864
Arsenic - A Hidden Poison in Water-Soil-Rice Crop
Continuum
Hema Singh, Dr. Sangeeta GoomerAbstract - Arsenic a toxic ubiquitous metalloid is termed as ‘poison’ among general public. It is present and cycles in all four spheres of the ecosystem i.e., atmosphere, lithosphere, hydrosphere and biospheres. The concentration of arsenic in the environment has been increased due to natural sources, and/or through anthropogenic activities. Humans are mainly exposed to arsenic though consumption of As-contaminated water and foods. Rice crops are known to accumulate arsenic more efficiently at higher concentration (⁓10 folds), when compared with other crops like wheat and barley, because rice crops are grown under continuous flooded conditions. Under such conditions arsenic becomes mobile and rice crops easily uptakes arsenic. This is of major concern as rice is a staple crop for more than half of the world’s population. Therefore, presence of arsenic in rice will cause greater negative impact on the health of a large population. Nowadays bran products and brown rice are sold as ‘premier health food products’ in the market and are very popular among health conscious people. However, recent studies have shown that bran and brown (unpolished) rice also contains arsenic, the concentrations are even found higher than white rice. This can be a terrible irony for the population dependent on whole meal diets, as it can lead to a greatly increased exposure to arsenic. However, not much work has been conducted in this direction. Therefore, more extensive studies are required for building data on arsenic levels around the world. Proper remediation and mitigation steps should be implemented in those areas where arsenic concentration is above the safe limit. Further investigations are required to reveal the other arsenic exposure routes to humans. Practical guidelines on avoiding and reducing arsenic exposure are needed to be circulated among general public.
Index Terms - arsenic, occurrence, toxic, cycling, rice, bran, brown rice
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1 INTRODUCTION
Arsenic (As) is a toxic metalloid, present almost everywhere in the environment, also termed as “poison” among general public [1]. According to International Agency for Research on Cancer (IARC), arsenic is classified as Group 1 carcinogen to humans [2]. It ranks 20th element in the earth’s crust. It is colorless, odorless, and tasteless and its detection in water and biological samples needs expensive and sophisticated techniques that are only accessible to selected research laboratories and universities [3]. Arsenic is highly dependent on the conditions of the ambient environment. It may occur in four different oxidation states: As (V) (arsenate), As (III) (arsenite), As (arsenic), and as (– III) (arsine) [4, 5]. It can occur both in organic and inorganic forms in the environment where, organic forms associate with carbon and hydrogen, while inorganic forms usually associate with many minerals and other elements, especially oxygen, sulfur, and chlorine. The most important organic species are monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), while the most important inorganic species are arsenate As (V) and arsenite As (III). The toxicity of as is mainly dependent on the forms of arsenic. Inorganic forms are more toxic to living organisms than organic forms [6].
2 ARSENIC OCCURRENCE AND
MOBILIZATION IN THE ENVIRONMENT
2.1 Occurrence
Environment can be contaminated with arsenic either through natural sources and/or through anthropogenic activities. Arsenic was present ubiquitously, long before anthropogenic activities had any effect on the balance of nature [3]. Table 1 shows the background concentration of arsenic in the Earth’s Sphere.
TABLE 1
Background concentration of arsenic in the Earth’s sphere
Type Arsenic concentration Reference
LITHOSPHERE
Earth’s crust Terrestrial abundance Igneous rocks Metamorphic rocks Sedimentary rocks Muds
Clays Carbonate Stream/river Lake Soils
1.5–2 mg kg-1 1.5–3 mg 0.06 - 113 mg kg-1 0.0 – 143 0.1 – 490 3.2–60 4.0–20 1.0
5.0–4000 (mineralized area) 2.0–300
0.1 to 40 mg As kg-1 2 – 20 mg As kg-1(avg. natural levels)
[7] [3] [8] [9, 10, 11] [9, 10, 11] [11] [11] [12] [13] [14] [15] [16]
HYDROSHERE
Rainfall & snow water River water
Sea water Groundwater
< 0.03 µg As l-1
0.1 to 0.8 µg As l-1 0.001– 0.008 mg l-1 < 10 µg As l-1 (UK & USA) <0.5 – 5000 µg As l-1 (As-affected areas)
[17]
[17] [18] [17]
[17]
ATMOSPHERE
Air
< 0.02 µg/m3 (rural areas) 0.01to 0.16 µg/m3 (urban areas)
[20]
[20]
BIOSPHERE
Plants Marine animals Freshwater fish
<0.01 to about 5 µg g-1 (dw basis)
Up to 10 mg As kg-1 Up to 3 mg As kg-1
[7]
[7] [7]
2.2 Causes of arsenic mobilization
865 some of the most widely accepted hypotheses are detailed
below.
