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Entry of arsenic in the food chain

Arsenic contamination in groundwater is often considered as a grave environmental issue on global scale. However, it is not the only intake source of arsenic for humans. Cereals, pulses, vegetables and other crops grown on soils of the arsenic-contaminated zones can also act as a potential source of arsenic (Williams et al. 2005). Therefore, the total dietary' exposure to arsenic by humans as well as animals should be calculated considering arsenic intake through both drinking water and foodstuffs (Arslan et al. 2017). Since 85-90% groundwater is extensively used for purpose of irrigation, particularly in the arsenic affected belts of India and Bangladesh, the possibility of arsenic accumulation in agricultural soils and products is anticipated. Irrigation with arsenic laden groundwater can enhance the arsenic level in agriculnrral soils itp to five-fold than the normal unaffected soils (Alisan et al. 2009). It was reported that the soil arsenic levels could be raised by 1 pg g"1 per armiun due to irrigation with groundwater contaminated with arsenic (Meharg and Rahman 2003).

Entry of arsenic in the food-chain also results in diffused arsenic poisoning. On absorption by the agricultural crops, arsenic may substantially be added to the dietary intake through bio-magnification in food-chain. Consumption of these foods can affect a larger population even beyond the geographically affected zones. However, the uptake of arsenic in different crops varies widely. The concentration of arsenic in the edible parts of agricultural crops is reported to vary from 0.007 to 7.5 mg kg"1 (Dahal et al. 2008). Some hyper-accumulator plants can uptake huge amount of arsenic and also can translocate it fionr root to shoot. Among the cereals, rice accumulates good quantity of mobile and reduced form of arsenic (arsenite) as rice is grown in flooded submerged field condition (Srivastava et al. 2012). Rice grains are susceptible to the enrichment of arsenic in the affected belts of India and Bangladesh. The arsenic translocation factor of the rice plant (0.8) is also high (Xu et al. 2008). Leafy vegetables, grown in arsenic affected soils and irrigated by arsenic contaminated water, can also accumulate elevated amount of arsenic (Deb and Dutta 2017).

Bioremediation of arsenic toxicity in water-soil-plant system

Following the worldwide severe arsenic pollution scenario, remediation or treatments to remove arsenic fionr environment by physical, chemical and biological techniques are major challenges. The physical and chemical arsenic removal techniques attempt to convert mobile arsenite to less mobile arsenate to arrest it in soil and to restrict its entry in the water-plant systems. Adsorption, precipitation, coagulation, and filtration are some such physical arsenic removal processes. Physical adsorption can be performed using different types of clays like kaolinite and illite, which can adsorb and increase the oxidation of arsenite to arsenate (Manning and Goldberg 1997). Few other adsorbents like humic acid, activated alumina, granulated ferric hydroxide, etc. have also been used to adsorb arsenic. Chemical oxidation and precipitation reactions are also being used as the effective key to reduce arsenic toxicity. The oxidizing agents like potassium permanganate, chlorine, ozotre, hydrogen peroxide or manganese oxides play key role in reducing arsenic toxicity. Besides these, some agronomic management practices such as use of surface water (e.g., harvested rainwater) for irrigation during the lean period have also been suggested to reduce accumulation of arsenic in food crops. However, these physical and chemical approaches are proved to be expensive and labour-intensive. Therefore, scientists started to focus on biological remediation or bioremediation processes in last decades using biological agents like green plants, bacteria, fungi, etc. to remove or neutralize arsenic contamination in environment. The following section details a few such methods.

Phytoremediation

Phytoremediation of arsenic is the process of using green plants to clear hazardous arsenic from the contaminated soil and groundwater. This is a low-cost green technology and proper implementation makes it eco-friendly. This does not require luxurious equipment or highly-specialized human resources and is simple to implement. Among phytoremediation technologies, phytoextraction and phytostabilization are the two major approaches in remediation of soils contaminated with arsenic. The plants used for arsenic phytoremediation should be highly tolerant to arsenic and efficient in accumulation of arsenic into aboveground biomass.

Chinese brake fem (Pteris vittata L.) is a very efficient arsenic hyper-accumulator and can be used for phytoextracting arsenic from contaminated soils (Ma et al. 2001). This plant is a fast growing vegetation of tropical and subtropical areas. The mechanism of arsenic removal from soil by P vittata is principally credited to fast arsenic mobilization in rhizosphere, efficient uptake by the roots and unique metabolism characteristic of the plant (Mathews et al. 2010). As per study, a peiiod of 7-8 years is required for P. vittata to totally remove arsenic from top 15 cm soil (Ma et al. 2001). Root exudates and bacteria associated with P. vittata enhance arsenic solubilisation in the rhizosphere. Further, arsenic-resistant bacteiia (like Pseudomonas sp., Comamonas sp. and Stenotrophomonas sp.) present in the P. vittata rhizosphere exhibit incredible ability to boost arsenic concentration in the uptake solution by solubilizing insoluble FeAs04 and A1As04 (Ghosh et al. 2011). The pyochelin-type siderophores, produced by arsenic-resistant bacteria in rhizosphere, also play a role in arsenic solubilisation. There are also other arsenic hyperaccumulators like Pityrogramma calomelanosis, some species of Pteridaceae family, etc. The translocation and accumulation of arsenic is higher in younger plants than the older plants, possibly due to higher metabolic activity of younger plants (Gonzaga et al. 2007). This signifies the requirement of using young plants in phytoextraction and harvesting of the shoots before plant-death to minimize the return of arsenic to soil. Proper agricultural practices and plant management like application of specific fertilizers and rhizosphere manipulation also help to acquire efficient phytoextraction. For instance, application of phosphorus in soil can enhance arsenic uptake by increasing soil arsenic bioavailability (Fayiga and Ma 2006). Rhizosphere manipulation through application ofmycorrhiza can also change arsenic accumulation dynamics and the species of arbuscular myconhizal fungi plays an important role here (Trotta et al. 2006).

Phytostabilization is a method where plant roots and associated microbes limit the contamination of arsenic in soil through restricting its mobility and bioavailability. In this process, the amount of water percolating through soil matrix gets reduced by the plants and this acts as a hurdle towards direct contact of plants with arsenic. Plant species like Populusand salix is capable of this process. Such plants generally have high adaptability and tolerance to multiple metals and metalloids. In arsenic contaminated areas with poor soil nutritional status, adoption of leguminous plants is also a good option as their nitrogen fixing ability helps in revegetation. In addition, legume plants do not have the ability of shoot translocation of metals and thus results in low exposure tisk of arsenic to animals and human. The white lupin (Lupinusalbus) is a good nitrogen fixer plant for arsenic contaminated soil.

Uptake of arsenic from the soil, transforming it into volatile forms and releasing it to the atmosphere through transpiration is known as phytovolatilization. Here, the plants obtain the soil arsenic through water, passes it through the xylem towards leaves and convert it into non-toxic forms. These non-toxic forms (methylated and volatile arsenic compounds) are finally volatilized in the atmosphere (Heaton et al. 1998). The P vittata is considered as an efficient arsenic volatilizer.

 
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