Mercury Contamination in Rice: Sources, Impacts and Solutions

Tan Mei PengID: 012235School of BioscienceUniversity of Nottingham, Malaysia Campus43500, Jalan Broga, Semenyih Malaysia.

Many researchers have found rice is another pathway of MeHg exposure. Most of this rice contamination occurs in China since China is the largest source of mercury consumption (VCM industries) and producer in the world. Such contamination is spreading to other parts of the world as well which include Indonesia, Philippines and with the concern of Sub-Saharan Africa due to the increase of artisanal small-scale mining sectors (ASGM) which gold prices and demand are increasing. In UNEP Global Mercury Assessment 2013 report, ASGM has the highest level of atmospheric mercury emissions. This review paper contained the Hg contamination of rice in certain countries which leads to the question of the biogeochemistry of rice. Research on health impacts from mercury speciation exposure found that methylmercury is the most neurotoxic species to human and it has been reported to have severely damaged the IQ of the new-borns. Mercury use and gold consumption were compared and discussed as well as the rice and mercury production. Several mitigations has been suggested and applied. Hg contamination in rice has brought negative impacts to the human and the environment which should be continuously monitored to gain a better picture in solving the problem.

Keywords: Methylmercury, Rice, Gold, ASGM, health impactsIntroductionMany researchers have found several traces of heavy metals like arsenic (As), lead (Pb), cadmium (Cd), copper (Cu), Chromium (Cr) and zinc (Zn) are common contaminations in paddy rice around the world due to anthropogenic and industrial activities(YS et al. 2010; Meharg and Rahman, 2003; Liu 2011; Park 2011). Yet, heavy metal like mercury (Hg) is not uncommon contamination in paddy rice; however it is not been reported as much as As, Pb and Cd until recent years. All these heavy metals contamination are toxic to the environment and humans health.

Mercury contamination especially methylmercury (MeHg) is a potent toxicant which has been widely spread into the aquatic system as well as paddy rice. The first reported outbreak of MeHg contamination and poisoning was in Minamata in 1950s and Niigata in 1960s (Harada, 1995). It was found that the bioaccumulation of MeHg in fish and shellfish samples has caused severe negative health impact (Harada, 1995; Li, Feng and Qiu, 2010). Later on, it became a world issue which has been reported in Iraq, Canada and US in 1969 (Harada, 2011) and other parts of the world. In the recent research from IPEN and Biodiversity Research Institute (BRI), Japan and Uruguay fish samples contained more than 1.0ppm of mercury which is not recommended to consume according to USEPA guidelines (IPEN and BRI, 2013).

One of the earliest researches on mercury contamination in paddy rice was conducted by Morishita, Kishino and Idaka (1982) in Nifu Mine in Seiwa Village, Japan whereby they collected samples of soil, rice plant and human hair to measure the mercury level. It was found that soil sample near the pits and mine of Nifu contained higher mercury level (5.05ppm 98.6ppm) compared to soil samples distant from Nifu (below 1.0ppm). The mercury level in brown rice is higher near the mine compared to soil free from contamination.

Fish is usually the dominant source of Hg compared to rice. The issue of mercury contamination in paddy rice became a global issue when Hg was found in several countries paddy rice. In Guizhou Province, China; rice is the major pathway for meHg exposure rather than fish (Zhang et al. 2010). Severe Hg pollution was found in Guizhou Province due to Hg mining and smelting (Wanshan), zinc smelting (Weining), and heavy coal-based industry (Qingzhen). Other than China, paddy field in Naboc area, Philippines also contaminated with Hg due to irrigation using Hg contaminated river water nearby artisanal gold mining and 38% of the local inhabitants were classified as Hg intoxicated. (Appleton et al. 2005). In Indonesia, GunungPongkor, rice straw and rice grain contain high mercury due to traditional gold mining and smelting. Values were higher than the maximum permitted level of mercury in soils (0.5 ppm). The concentration of mercury in soil near the mining was higher than in more distant soils. (Setyorini, Prihatini and Kurnia, 2002)

Several studies have been done on the effects of mercury exposure and human health. Usually hair and blood samples were used for detecting methylmercury in the body. Research show that to measure recent exposure, mercury in blood (B-Hg) is a good biological indicator and hair is good measurement for the average exposure over the growth period especially on pregnant woman and infants (Apostoli et al., 2003; Cernichiari,1995 as cited in Li et al. 2010).