2.2.1. Reductive dissolution: It is by far the most important mode for arsenic pollution in South and South Asia. In this mechanism arsenic gets released into the groundwater, as buried peat and other organic deposits reduce the As-rich Fe oxides [21, 22, 23].
2.2.2. Alkali desorption: Desorption of arsenic occurs at pH > 8 from metal oxides, especially Fe and Mn which can lead to a high arsenic concentration in the groundwater [4, 24]. 2.2.3. Geothermal action: In some locations, geothermal solutions and fresh groundwater merge which can lead to high arsenic concentrations [4].
2.2.4. Sulfide oxidation: Atmospheric oxygen invades the aquifer which leads to oxidation of arsenical pyrite in the alluvial sediments. This mechanism is commonly referred to as the “oxidation- hypothesis” [25, 26, 27].
In Asia, when arsenic contamination scenario was observed, it appeared that the flood plains of many rivers originating from the Himalayan Mountains and the Tibetan plateau are affected with arsenic [28]. Based on this basis, the Ganga-Meghna-Brahamaputra plain is contaminated with arsenic and the source is geologic. Various theories have been postulated on sources of arsenic and mechanism of mobilization [21, 22, 27, 29, 30]. Still the exact nature of mobilization process in this region is unknown.
3 ARSENIC SOURCES AND CYCLING IN
THE ENVIRONMENT
Some of the common natural sources of arsenic can be fossil fuels including coals and petroleum, hydrothermal ore deposits and its associated geothermal water, marine sedimentary rocks, and volcanic rocks specially their weathering products and ash [4, 31]. Some of the anthropogenic activities which causes an increase in the concentration of arsenic in the environment are fossil fuel processing and combustion, disposal and incineration of municipal and industrial wastes, non-ferrous metal mining and smelting, wood preserving, and pesticide production and application [32, 33]. Arsenic cycling depends on the sources of arsenic (natural and/or anthropogenic), their availability, oxidation state, speciation and on other environmental factors [34, 35, 36]. Figure 1 show the arsenic cycle in the environment. Arsenic enters into the atmosphere through emission from burning of fossil fuels
and waste materials, smelting of coal, and volcano ashes in the form of arsenic gas or as dust particles. After rainfall, arsenic mostly in the form of dust precipitates back into the hydrosphere and lithosphere [37, 38]. In hydrosphere, it enters from various sources such as weathering of rocks and minerals [39], volcanic activities, agriculture and industrial establishments, heavy rains, floods and landslides acts as a source of arsenic to the surface water. Arsenic reaches to groundwater from arsenic contaminated soil (lithosphere) through seepage via soil washing and runoffs or leaching via sink into the soil. Arsenic contaminated water enters the lithosphere (soil) through irrigation, volatilization and other domestic uses. In lithosphere, arsenic is generally associated with pyrites and other metals [40]. Anthropogenic sources such as pesticides, wood preservatives, landfilling of electronic waste or disposal of industrial wastewater and sewage are responsible for contamination to both lithosphere and hydrosphere [38]. Arsenic accumulated in the top soil (lithosphere), is taken up by plants and therefore enters the biosphere [41]. Another mode of arsenic entry into the biosphere is irrigation of agricultural crops with arsenic-contaminated water (e.g., rice), which affects humans, plants and animals [38]. Plants translocate arsenic from As-contaminated soil, which is then entrenched in human beings and animals [42, 43]. The amount of arsenic in a plant is almost solely dependent on the amount of arsenic it is exposed to. Its concentration varies widely from 0.01 to 5 mg As kg-1 [3]. Further, arsenic is introduced into the human
body either through the ingestion of arsenic-contaminated water and food or through dermal contact (hand-to-mouth) and causes serious arsenic toxicity in humans [44, 45]. Arsenic cannot be degraded nor be destroyed once it is released into the environment and therefore it cycles in the environment [38]. This suggests that arsenic is present almost everywhere in the environment, and is linked to the humans and animals either directly or indirectly.
