (15-42 mg/kg) than that of As(V) (20-800 mg/kg) (Kaise T& Fukui S, 1992).

 

Arsenic and the food chain

One of the major routes for arsenic exposure to humans is through drinking water where it is typically present in inorganic forms of either arsenite(AsIII) or arsenate (AsV).

Over 200 million people across 70 Asian countries are affected by As-contaminated groundwater at concentrations greater than the World Health Organization (WHO) guideline value for arsenic of 10 micrograms per litre (10 mg/L). In India, the maximum groundwater arsenic concentration was detected in a tubewell from West Bengal as 3,700 μg/L, which is 370 times higher than the WHO guideline value and 74 times higher than the Indian standard of arsenic (50 μg/L) in drinking water!

Elevated concentrations of arsenic in groundwater were also detected in other states such as Uttar Pradesh, Bihar, Manipur, Assam and Jharkhand. Altogether, from all surveyed states,13.85 million people were exposed to arsenic concentrations greater than 10 μg/L and 6.96 million to concentrations greater than 50 μg/L (ChakrabortiD et al., 2018).

The other significant pathway of arsenic contamination is through the food chain.

When contaminated groundwater is used for staple-food crop irrigation (rice, pulses, fruits and vegetables), and for cooking, it enters the food chain where inorganic arsenic gets methylated and gives rise to organic forms like monomethylarsine (MMA), dimethylarsine (DMA), and trimethylarsine (TMA). Among the organic arsenic forms, MMA and DMA are more toxic than TMA. Thus, the LD50 values for MMA (1.8 g/kg) and DMA (1.2 g/kg) are lower than that of TMA (10 g/kg; [Shrivastava Aet al., 2015]).

 

White rice OR brown rice? 

While the inorganic forms of arsenic are more toxic than the organic forms, crop plants contain both inorganic and organic arsenic forms and when the crops are being consumed, the people are exposed to arsenic.

In general, while accumulated arsenic concentrations greater than 3.6 mg/kg is toxic to most plants growing in natural soil (Gebel. T, 1997), plant species cultivated on contaminated sites can adapt by restricting AsV uptake (Abbas G et al., 2018; Meharg AA et al., 2002).

Thus, even in heavily contaminated soils, such as in the Ganga-Meghna-Bramhaputra (GMB) basin which is one of the major arsenic-contaminated hotspots in the world (Bhattacharya Pet al., 2010), not all plants accumulate arsenic in the same manner nor to the same extent. In the GMB basin, the highest mean arsenic concentration was found in potato (0.654 mg/kg). The other higher mean arsenic contents (milligrams per kilogram dry weight) were observed in Boro rice grain (0.451), arum (0.407), amaranth (0.372), radish (0.344), Aman rice grain (0.334), lady’s finger (0.301), cauliflower (0.293), and in brinjal (0.279). Comparatively lower arsenic concentrations (milligrams per kilogram) could be seen in wheat (0.129), garlic (0.126), lentil (0.096), beans (0.091), green chili (0.085), tomato (0.084), bitter gourd (0.021), lemon (0.012), and the lowest content was in turmeric(0.003).

Arsenate (AsV) is found in aerobic conditions, while arsenite (AsIII) is more prominent in paddy soil or submerged soil. Rice has a higher tendency for As uptake than other cereals as it is grown in submerged soil conditions where arsenite is more available than arsenate (Zhao FJet al.,2010). Thus, rice grain contains arsenite (AsIII),arsenate (AsV) as well as dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) (Meharg AA et al. 2009) with approximately 80% of the As is in inorganic form (Ma L et al., 2016).

Hence, the accumulation of arsenic in rice is currently viewed as a disaster for South-East Asia, where rice is a staple food.

However, the As content in the marketed rice differs from the As content in rice from the field, as the marketed rice is a collection of grains from large parts of a country, while rice collected from a specific site mirrors the local As contamination (water and soil). Furthermore, rice exhibits varietal differences in arsenate tolerance and accumulation in grains (Wang P et al., 2016).