According to EPA (1999), mercury exists in three chemical forms, methylmercury (Organic mercury), elemental mercury and inorganic mercury. Exposure of elemental mercury through inhaling the vapour will cause toxicity in lungs, gingival, skin and eyes. It also causes insomnia; weaken central nervous systems, kidney and lower immune system. High exposure of inorganic mercury (released from industries, eg. Mercuric chloride) may experience skin rashes, dermatitis, mental disturbances, and central nervous breakdown and weaken muscles.

Li, Feng and Qiu (2010) stated MeHg (organic mercury) was formed through the methylation of inorganic Hg by microorganisms present in sediments which are bioavailable and biomagnifies in the aquatic food chain. MeHg also formed in water-saturated soils like paddy field whereby MeHg will be bioaccumulated in the paddy rice.

Ingestion of MeHg food has caused negative impacts on cognitive thinking, memory, attention, language, and fine motor and visual spatial in fetal (1999). 95% of the ingested MeHg will be absorbed in the gastrointestinal tract where it will be distributed readily in all tissues (Harada, 1995). MeHg has the ability to cross diffuse the barriers and penetrate all membranes easily (Mangal, 2001). 5% of adults contained 200 g/L of Hg in the blood would experience neurological effects (WHO, 1990).

Rice does not contain a lot of beneficial micronutrients (eg. n-3 LCPUFA, selenium and essential amino acids) like fish. Rice contains less protein than fish that can prolong the survival after ingestion of MeHg (Adachi and Hirayama, 2005). This concludes more health risks with the consumption of MeHg contaminated rice than fish if both were to be taken at the same rate.

Sources and Biogeochemistry of Mercury in Rice

Many researchers have found the sources of mercury contamination in paddy rice were released from Hg mining, non-ferrous metal smelting, coal combustion industries and artisanal and small-scale gold mining (ASGM) and most of the contaminated paddy rice were reported in Wanshan mining areas, China (Anderson et al. 2012; Appleton et al. 2005; CCICED, 2011; Li et al. 2010; Zhang and Wong, 2006; Zhang et al. 2010).

Currently, China is the worlds biggest Hg emitter with the Hg emission increasing rapidly in Asia (Lin, 2011). UNEP global mercury assessment (UNEP, 2013) indicated East and Southeast Asia have contributed 40% of the global Hg emission seen in Figure 1. The demand for Hg in China is increasing due to usage of Hg as catalyst for PVC production (CCICED, 2011). The emission of Hg in China has caused China to be one of country with highest Hg pollution.

In general, mercury can be released through natural sources (volcanoes), anthropogenic activities and re-emission. In figure 2, it showed the simplified version of anthropogenic activities and the mercury cycle. The biogeochemistry of mercury has received attention from many researchers because the methylmercury (MeHg) is a neurotoxic component to human and wildlife; moreover it is easily accumulated in biota and biomagnified in the food chains (Horvat et al., 2002).

Figure 1: Global Distribution of Atmoshperic Hg Emission (2010)

Feng, Qiu, Shang and Wang (2011) did research on mercury contamination in Large Scale mercury mining (LSMM) and Artisanal Mercury Mining (ASMM) in Guizhou. The results indicated there were heavy Hg contaminations from LSMM and AMM. The Hg released from LSMM were mainly concentrated in the historical mining and smelting facilities, while Hg pollution resulted from AMM was found to be distributed everywhere in the mining area. Moreover, newly deposited Hg (mostly IHg) on Paddy field from AMM was more ready for methylation than the bulk of old Hg in soil. This suggested the significant impact of AMM on MeHg accumulation in rice, which can pose a health risk to human.