866
Fig. 1: Pictorial representation of arsenic cycling in the environment
4 HEALTH EFFECTS OF ARSENIC ON
HUMANS
Prolonged exposure to arsenic causes chronic diseases in humans called Arsenicosis. Chronic arsenic toxicity (CAT) adversely affects multi organ systems in humans. The symptoms are insidious in onset with non-specific symptoms of diarrhea, abdominal pain and sore throat [46, 47]. Consumption of arsenic even at low concentration can leads to carcinogenesis [3]. Wide variations of symptoms have been observed in arsenic affected population. Some of the specific symptoms of chronic arsenic toxicity in humans are:
4.1. Skin manifestations – Skin lesions such as pigmentation and keratosis are characteristic symptoms of chronic arsenic toxicity. Patterns appears in pigmentation include “raindrop” pattern, hyperpigmentation and leucomelanosis [48, 49, 50]. Arsenical keratosis is subdivided into mild (minute papules < 2 mm), moderate (lesion 2-5cm) and severe (wart-like or horny appearance > 5 mm) elevations on palms and soles [50, 51]. In West Bengal, first population based study was conducted to ascertain pigmentation and keratosis prevalence in relation to arsenic exposure. The study showed men compared to women and subjects below 80% of the standard body weight for their age and sex were more prevalent to both keratosis and pigmentation [49].
4.2. Systemic manifestations - According to a report conducted in West Bengal, 156 cases on the clinical features that were chronically drinking arsenic-contaminated water, produces various systematic manifestations over and above skin lesions [49].
4.3. Respiratory disease - Restrictive and obstructive lung disease was reported in a study from West Bengal [49]. Patients with the characteristic skin lesion of Chronic arsenic toxicity showed association with respiratory disease [52]. Consumption of arsenic contaminated water was found to be
associated with reduced pulmonary function and bronchiectasis [54, 55, 56].
4.4. Liver disease – Many researches have reported that people who were drinking arsenic contaminated water were found to have portal hypertension associated with portal fibrosis, hepatomegaly and liver enlargement [48, 56, 57, 58, 5, 60].
4.5. Cardiovascular disease - Increased risk of cardiovascular disease is reported in several studies [61, 62, 63] in patients who were exposed to arsenic. Several investigators have also shown a link between diseases like hypertension and ischemic heart disease associated with long term arsenic exposure. Arsenic causes direct myocardial injury [64], cardiac arrhythmias [65] and cardiomyopathy [64].
4.6. Diseases of nervous system - Chronic exposure of arsenic through drinking water causes cerebrovascular disease [59, 66, 67], Cognitive and memory impairment [68], peipheral neuritis characterized by paresthesia (tingling, numbness, limb weakness, etc.) [69], peripheral neuropathy [48, 59, 60, 67, 68, 70], sensory neuropathy [71], sleep disturbances, and weakness have also been reported. In West Bengal, arsenic affected patients also showed symptoms of depression, headache, irritability, lack of concentration, sleep disorders, and vertigo [72].
4.7. Diabetes - In Taiwan a dose-response relation between cumulative arsenic exposure and prevalence of diabetes mellitus was observed, and reported a significant relation between them [73]. A study in Bangladesh reported significant increase in the prevalence of diabetes mellitus due to drinking arsenic contaminated water among patients suffering from keratosis [74]. Hence, these results indicate that CAT may induce diabetes mellitus in humans.
