Contamination of Arsenic and Other Heavy Metals in Rhizospheric Soil

The contamination of arsenic (As) and other heavy metal (HMs) in soil causes serious health hazard to the ecosystem. In this work, the contamination of As and other heavy metals (i.e. Ti, V, Cr, Mn, Fe, Ni, Cu, Zn and Pb) in rhizospheric soil of 14 plants of the severely As contaminated area of Central India is described. Among them, high content of As in the rhizospheric soils was observed, ranging from 0.22 to 4.60 g/kg with mean value of 1.6 ± 0.7 g/kg. The concentration variation, enrichment indices and toxicities of the metals in the soil are described.


Introduction
Arsenic and its compounds are especially potent poisons linking with a broad variety of neurologic, cardiovascular, dermatologic, and carcinogenic effects; including peripheral neuropathy, diabetes, ischemic heart disease, melanosis, keratosis, and impairment of liver function [1].Chronic arsenic pollution has been now recognized as a worldwide problem, spreading in several countries of Asia, Australia, Africa, North and South America due to geogenic and anthropogenic emissions [2]- [7].Severe arsenic contamination in environment of the central India (21˚6'N & 81˚2'E) has been reported [8]- [11].High concentration of As and other HMs in the rhizospheric soils interfere the biological activities of plants [12]- [14].The composition of rhizospheric soil is different from those of bulk soils due to biological activities [15]- [17].Plants were reported for the phytoremediation of the HMs in soils [18]- [22].In this work, content, enrichment and toxicity of As and other metals i.e.Ti, V, Cr, Mn, Fe, Ni, Cu, Zn and Pb in rhizospheric soils of 14 common plants of the arsenic contaminated area, Koudikasa, CG, central India are described.

Collection of Soil
The sampling network for the soil collection is presented in Figure 1.A 100 g rhizospheric soil was collected in January, 2012 from the arsenic contaminated area, Koudikasa, Ambagarh Chowki block, Rajnandgaon, CG, India (21˚6'N & 81˚2'E) as prescribed in the literature [23].The bulk soil was also collected from the 14 sites for the comparison studies.The soil samples were kept in the polyethylene bottle and dried in an oven at 60˚C for 12 hrs.The samples was milled and sieved out particles of ≤1 mm for the analysis.

Soil pH
The soil sample (10 g) was mixed with 20 mL of deionized water in a 100-mL conical flask and allowed to stand for overnight.The pH value of the extract was measured by Hanna pH meter type-HI991300.

Soil Analysis
The Bruker S2 Picofox TXRF portable spectrometer equipped with poly capillary lens and the X-ray beam 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 in ultrasonic bath for 15 min.Gallium was added to suspensions as an internal standard with a concentration of 10 mg/L.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.Al, 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.

Pollution Indices
The pollution indices i.e. enrichment factor (EF), contamination factor (CF) and geo-accumulation index (GI) are used to determine element concentration in the soil samples with respect to the base line concentration.These relate the concentration of an element to a crustal element (e.g.Al) in the soil sample, and this ratio is then normalised to the ratio of those elements in the earth's crust.The following equations are used for the calculation of the pollution indices [24] [25].
where, X s , Al s , X e and Al e are concentrations of metal and Al in the soil and earth crust, respectively.

Results and Discussion
The traditional and botanical name of the plants selected for the proposed studies is presented in Table 1.The soil is yellowish colored with alkaline nature.The pH value of rhizospheric soil extracts (n = 14) was ranged from 7.3 -8.6 with mean value of 7.9 ± 0.2.

Concentration of Metals in Bulk Soil
High content of metals i.e.K, Ca, Al, Fe and Ti was present in the bulk soil.Among them, Fe exhibited the highest content in the soil, followed by Al, K and Ca.The mean concentration of Fe, Al, K, Ca, Ti and P in the soil was found to be 203, 114, 75, 52, 31 and 1.6 g/kg respectively.The mean value of other HMs i.e.Mn, Cr, Zn, V, Ni, Cu, As and Pb in the bulk soil was 5.93, 0.88, 0.60, 0.51, 0.38, 0.36, 0.20 and 0.04 g/kg, respectively.The As content in the field soil of the study area was found to be much higher than in the soil of other countries i.e.Bangladesh, Nepal and China [1]- [7].

Concentration of Metals in Rhizospheric Soil
Similarly, elevated level of metals i.e.  2. Among them, a fair correlation (r = 0.89) of the As with the K was observed, indicating their existence as K 3 AsO 4 in the soil.
Remarkably high content of the As (4.60 g/kg) was seen in the rhizospheric soil of Mentha spicata (MS), may be due to interaction of As with the carvone [26].Significance content of V, Zn and Pb was observed in the in the rhizospheric soil of Solenum nigrum (SN), may be due to interactions of As with the glycoalkaloids [27].Similarly, high content of Cr was seen in the rhizospheric soil of Achyranthes aspera (AA), and Cynodon dactylon (CD).The contamination of As and other HMs in the tree rhizospheric soil (TS), herb rhizospheric soil (HS) and bulk soil (BS), Figures 2(a)-(c).Arsenic in the TS and HS with respect to BS was extremely enriched (>8-folds).Other HMs i.e.V, Cr, Mn Fe, Ni, Cu, Pb and Zn were poorly to moderately enriched (>1 ->2) in the rhizospheric soils.

Toxicities
The allowable limits reported for the HMs i.e.As, Cr, Mn, Cu, Ni, Zn and Pb in the soil are 5, 200, 55, 50, 50, 400 and 70 mg/kg, respectively [29] [30].Among them, the concentration of As, Mn, Cu, Ni and Zn in the rhizospheric soil was found to be several folds higher than the reported values.The rhizospheric soils were contaminated with As at dangerous levels, and can cause toxic effects i.e. replacement of inorganic phosphate in biochemical reactions, inactivation of S-enzymes, distribution of N-assimilation in plants, etc. [31].

Conclusion
The higher content of the As and other HMs was more enriched in the rhizospheric soil than the bulk soil due to biological interactions.The HMs (i.e.As, Mn, Cu, Ni and Zn) content in the rhizospheric soil was found above the permissible limits.Among them, As is extremely enriched in the soil.The higher content of HMs (i.e.As, Cr, Mn, Cu, Ni, Zn and Pb) in the rhizospheric soil of herbs i.e.MS and OS was observed.

Figure 1 .
Figure 1.Representation of sampling locations in the map.

Table 1 .
Description of plants.

Table 2 .
Distribution of metals in rhizospheric soil, g/kg.

Table 4 .
Contamination factor (CF) of metals in rhizospheric soil.