Several paddy fields were contaminated with Hg from mine tailings and irrigation with mercury contaminated water (Appleton et al., 2005; Zhang et al., 2009). Even though mercury level in paddy soil has positive relationship with the mercury level in the river/water being used for irrigation but that only apply to total mercury (THg) and not MeHg. Zhang et al. (2009) research showed weak correlation (P>.05) between MeHg concentrations in river and paddy soil and suggested Hg methylation is more important in the paddy soil than transport of MeHg from river to the soil. There was also weak correlation between THg concentration and MeHg concentration in paddy soil which showed that there are more factors to be considered. For instance, Frohne et al. (2012) studies found that incresed Hgt (total dissolved mercury content) concentration has an inhibitory effect on Hg net methylation

Another research suggested agricultural wetland like rice field is one of the potential sources for bioaccumulation of methylmercury. Ackerman and Smith (2010) introduced caged mosquitofishes into the agricultural wetlands; white rice fields, wild rice fields and permanent wetlands. After 2 months, it was found that mercury was increased tremendously in all the caged fishes. The total concentrations of mercury increased tremendously from 135% to 1,197% while mercury body burdens rose from 29% to 1,566%. Fish mercury concentrations are the highest in white rice fields (12 times higher than the initial concentrations), wild rice fish mercury concentrations were 6 times greater and permanent wetlands were 3 times greater. 82% of caged fish and 59% of wild fish have mercury concentrations exceeded 0.2 g/g, a common fish toxicity threshold.

Boening (2000) and Rinklebe (2010) found that many floodplains soils contain high Hg content are due to atmospheric deposition and transport from the watershed. In paddy field, Meng et al. (2010) reported Hg in ambient air is the source for the accumulation of Inorganic Mercury (IHG) on the above ground parts while IHG concentrations in the root were related to Hg concentrations in the paddy soil. Positive correlation of THg and MeHg in rice does not concluded MeHg in rice was due to methylation of IHg in the paddy soil. Zhang et al. (2009) stated the high concentration of THg and MeHg in rice plants may be due to external factors whereby THg might be absorbed from the ambient air through leaf surface while MeHg was due to the uptake from the soil via roots. Machiwa (2010) research also found the rice husks contained higher concentration of Hg which may due to significance of atmospheric transport of Hg.

In order to determine if rice is the bioaccumulator for methylmercury in paddy soil, Zhang and his team (Zhang et al. 2009) went and collect the samples of Qiu et al. (2008) research showed the rice grown at abandoned mercury mining areas in China contained methylmercury of > 100 g/kg in its edible portion which were 10-100 times higher than other crops. This showed that methylmercury is easily bio-accumulated in paddy rice and there is a positive relationship of MeHg in paddy soil and rice plants.

There are a few biogeochemical cycles that have large effect on MeHg production. Maryland Department of Natural Resources (2008) concluded Hg and S cycles are closely linked together and the key to control Hg methylation rate is the balancing of sulphate and sulfide. According to Frohne et al. (2012) research, sulfate reducing bacteria (SRB) such as Desulfobactor species or Desulfovibrio species helps to promote Hg methylation. The bacteria process the sulfate in the contaminated floodplain soils to take up inorganic mercury and convert it to methylmercury through metabolic process. Toxicity became greater when inorganic mercury been converted to methylmercury thus biota require longer time to eliminate the methylmercury (USGS, 2000). On the other hand, sulphide creates mercury complexes that are not bioavailable for Hg methylation. Frohne et al. (2012) have also found that high concentration of dissolved organic carbon (DOC) may promote Hg methylation by enhancing the activity of SRB. DOC served as an essential carbon source for bacteria. The positive relationship of DOC/Hgt plays an important role for methylation.