867 showed incidence of anaemia, leucopaenia and
thrombocytopaenia [49, 75].
4.9. Pregnancy outcome - In the high arsenic exposed area, cases of spontaneous abortions, Stillbirths, neonatal and post-neonatal infant mortality rates were found to be increased [76, 77, 78, 79]. Kile et al. [80] studied the effect of arsenic exposure during pregnancy on perinatal outcomes in Bangladesh and reported that arsenic exposure during pregnancy was associated with lower birth weight.
4.10. Other effects - Many workers who were chronically exposed to drinking arsenic contaminated water reported high incidences of weakness and fatigue [48, 49, 57, 68, 75]. In West Bengal and Bangladesh, conjunctival congestion and non-pitting oedema of the legs and hands have also been reported in patients of chronic arsenic toxicity [49, 60].
4.11. Arsenicosis and cancer - International Agency for Research on Cancer (IARC) [2] reported that arsenic is potentially carcinogenic for skin cancer. Malignant arsenical skin lesions may be Bowen’s disease (intraepithelial carcinoma, or carcinoma in situ), basal cell carcinoma, or squamous cell carcinoma. The lesions are usually erythematous, pigmented, crustated, fissured and keratotic. Some may be nodular, ulcerated or eroded [81, 82]. Arsenic is associated with skin, lung, liver, kidney, and bladder cancers [83]. In animals’ high levels of arsenic are teratogenic [84].
5 ARSENIC CONTAMINATIONS IN
GROUNDWATER: A GLOBAL PERSPECTIVE
Arsenic contamination in the drinking water has now turned to be a major environmental concern and global public health issue [85, 86, 87, 88, 89, 90, 90, 91, 92, 93, 94, 95, 96]. Humans exposed to arsenic were responsible for the largest mass poisoning of a population in the human history [86, 91, 96]. In many countries millions of people (more than 200 million) are probably at risk of exposure to As-contaminated drinking water for drinking purposes. Table 2 shows the estimated number of people exposed to arsenic in South Eastern Asia through consuming As-contaminated drinking water. Still in many areas water wells that supply drinking water have not been tested for As-contamination, therefore the information on arsenic-affected population are incomplete [97].TABLE 2
Estimated number of people in several South Eastern Asian countries exposed to arsenic in drinking water at concentrations greater than 10
mg/L or 50 mg/L
Country Total population [98]
Population at risk of exposure to drinking water as exceeding 10 µg/L or 50 µg/La
References
Bangladesh 163 million 28 milliona – 60 million [99, 100, 101, 102] Cambodia 15.8 million 320,000a – 2.4 million [101, 103] China 1.38 billion 2 milliona – 5 milliona [101, 104] West Bengal 91 millionb 6 milliona [104]
India 70-80 million [104]
Myanmar 52.9 million 3.4 milliona [101]
Nepal 29.0 million >3 million [105]
Pakistan 193.2 million
47 million – 60 million [106]
Vietnam 94.6 million 10 milliona [101]
Retrieved from Uppal et al. [97].
a Population at risk of exposure to arsenic concentrations in drinking water over 50 mg/L.
b Obtained from population of India 2018 [107].
In groundwater, a very wide range of arsenic concentration can be found from < 0.5 to 5000 µg As L in more than 70 countries [108]. Countries like Argentina, Chile, China, Hungary, Mexico, and USA [4, 33] also in the Indian state of West Bengal, Bangladesh, and Vietnam, elevated levels of arsenic contamination in groundwater has been well reported [33, 21, 109, 110, 111, 108]. Table 3 shows the As-contaminated groundwater across the globe, which varies considerably depending on the geographic conditions.