In the European Union (EU), the regulatory maximum level for inorganic As content in rice and rice-based products is established at 0.20–0.30 mg kg−1 for adults and 0.10 mg/kg for rice-based baby food ([Ankarberg EH et al.2015, National Food Agency, SWEDEN (Livsmedelsverket) Report no. 16/2015,1-28]).

In most rice-growing areas, besides local rice cultivars, high-yielding varieties (HYVs) are also cultivated. For example, in Bangladesh, an arsenic hotspot area, at least 57 HYVs have been launched by the Bangladesh Rice Research Institute (BRRI) (M Hossain et al. BRAC Printers,2013; 1-118). A few of the local rice varieties (e.g. kalijira, chinigura) have a distinct aroma (local aromatic rice: LAR)that attracts a price premium and, as compared to most of the HYVs, require much less extensive irrigation for maturity and production.

As demonstrated by numerous analyses, including those carried out on rice from West Bengal and on HYV and LAR varieties from Bangladesh, the accumulation of arsenic in the grain of all studied de-husked samples was between 0.06 and 0.78 mg/kg dry weights of arsenic, which did not exceed the WHO recommended permissible limit in rice of 1.0 mg kg-1 (Abedin MJ et al., 2002).

However, while most de-husked rice samples exhibit lower arsenic contents than unpolished brown rice, local Bangladesh aromatic rice varieties (brown rice), which are difficult to entirely de-husk, while accumulating more total As than the de-husked grains of HYV rice (0.103 versus 0.048 mg/kg), have a much lower arsenic accumulation factor (the propensity to accumulate arsenic) than HYV varieties even though the soil arsenic concentration in LAR fields was 2 to 5-fold higher than in fields with the HYV cultivars (Sandhi A et al., 2017).

Hence, if total arsenic content is used as a guide, HYVs would appear as the safer rice since LAR varieties transferred 3 to 4-fold more total As to the human food chain than the HYV rice.

However, if it is the accumulation factor (AF) that is used as a guide, then LAR is safer for human consumption than HYVs since, when grown in fields with similar levels of As-contaminated, arsenic uptake by LAR is lower than that of HYV due to its smaller AF value.

 

Our recommendations and conclusions 

The above clearly reveals that alternating consumption of white and brown rice varieties, as opposed to systematically consuming only one variety, be it white or brown, could be the safest route out of this conundrum.

One major problem remains, however: arsenic-contaminated groundwater at concentrations greater than the World Health Organization (WHO) guideline value for arsenic of 10 mg/L, which further distinguishes urban from rural communities with respect arsenic exposure.

While in urban centres arsenic contamination in drinking and cooking water can be largely removed via water treatment plants, this is unfortunately far from being the case in rural areas, and particularly so in regions where drinking, cooking and crop production is dependent on groundwater availability.

Thus, whereas in urban centres, most of the rice consumed, be it brown or white, is a mix of different origins that has been bulk treated and packaged, in rural areas it is the locally produced rice crops that will be preferentially consumed. Hence, while in urban centres some grains may show higher arsenic concentrations than others, the overall arsenic content will average at low levels whereas, in rural communities, the local arsenic contamination levels in irrigation water and soil will be directly reflected by the arsenic contents of the rice consumed, be it brown or white.

There are five ways to remove inorganic contaminants such as arsenic from drinking and cooking water: activated carbon (newly produced charcoal) filtration, reverse osmosis, activated alumina, ion exchange and distillation. Filtration through activated carbon will reduce the amount of arsenic in drinking and cooking water from 40 – 70%. Reverse Osmosis has a 90% removal rate, while distillation will remove 98% and anion exchange can reduce it by 90 – 10%.

While none of the above treatments could be considered for crop irrigation water, most rural communities can probably implement charcoal filtration systems to decontaminate their drinking and cooking water, the exhausted charcoal remaining usable as fuel after drying.

Hence, a simple precaution could considerably attenuate the problem. Simply washing the rice with arsenic-free water before cooking removes 3-43% of the arsenic in both white and brown rice, thereby reducing arsenic concentration and bio-accessibility sufficiently so that most rice would now be safe according to the EU maximum arsenic value (Althobiti RA et al., 2018; Liu K et al., 2018).

 

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