Liu et al. (2012) stated the exchange of MeHg between paddy soil and rice plants mainly occurs in pore water and it was found there was a significant correlation of MeHg uptake flux from soil to the roots to the top of the rice plants using the diffusive gradient in thin films (DGT) technique. Hg cycling of flooded rice paddy in Wanshan Hg mining region was investigated by Rothenberg and Feng (2012) in 2008 and 2009. The rice-planted section pore water contained higher concentrations of MeHg compared to the fallow section of the same paddy. Pore water in rice-planted section was more acidic which indicated that the low pH value has some effect on the mobility and methylation of Hg (Boening, 2000). Nevertheless, Rothenberg and Feng (2012) research showed that the fallow section of paddy field act as a sink for FeS(s) and no presence of FeS(s) in rice-planted section and both sections contain low sulphide concentrations. It was suggested FeS decreased the bioavailability of Hg by interacting with inorganic sulphur ligands. Fe (III) reduction has increased electron acceptors for SRB which indirectly enhanced Hg(II)-methylation. Frohne et al. (2012) also reported Fe reduction through Fe reducing bacteria like Geobacter, able to decrease demethylation.

The distribution pattern and content of methylmercury (MeHg) and inorganic mercury (IHG) in rice plants tissues (Oryza sativa L.) became the interest and focus for researchers (Meng et al., 2010). The research concluded rice plants cultivated in Hg mining sites have higher concentrations of IHg and MeHg than the control sites and rice seed (brown rice) has the highest MeHg being accumulated compared to the other parts of the plants (hull, root, stalk and leaf.) Meng et al. (2011) research suggested newly deposited Hg is more easily transformed to MeHg and accumulated in the tissues of rice plants compared to Hg which has been deposited for a long time. Orihel et al. (2006) research also suggested aging Hg in sediments and soils decreases the bioavailability of Hg for methylation. Meng et al. (2011) research confirmed soil is the potential source of bioaccumulation of MeHg whereby MeHg was first absorbed by the roots and then translocate to the leaf and stalk. During ripening period (harvest season), most of the MeHg was transferred to the rice seed and very small amount was being retained in the root section. Except for premature plants, most of the MeHg will retain in the leaf and stalk.

Main Geographic Trend in Rice production, Mercury Production and Emissions

In 2012/2013 forecast from FAO (2013), rice was third highest ranking cereal being produced with the annual production of 486 million tonnes (m/t). It is a staple food for nearly half of the current world population. Adapted from 2011 FAO statistical data, China (202.6 m/t), India (155.7 m/t), Indonesia (65.7 m/t), Vietnam (42.3 m/t) and Bangladesh (50.6 m/t) are the top five global rice producers countries since 1961.

Nonetheless, Asia consumed 90 % of the rice especially for the low income population as they spent 50% of their income on rice (Zeigler and Barclay, 2008). According to IRRI (2013), about one billion including 640 million in Asia were living in poverty where rice is their main food source. FAO (2004) reported rice provided 27% of energy supply, 20% of protein and its a good source of niacin, thiamine and riboflavin in the developing countries. It was noted that rice is one of the basic food that provide nutrients yet places like Wanshan in Guizhou China, people are getting intoxicated from the rice consumption.

Figure 3: Rice Production from 1961-2011 (tonnes). FAO Stat.

Rice is also linked to poverty when rice prices have tripled in 2008; about 100 million people were being forced to live in poverty. When rice prices rose to 71% from 2007 to 2010, about 2 millions of people were impoverished in Indonesia (Dartanto, 2010). It was thus concluded the increase/decrease rate of poverty was highly affected by the fluctuations of the world price.

The global rice consumption since 1990s per capita is 65kg due to stronger economic in many Asian countries. IRRI (2013) reported rice consumption in China, India and Indonesia have declined as higher income opted for larger variety of food choices. Based on FAO statistics, global rice consumption does not decreased for there was a rising demand in other parts of Asia, Latin America and Sub-Saharan Africa (Rejesus et al. 2012). Since 2011, rice production in Sub-Saharan Africa has risen by 4.4% (FAO, 2013). The newly developed rice variety New Rice for Africa (NERICA); collaboration and funding from NGOs, Africa Rice Center and IRRI has successfully sustained 2.28% population growth per year (Wailes and Chavez, 2012). Rejesus et al. (2012) made a forecast suggested global rice consumption will continue to rise from 450 (m/t) in 2011 to about 650 (m/t) by 2050.