TABLE 3
Status of as contamination in natural groundwater in various countries
Country Region Groundwater As level (µg As L-1)
Permissible limit (µg As L-1)
References
Argentina Chaco Province 0.7 - 1900 10 (WHO) [112]
Afghanistan Ghazni 10–500 10 (WHO) [113]
Australia
Victoria (around the gold-mining regions)
1–12 (Groundwate r);
1–73 (Drinking-water); 1–220 (Surface water)
10 [89, 91, 113]
Bangladesh Noakhali <1–4730 50 (WHO) [113, 114, 115]
Brazil
Minas Gerais (Southea stern Brazil)
0.4–350 (Surface water)
10 (WHO) [89, 91, 93]
Cambodia Prey Veng and Kandal-Mekong delta
Up to 900
1–1610 10 (WHO) [113, 116]
Canada
Nova Scotia (Halifax county)
1.5–738.8 10 (WHO) [91, 113]
China — 50–4440 50 (WHO) [117]
Chile 100-1000 [118]
Finland Southwest Finland 17–980 10 (WHO) [89, 91, 93]
Greece Fairbanks (mine tailings)
Up to 10,000 10 (WHO) [4, 113]
Hungar,Ro
mania <2-176 10 [118]
India West Bengal Uttar Pradesh
< 10–3200 50 (WHO) [4, 89, 113, 119, 120]
Japan
Fukuoka Prefectur e (southern region)
1–293 10 (WHO) [89, 91]
Mexico Lagunera 8–620 25 [4, 89, 113]
868 Pakistan
Muzaffar garh (southwe stern Punjab)
Up to 906 50 [9, 91, 121]
Spain - <1-100 [118]
Taiwan — 10–1820 10 (WHO) [4,89, 113] Thailand Ron Phibun 1–>5000 10 (WHO) [4,89, 113]
USA Tulare Lake Up to 2600 10 (USEPA) [113, 122, 123]
U.K. - <1-80 10 [118]
Vietnam
Red River Delta (Northern Vietnam) Mekong Delta (Souther n Vietnam)
<1–3050 10 (WHO) [4, 117]
Source: Adapted from Shankar & Shankar [124] and Sarkar and Paul, [123]
6 ARSENIC IN IRRIGATION
WATER-SOIL-CROP CONTINUUM
Most of the researches till date are focused on the arsenic contamination in drinking water. Still less attention has been paid towards the risks of As-contaminated groundwater used for irrigation purposes. Some of the surveys and research studies reported different ranges of As concentration in the irrigation water. Imamul Huq et al. [125] reported that As concentration in the irrigation water varied from 0.14–0.55 mg L-1. Long-term use of As-contaminated groundwater for
irrigation could cause accumulation of arsenic in the topsoil of the agricultural field. Soil acts as a sink to arsenic, where concentration of arsenic builds up in soil either through natural sources or anthropogenic activities or both [3, 126]. As deposits to soil by natural processes such as weathering of As-bearing rocks or use of As-contaminated groundwater for irrigation or through anthropogenic activities such as mining [127], smelting [128], landfilling of industrial wastes, disposal of chemical warfare agents [129], agriculture practices [130] in many areas of the world, application of As pesticides, accident [131], burning coal [132], preservation of wood [133, 134, 135] and illegal waste dumping [136]. Arsenic deposited in the soil accumulates rapidly as it is depleted from the soils only through plant uptake, leaching, methylation, or erosion [137] (Fig 2).
Fig. 2:Sources of arsenic in soil and its build up and depletion ways
Plants can uptake As present in the surface soils. As contaminated soils and irrigation waters may increase As in the food crops via plant uptake mechanisms. Onken and Hossner [138] observed that plants grown on arsenic contaminated soil had higher rate of arsenic uptake as compared to plants grown in soils not contaminated with arsenic. Several studies have reported that crops like wheat, maize and paddy, when grown on such As-contaminated fields may accumulate arsenic, and can add substantially to the dietary arsenic intake and additional health risks for humans [139, 140, 141, 142].
