Contamination of Paddy Soil and Rice with Arsenic

The drinking water and food are main pathway entry of the As in humans and animals. Their intakes cause diseases i.e. skin cancer, vascular disorder, etc. A wide variety of the rice is cultivated in the central India. The field soil and rice cultivated in the summer season at Koudikasa village, central India were selected for the As contamination studies. The concentration (n = 20) of totalAs (AsT) in the field soil, rice grain, husk, straw and root was ranged from 44 270, 0.17 0.72, 0.40 1.58, 2.5 5.9 and 204 354 mg/kg with mean value of 126 ± 28, 0.47 ± 0.07, 0.83 ± 0.15, 4.2 ± 0.5 and 276 ± 21 mg/kg, respectively. The total arsenic, monomethylarsenonate, dimethylarsinite and inorganic As in the rice grain are quantified.


Introduction
Arsenic is a toxic metalloid, and its long exposure causes diseases i.e. hypopigmentation, melanosis, keratosis, skin, bladder, lung, and prostate cancer, etc. [1].Rice is a predominant staple food in many countries i.e.Asia, Pacific, America and Africa.The rice husk and straw are used as fodder for feeding cattle and fuel.The exposure of arsenic from rice has been reported a global health issue [2]- [4].The elevated levels of As in the field soils in various regions of the World were reported [5]- [12].Arsenic is a bioactive toxic element, accumulated in rice of several regions of the World [13]- [19].In the present work, the contamination of As in the paddy soil, rice grain, husk, straw and root grown in the contaminated environment, Kaudikasa village, Ambagarh Chowki, central India is described.The speciation and translocation of As in various parts of the rice plants are discussed.

Choice of Study Area
The Ambagarh Chowki block, Rajnandgaon district, Chhattisgarh state, India is a hot spot for the As contamination research due to the huge mineralization of As in the environment [20] [21].The studied area falls in a tribal belt with population of ≈0.1 million over 155 villages.Among them, Koudikasa village (area ≈ 5 km 2 ) was selected for the proposed studies due to As contamination of the environment at the hazardous levels.

Sampling of Soil and Rice Samples
The sampling network for collection of field water, soil and rice is presented in Figure 1.The water samples were collected as prescribed in the literatures [22].Twenty water samples (once in a month) in duplicate from August-November, 2012 were taken from 20 different rice fields.A total of 4 samples, from each field from the period: August-November, 2012 was taken for the investigation.They were collected directly from rice field in duplicate, and placed in 250-mL polypropylene plastic bottle.Bottles were first rinsed thrice with the water and then, completely filled with the same water.The first sample was acidified with concentrated nitric acid (0.1%, v/v) for analysis of the As.The second sample was left free for analysis of the ions.The physical parameters i.e. pH and electrical conductivity (EC) of the water were measured at the spot.
Twenty composite soil samples (0 -10-cm depth) were collected after harvesting of rice paddy (December, 2009) from 20 fields of Koudikasa village as prescribed in the literature [23].The samples were stored in polyethylene bottles and dried in open air under diffused sunlight followed by drying in oven at 50˚C for 24 hr.Rice grains were separated from the plants by hand picking in December, 2012 from the field in the polyethylene bag.They were dried in the oven at 50˚C for 24 hr and their husk was separated manually.Similarly, the straw and root of the rice plant were collected.They were washed with the deionized water several times to remove the soil particles.The dried soil, rice grain, husk, straw and root samples were ground to a fine powder with mortar and passed through a mesh sieve of <1 mm.

Analysis
The Bruker S2 Picofox TXRF portable spectrometer was used for the analysis of the elements in soil.A suspended solution was prepared by mixing 10 mg of soil sample with 10 ml of a water solution containing 1% (w/v) triton and 10 µg/mL Ga in ultrasonic bath for 15 min.For each measurement, 10 μL of sample solution was sprayed on the quartz filter with subsequent drying.The X-ray source was focused on the filter for quantification of the elements.The peak area of the signal was computed.The three replicate measurements for each sample were carried out.The content of 14 elements (i.e.P, K, Ca, As, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn and Pb) in each soil was analyzed.The standard soil sample (NCS DC 73382 CRM) was used for the quality control.
The As content in the soil and rice samples were analysed by the ICP-MS and AFS techniques.The samples were digested with nitric acid and H 2 O 2 in Perkin-Elmer microwave system.The total, inorganic and organic As species in the rice grain samples were quantified by using method proposed by Williams et al. [24].The Rice flour SRM 1568 was used for the quality control.

Rice Morphology
The details of the rice morphology (i.e.cultivation period, height of plant, grain and husk content) are summarized in Table 1.The cultivation period, plant height and yield of rice varies by variety and environmental conditions, ranging from 80 -145 day, 90 -140 cm and 33 -57 Q/ha with mean value of 124 day, 110 cm and 44 Q/ha, respectively.Among them, the plant height was found to be partially correlated (r = 0.42) with the cultivation period.The single grain weight, rice and husk content were varied from 14 -30 mg, 65% -87% and 13% -35% with mean value of 21 mg, 74% and 26%, respectively.