Western countries like Spain, Slovenia, California USA and Italy used to be the major Hg mining areas responsible for over 70% of the total global production. Nevertheless in 2000, Hg decreases to 1800 tons from these mines. However, the trend of Hg mining appeared to be shifting focus to the east, the Asia countries with the estimated total production of 1930 tons in 2011 (Hylander and Meili, 2003; USGS, 2012). China being the major staple of rice production is also one of the countries with highest Hg production; 1,400 metric tons in 2011 (USGS, 2012).

As there will be increasing trend for global rice production and consumption, the mercury production and the graph of the global rice production has been compared with global mercury production shown as below in Figure 4. The mercury production only covers Hg mining and by-products from smelting activities (USGS, 2012). There may not be drastic mercury mining due to less expensive Hg sources could be gained from recycling; eg. Hg from closed chlor-alkali plants and by-product from zinc, copper could replace if new regulations restrict Hg emissions from ore refining (Swain et al. 2007).

According to Lin (2011) and Maxson (2009), the demand of Hg in China was predicted to increase as PVC/VCM production is expanding with the intentional use of Hg as catalyst. Thus, Hg mining in China might increase but only for domestic use.

Figure 4: Global Mercury (USGS) and Rice Production (FAO Stat) 1961-2011 (tonnes)

The trend of anthropogenic mercury emissions to air also shifted from Europe and North America to East and Southeast Asia and China is the largest contributor (CCICED, 2011; UNEP, 2013). In CCICED (2011) report, China has released 643 tonnes of mercury into the air in 2007 whereas in UNEP (2013) report, China is responsible for three-quarters of atmospheric mercury released of East and Southeast Asia (average of about 583 tonnes). Thus, high mercury production and emission have caused concern of Hg contamination in paddy rice.

It was known that atmospheric mercury is easily dispersed, travel and deposit on soil or water bodies within the range of 100-1000km. With such increasing production of rice, it has been a concern to the rice consumer (UNEP, 2013).

Artisanal Mining Gold Production

UNEP global mercury assessment (2013) showed the atmospheric Hg emission has slightly increased by 30 tonnes since 2005 (1930 tons) to 2010 (1960 tons), Figure 5. Comparing 2005 and 2010 atmospheric mercury emission, artisanal small-scale gold mining (ASGM) has doubled from 350 tons to 727 tons. This may due to the increment of gold prices in the market, rising rate of rural poverty, and better data was obtained for the mercury emissions level. China was considered as one of a significant contributor in ASGM even though the recent information was limited due to China had banned such activities in 1996. Nevertheless, East and Southeast Asia, South America and Sub-Saharan Africa are the main contributors in this sector (Table 1).

Table 1: Hg emissions from ASGM sectorRegionCountriesASGM Hg Emissions (Tonnes)East and Southeast AsiaChina652Indonesia200Philippines 90South AmericaColombia200Bolivia150Peru100Sub-Saharan AfricaGhana120Tanzania 60

Figure 5: Global Atmospheric Hg Emission in 2010 (tons)

London-based International Institute for Environment Development (IIED) new report suggested Artisanal Small-Scale mining (ASM) is one of the sectors where 20 to 30 million of the worlds poorest people are depending on it to support the livelihood more than five times of that number (IIED, 2013). The International Labour Organisation (ILO) statistics showed ASGM supports livelihood of more than 13 million people in the developing world (Phiri, 2011). ASM employment rate is 10 times higher than large-scale mining and people involved in this sector include women and children as labourers too. ASM brings contribution to the society yet it does not sustain the environment and it is illegal in the society. The labourers are usually unskilled and not knowledgeable especially on the danger of using mercury for amalgamation method for purifying the gold. ASGM has produces 20% gold to the market every year (IIED, 2013). However most of their contributions were informal and not being taken account from the government. ASGM miners wages could only be $2 - $5 dollars a day depending on the countries and geographic area (Siegel and Veiga, 2010 as cited in IIED, 2013)

In Ghana, artisanal miners have shifted their attention from diamond mining to gold mining (Nyame and Grant, 2012). This may be due to the banning of most of the export of diamonds in 2006 under Kimberley Process Certification Scheme with more strict production and export regulations. This regulation has caused the increased price of small-scale diamond mining. The diamond artisanal small-scale miners unable to find alternative employment were forced to shift attention other mining sector. Such changing patterns have contributed to an increase use of mercury and increase ASGM mercury emissions.