7 ARSENIC IN PADDY CROPS VIA IRRIGATION
WATER AND SOIL
Paddy crops can accumulate higher concentration of arsenic more efficiently (⁓10 folds higher) as compared to other cereal crops like wheat and barley when grown in As-contaminated soil [143, 144]. According to Meharg and Rahman [145], 150-200 (up to 900) mm and 500-3000 mm of irrigation water is required for land preparation before planting and during crop growth, respectively, and assumed that 1000 mm groundwater per year (1000 L m-2 per year) conservatively. If
As-contaminated irrigation water (100 µg As L-1) would be used
for irrigation and it retains in the soil, the water input would cause a yearly increase of 1000 µg As kg-1 in field soil. They
also mentioned that wheat, maize and vegetables require much less water for growth. Hence, the deposition of arsenic in the field soil would be less. When paddy fields were irrigated with As-contaminated water for ten years, 5000 – 10000 µg As kg-1 was added approximately to soil [146].
Such higher accumulation of arsenic in paddy crops can be attributed to many factors, such as:
7.1. General Rice cultivation practices - Continuous flooding of the irrigation land for the cultivation of rice requires excessive amount of water; and if this irrigation water is arsenic-rich, it can result in to heavy accumulation of arsenic in rice crops (45, 145, 147).
869 arsenic in the paddy soil through shift in As speciation
(arsenate to arsenite) (148, 149).
7.3. Rice plants are known as “hyperaccumulator” as it can accumulate huge amount of arsenic from the irrigated water and soil, and can also translocate it to shoots and grain (Fig. 3). This is because rice plants have high arsenic translocation factor for arsenic often approaching to unity whereas, wheat and barley plants have low translocation factors of arsenic because of their restricted translocations from roots to shoots, hence they are called excluders [150, 151, 152, 153, 154]. Arsenic translocation factor (TF) of rice (0.8) is greater than wheat (0.1) and barley (0.2) [149].
8 ARSENIC CONCENTRATIONS IN RICE: A
GLOBAL PERSPECTIVE
In several Asian countries, paddy fields irrigated with groundwater contains arsenic-rich sediments pumped from tube wells. As a result, people in countries where rice is the staple diet, such as Bangladesh, India, China and Vietnam, face health risks from arsenic. Table 4 shows the arsenic contamination in rice around the world. Major Rice producing and exporting countries in the world shows the highest contamination of arsenic in rice, ranging from 0.0 – 1.835 mg/kg. Rice produced in India also contains high arsenic concentration. Arsenic levels in these major rice producing countries are mostly above FAO/WHO recommended safe limit (0.2 mg/kg) [155].
TABLE 4
Arsenic levels in Rice around the World
Country Range (mg kg-1) References
Bangladesh 0.010 – 1.835 [44, 117, 145, 156, 157, 158, 159, 160, 161, 162, 163, 164] India 0.01 – 0.961 [165, 166, 157, 158, 167, 44,
153, 163, 168] China 0.0 – 0.624 [44, 169, 170, 171, 172] Thailand 0.01 – 0.39 [158, 173, 44, 163] Philippines 0.00 – 0.025 [158]
Pakistan 0.073 – 0.088 [163] Japan 0.07 – 0.42 [44] Korea 0.104 – 0.774 [174] Taiwan 0.12 – 0.76 [157, 175] Australia 0.02 – 0.438 [158, 163] Italy 0.07 – 0.547 [44, 163, 176, 177] Spain <dl – 1.16 [44, 173, 178, 179] France 0.09 – 0.56 [44]
Portugal 0.22 – 0.78 [180] Venezuela 0.19-0.46 (0.3) [181] Egypt 0.02-0.08 (0.05) [182] Europe 0.13-0.2 (0.15) [157] USA 0.16-0.71 (0.29) [183] Ghana <0.01-0.15 [184]
Canada 0.02 [157]
Lebanon 0.01-0.07 (0.04) [184]
Most of the published works have been done on the market-based surveys, which cannot show the actual level of arsenic contamination in paddy rice. More field based studies are required to know the actual level of arsenic contamination in paddy rice of a particular area.