Concentration of As and Other Elements in Soils
The distribution of As and other elements i.e.P, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn and Pb in the field soil is shown in Table 3   respectively.Among them, the highest concentration of Fe was observed in all soil samples.They were found to occur in the following increasing order: Pb < As < Cu < Ni < Zn < Cr < P < Mn << Ca < K < Fe.Among them, a good correlation (r = 0.73) of the As with the K was observed, indicating their existence as K 3 AsO 4 in the soil, Table 4.The Fe showed good correlation with the heavy metals i.e.Cr, Mn, Ni, Cu and Pb in the soil, indicating origin from similar sources, Table 4.The As concentration in the soil of studied area was found to be higher than reported in other regions of the country and World [5]- [12].

Distribution of As in Rice Grain, Husk, Straw and Root
The distribution of As in rice grain, husk, straw and root is summarized in Table 5.The concentration of As in the rice grain, husk, straw and root (n = 20) was ranged from 0.17 -0.72, 0.40 -1.58, 2.5 -5.9 and 204 -354 mg/kg with mean value of 0.47 ± 0.07, 0.83 ± 0.15, 4.2 ± 0.5 and 276 ± 21 mg/kg, respectively.The As content in husk was found to be higher than the rice grain, may be due to external contamination from the environment.The high yield rice varieties i.e.Kalinga, IR-64, G. Gurmatia, Shyamla, Ek Hazar Das, M2, etc. were found to be more sensitive to the As-accumulation.The concentration of As in the rice of the studied area was found to be higher than reported in the other region of the country and World [13]- [19].The As content in the straw and root was found to be comparable to the values reported in the Taiwanese rice plants [14].

Biological Absorption Coefficient and Concentration Factor
The biological absorption coefficient, BAC (i.e.plant to soil metal ratio) and concentration factor, CF (i.e.plant to water soluble metal ratio) of As are presented in Table 6.The CF and BAC values depend on the physical   and chemical composition of the field water, type of rice species and their morphology.The CF values in the rice grain, husk, straw and root was ranged from 3 -15, 7 -32, 40 -120 and 2934 -7656 with mean value 9 ± 2, 15 ± 3, 75 ± 11 and 5065 ± 1646, respectively.Relatively very low BAC values for the rice grain, husk, straw and root were observed, ranging from 0.001 -0.009, 0.003 -0.015, 0.015 -0.073 and 1.0 -5.0 with mean value of 0.004 ± 0.001, 0.008 ± 0.002, 0.039 ± 0.007 and 2.7 ± 0.5, respectively.The remarkably higher FC and BAC values with new rice variety i.e.Kalinga, Masuri, Ek Hazar Das and M2 were seen, may be due to smaller grain size and higher cultivation period, Figure 2 and Figure 3.In addition, the FC and BAC values for different plant parts were found to be fairly correlated (r = 0.76 -0.96).The As was found to be poorly translocated from the root to grain and its concentration was found to increase in order: grain < husk < straw << root.The As content was found 1.9 ± 0.3, 9 ± 1 and 108 ± 15 folds higher in the rice husk, straw and root with respect to the respective grain.The higher As content in the husk, straw and root of the rice species i.e.Masuri, IR-64, Luchai, Safari and HMT was observed.

Speciation of As in Rice
The As species concentration in four new fast growing rice grains is summarized in Table 7.The main As species detected in the rice extract were monomethylarsenonate (MMA) and dimethylarsinite (DMA) and As(III).Total arsenic concentration of the four rice samples was varied from 207 -548 µg/kg.The inorganic As(III) concentration was ranged from 46% -66%.The higher DMA fraction was observed in the rice grain containing higher As content.Amongst them, the lowest concentration of the MMA was marked.The inorganic and organic   As content was found to be fairly correlated with the AsT content in the grain.The distribution pattern of Asspecies in the rice grain of this region is observed similar to Bangladeshi rice [24].

Conclusion
The field water and soil were found to be contaminated with As at dangerous levels, may be due to geogenic origins.The high yield rice variety i.e.IR-64, Kalinga, Ek Hazar Das, M2, Zero JR, etc. was found to be more sensitive to the As accumulation.Several folds higher As contamination of the straw and root of the rice plants than the grain was marked.The feeding of straw to the domestic animals seems to be a potential pathway entry of As.The rice root was marked as hyper phytoextractants for accumulating As from the surface soil.

Figure 1 .
Figure 1.Sampling net-work for collection of the field soil and rice samples in Koudikasa village.

Figure 2 .
Figure 2. The FC value of rice plants in Koudikasa village.

Figure 3 .
Figure 3.The BAC value of rice plants in Koudikasa village.

Table 2 .
Characteristics of field water in Koudikasa village.

Table 3 .
Chemical characteristics of field soil in Koudikasa village, mg/kg.

Table 4 .
Correlation matrix for elements in field soil.

Table 5 .
Distribution of As in various parts of rice in Koudikasa village, mg/kg.

Table 6 .
BAC and CF value of rice grain, husk, straw and root.