Many ASGM sectors use mercury for amalgamation due to it is fast, inexpensive and effective where it is easy to be used by unskilled artisanal miners (Phiri, 2011). Amalgamation method has caused Hg released into the environment in several ways which causes negative health impacts to those who live within the mining area. For example, elemental mercury vaporizes when blow-torching the amalgam in an open vessels or through burning. Direct amalgamation of ground ore without the practice of gravity concentration and over usage of mercury have caused the Hg residues (Inorganic mercury) together with other wastes to be discharged into the waterways and tailings (Beinhoff and Calvez, 2001). When mercury was being used together with cyanide to extract gold, cyanide dissolves mercury to form cyano-mercuric complexes which are more bioavailable causes higher rate of methylation (Telmer and Veiga, 2008 as cited in Pirrone and Mason, 2009). UNIDO (2006 as cited in Pirrone and Mason, 2009) found fishes which contaminated with cyanide and mercury contain higher level of MeHg than mercury alone.

It was noted soils around gold mining areas in general are naturally rich in mercury and the formation of soil naturally concentrates and sequesters mercury (Jonasson and Boyle, 1972 as cited in Pirrone and Mason, 2009). Furthermore, soil erosion from mining releases mercury from the soil into the water bodies in an accelerated rates will be more ready to be methylated and bioaccumulated where it likely becomes available to be methylated and bioaccumulated in floodplains. Vaporizes mercury would be inhaled by the people living in mining area while and ingest MeHg contaminated rice and fishes. Both ways of emissions cause serious issues to the health.

The increase demand of gold especially for jewellery about 43% in 2012 was demanded (World Gold Council, 2013). The demand and soaring gold prices has caused ASGM to use more mercury to produce gold. Swain et al. (2007) stated Hg is an actively traded commodity whereby in year 2000 about 9000t of metallic Hg were bought and sold. In 2008, about 1000 tonnes/year of mercury being released into the air while in 2011, it has increased to 1465 tonnes/year (Telmer, 2011). It was suggested about 1.3:1 (Hg:Au) being used to produce 27 tonnes of gold through concrete amalgamation. Obtained data from USGS for gold and mercury prices comparison (Figure 6) showed both mercury and gold prices increases simultaneously. This is one of the factors why atmospheric mercury emissions are increasing.

Figure 6: Global Mercury and Gold Prices 1900-2011.

Australias Bureau of Resources and Energy Economics (2013) has forecast world gold mine production in 2013 to be increased by 4 per cent, compared to 2012 (2836 tonnes), to total 2956 tonnes but the gold prices would most likely decrease due to economic downturn and reducing demand from main consumer (India).This forecast does not include ASGM.

The increase of gold demands, ASGM activities, increasing Hg consumption, rice production booming in Sub-Saharan Africa and other parts of the world has summed up to possible increasing Hg contaminated rice in the world.

Mitigation

In 10th January 2013 (UNEP, 2013), 140 nations have signed a legally binding treaty known as Minamata Convention to reduce mercury. Some notable actions were discussed and will be taken into account. For example, The UNEP Mercury Products Partnership has decided to totally cut off of long line products which contained mercury by 2020. 70% of mercury thermometers and blood pressure devices will be reduced by 2017. 20 tonnes of mercury emissions should be reduced by 2016 according to Mercury and Air Toxics Standard. Since 2011, European Union has banned mercury exports and on 1st Jan 2013, USA has set the rule of banning mercury exports as well. This is to reduce mercury being transferred to developing countries for activities like ASGM.

A few methods have been discussed in UNEP (2008) report on using alternative in replacing the mercury. Since ASGM is the biggest mercury emissions sector, more efforts should be made. New treaty Minamata Convention stated they do not plan to ban the usage of mercury but there will be stricter regulations on supply and demand of mercury. (UNEP, 2013). UNEP has provided the guidelines to reduce mercury use for ASGM.