9 DISTRIBUTION OF ARSENIC IN DIFFERENT
FRACTIONS OF RICE GRAIN
Significant variations have been reported in the total arsenic concentrations in different fractions of rice (hull, polished rice, whole rice, and bran). Meharg et al., 2008[185] reported that brown rice generally contains higher levels of arsenic as compared to white (polished) rice. They also reported that total arsenic levels were much higher in bran than in polished (white rice) rice, obtained from the same whole grain rice, due to the localization of arsenic in the bran layer. Rahman et al. [142] studied total arsenic concentrations in different fractions of raw rice collected from arsenic-contaminated area (Satkhira district) of Bangladesh. Results showed highest arsenic concentrations husk (range of 0.7–1.6 µg/g d. wt.) followed by bran (0.6–1.2 µg/g d. wt.), whole grain (0.5–0.8 µg/g d. wt.), and polished rice (0.3–0.5 µg/g d. wt.). Thus, the order of arsenic concentrations in rice fractions was husk > bran > whole rice > polish rice. Ren et al. [186] also determined the total arsenic concentration in fractions of Chinese whole grain rice, and found that arsenic concentrations were highest in bran (range of 0.55–1.20 µg/g d. wt.), followed by whole grain (0.14–0.80 µg/g d. wt.) and polished rice (0.07–0.4 µg/g d. wt.), showing the same trend reported by Rahman et al. [147]. Sun et al. [159] also determined total arsenic concentrations in different fractions (endosperm, whole grain, and bran) of freshly milled Chinese (two varieties) and Bangladeshi (four varieties) rice grains. Results showed that the mean (n=6) arsenic concentrations in endosperm, whole grain, and bran were 0.56± 0.08, 0.76± 0.12, and 3.3± 0.6 µg/g d. wt., respectively. The trend of total arsenic concentration in fractions of rice grain was endosperm<whole<white grain<bran, which is in agreement with the previous studies of Rahman et al. [147] and Ren et al. [186]. In the latest study, conducted by Devenda et al., [187] the distribution of arsenic in rice plant was also in the order – roots > straw> husk > grain. They also reported that the mean arsenic concentrations in grain, husk, straw and root increases with increasing concentration of arsenic in irrigation water.
870 intended for human consumption can contain up to 2000 µg/kg
of Arsenic, and were typically over 1000 µg/kg. Even pure untreated bran contains arsenic levels approaching 1000 µg/kg. It is unlikely that consumers of these products are aware of the dietary arsenic that they are being exposed to. When conforming to manufactures daily ingestion rates-17 g per serving, equated up to 34 Asi µg/d intake when bran solubles contained 2000 µg/kg of arsenic.
10 CONCLUSION AND RECOMMENDATIONS
This study has discussed distribution and transport of arsenic in the environment leading to better understanding of this particular problem. However, research focused only on the general distribution and cycling of arsenic in the environment. More and detailed studies are needed to focus on assessing arsenic levels in the environment to gain full picture of arsenic contamination around the world. It is very important to raise awareness of the toxicity and the potential environmental impacts of arsenic contamination via various routes. Since, Rice is a major staple food, so accumulation of arsenic in rice grain and its different fractions can pose immense health hazards to humans. The interest in rice as a potential source of exposure to arsenic came into notice due its large consumption across the population. Total arsenic (tAs) content of rice and its different fractions varies geographically depending on the Arsenic content of soil and irrigation water therefore on a massive scale arsenic contamination in rice and in different fractions of raw rice is a newly uncovered disaster. New and effective remediation technologies are required which are cost effective, promising, and environmental friendly. Long-term monitoring is required in areas prone to arsenic contaminated groundwater. Further researches focusing on assessing arsenic concentration in soil and food crops in vicinity of arsenic contaminated groundwater is required. Further investigations are required to reveal the other arsenic exposure routes to humans.ACKNOWLEDGEMENTS
The authors declare that there is no conflict of interests regarding the publication of this paper.
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