As such, method like gravity separation which includes all processes that separate gold from ore based on density. One of the method known as Gravity separation process which does not require the use of mercury and it can be applied to whole ore and ore concentrate. The separation processes comes in four methods including sluices, vibrating tables, hand screening and centrifuges.

Another way to replace mercury in ASGM is using the iGoli process which was developed by Mintek, a mineral technologist working with government of South Africa. iGoli process can produce 99% pure gold, a better method of extracting gold compared to amalgamation process which contained 5% residual mercury in the extracted gold. This process can be applied to free-gold ore and refractory ore. This method has been applied in Peru, Tanzania and Mozambique.

Chinas VCM industries consume and released most mercury in the world. Alternatives like using ethylene-based VCM process which uses ethylene as the primary feedstock. Ethylene-based process need only 3,500 kw/hours to produce 1 tonnes of PVC compared to use acetylene-based process which required 6,500 7,000 kw/h (ICIS, 2003; as cited in UNEP, 2008). On the other hand, Foreign Economic Cooperation Office of China (FECO) has proposed by 2015 the use of low-mercury accelerants will be applied to all calcium carbide process-based PVC industries (UNEP, 2011).

In terms of reducing methylmercury concentration in rice, Peng et al. (2012) reported rice grown aerobically had reduced THg and MeHg concentrations remarkably and it also reduced the MeHg content in the grain. Different rice cultivars have different level of THg and MeHg. Rothenberg and Feng (2012) research showed that among the 50 indica rice cultivars in three different sites (highly-contaminated, moderately-contaminated and background sites), moderately-contaminated sites contained higher THg and MeHg level. This is due to alkaline conditions in highly-contaminated site has reduces the uptake of Hg species by decreasing the micronutrients solubility which required for plant growth. Perhaps cultivating low Hg accumulating rice would be the key to cultivate in high mercury contaminated sites.

Certain measures of reducing methylmercury in terms of healths perspective have been taken into account. European Food Safety Authority (2012) has set a new standard for methyl mercury intake per week to 1.3 g/kg bw (lower than previous JECFA safes level 1.6g/kg) due to the new findings considered long chain omega 3 fatty acids present in fish may have previously led to an underestimation of the potential adverse effects of methylmercury in fish. The consumption of inorganic mercury is 4 g/kg bw. However, no tolerable safe level for consumption of methylmercury contaminated rice. Rice does not contain the omega 3 fatty acids benefits which play an important role in growth, development and the function of the brain. Transande et al. (2005) and Bellanger et al. (2013) reported pregnant women exposed to MeHg, will reduce the IQ of new-borns that may last for a lifetime. Using cost analysis studies, it showed the loss of intelligence will cause lower economic productivity to the countries.

Ballatori et al. (1998) and Aremu et al. (2008) research found N-acetylcysteine (NAC, a non-toxic amino derivative) could be the antidote in reducing MeHg level in body through urinary excretion. It is effective when taken orally or intravenously and about 5% MeHg doses eliminated through urine within 2 hr using 1 mmol/kg of NAC. Furthermore, NAC significantly reduced the body burden of MeHg in pregnant rats. However, more research has to be done.

Economic analysis should be used in making policies. For example, evaluation of health using benefit-cost analysis (BCA) method based on monetary values or cost effectiveness analysis (CEA) - comparative cost and consequences of alternative decisions would be taken into account (Swain et al. 2007; Levin, 1995).

Public sector policies are important but it was suggested private sector and NGOs should make policies too as a guidelines in curbing mercury (IIED, 2013). Especially ASGM, it is important to get involve in local communities to make a better policies based on their lifestyle, usage of mercury, incentives, challenges and more. Integrated cross-scale governance like could be one of the way to tackle mercury issue. In conclusion, policies making should be integrated with multiple levels of governance, an integration of local, regional and global (Selin and Selin, 2006).

AcknowledgementsI thanked Dr. Lawal Billa and Dr. Paul Williams for their patience and guidance. It was a tedious work that I would not be able to complete it without any aids. This paper has gave me more insight on the impacts of mercury contamination in paddy rice and a bigger picture of how environmental scientists do.

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