Phyto-extraction ability of Digitaria exilis ( Fonio ) to heavy metals

Among the different contaminants in the environment, heavy metals are unique due to the fact that they cannot be broken down to non-toxic forms. The research was conducted to determine the remediation potential of Digitaria exilis and to identify the varieties that can tolerate heavy metals contamination. Soil was collected in an Automobile mechanic workshop, Government Reserved Area (GRA) Ado-Ekiti. Five kilograms (5 kg) of soils collected was air dried, sieved and filled into plastic buckets. The experiment was a completely randomized designed with three replicates. Three different accessions of Digitaria exilis namely; Jakah D Iburua (JAK), Dinat D Iburua (DID) and Jiw D Iburau (JIW) were planted. Soils were watered till field moisture capacity. Plant height was determined every two weeks after planting. Soil analysis was carried out to determine the physical and chemical properties before and after the experiment. Heavy metals including Cd, Fe, Zn, Pb, As and Cu were determined in the plant roots and shoots using Atomic Absorption Spectrophotometer (AAS). Accession JAK Bio-accumulated heavy-metals in their root than shoot, consequently, significant high plant height was recorded. However, shoot of accession JAK accumulated 22 % Pb, 46.78% Cu and 37.5% Cd and 20.3% Zn than others whereas accession DID accumulated As than accession JAK. Therefore, accession JAK demonstrated highest phyto-extraction of heavy-metals potentials at the root. Variety JIW has the lowest plant height, root and shoots heavy metals accumulation. Although heavy metals contaminated soil affected the growth of D. exilis, nevertheless accession JAK has the highest productivity under this condition and can be further recommended in phyto-extraction of heavy-metals in polluted sites.


Introduction
. Disposal of high metal wastes, leaded The need for economic growth especially gasoline and paints, land application of in developing countries has led to increase in fertilizers, animal manures, sewage sludge, different industries which emit heavy metals pesticides, wastewater irrigation, coal (Ahmadpour et. al., 2012).Soils may become combustion residues, spillage of petrochemicals, contaminated by the increasing accumulation of and atmospheric deposition are the various heavy metals and metalloids through emissions activities leading to contamination of our soils from the rapidly expanding industrial areas (Sahibin et. al., 2002;Jadia & Fulekar, 2009;(Wuana & Okieimen, 2011;Ahmadpour et. al. Wuana & Okieimen, 2011).The heavy metal fonio in soil contaminated with petroleum contents of these stated pollutants cannot be derivatives in order to understand its mechanism oxidized by microbes and chemically degraded of remediation and to identify the varieties which as in the case of organic contaminants can tolerate heavy metals contamination.(Ahmadpour et. al. 2012).The average half-life of these metals is as high as 20 years (Ruiz et. al., Materials and Methods 2009).
Soil preparation and Planting Humans and ecosystem in general, are at The experiment was conducted at the receiving end of the risks and hazards posed Federal University Oye-Ekiti screen house.by heavy metal contamination of soil Contaminated soil from an automobile mechanic (McLaughlin et. al. 2000;Yoon et. al., 2006), with workshop (AUMS) along with agricultural field as more than 10 million people's health under control soil were collected in Ado-Ekiti and air threat in many countries (Ahmadpour et. al. dried for 3 days which were sieved using a 2 mm 2012).It has become necessary to reduce the mesh to remove debris.Five kilogram (5 kg) air concentrations of these heavy metals in the dried and sieved soil was weighed into each environment to a safe level in a sustainable and plastic buckets with perforated at the base to cost-effective way.Current conventional drain water and increase soil aeration.The methods to remediate heavy metalcontaminated soil was watered immediately for a contaminated soil and water, such as ex situ week to allow homogeneous mixture of the oil.excavation, landfill of the top contaminated soils Three different varieties of Digitaria exilis (Zhou & Song, 2004), detoxification (Ghosh & (Dinat D Iburua (DID), Jakah D Iburua (JAK) and S i n g h , 2 0 0 5 ) , a n d p h y s i c o -c h e m i c a l Jiw D Iburua (JIW)) were planted on the plastic remediation, are expensive (Danh et. al., 2009), bucket each having three (3) replicate each (2 time consuming and labor exhaustive.contaminated, 1 control) which were then The use of plant to control the watered.Growth was observed on the third day concentrations of these metals has proved of planting.Growth parameters were accessed effective, both on land and in water bodies, second and third week after planting.Plants through the process of phytoremediation and were watered regularly and the buckets were rhizo-filtration respectively.It is a low cost, long maintained weed free throughout the term, environmentally and aesthetically friendly experiment.method of immobilizing/stabilizing, degrading, transferring, removing, or detoxifying Measurement of Growth parameters contaminants, including metals, pesticides, The plant heights were measured hydrocarbons, and chlorinated solvents (Susarla weekly.The experiment was terminated 4 weeks et. al., 2002;Jadia & Fulekar, 2008;Zhang et. al., after planting.Thereafter, both shoots and roots 2010).Several hyper-accumulators have been were analyzed at Center for Energy, Obafemi found by several workers, they include Thlaspica Awolowo University, Ile-Ife, Osun, Nigeria erulescens (Pence et. al., 2000), Sedum alfredi (OAU).The plants were dried using an oven at a 0 temperature of 60 C to constant weight.(Yang et. al., 2004), Arabidopsis halleri (Bert et. al., 2003), Brassica campestris (Glick, 2003) and Soil and Plant Chemical Analysis Pistia stratiotes (Moodley et al., 2007;Abubakar One gram (1g) of the dried soil sample et.al . 2014).
was accurately weighed into a teflon beaker and Fonio (Digitaria exilis and Digitaria 20ml Hydrofluoric acid was carefully added.The iburua) is probably the oldest African cereal.For beaker was heated until the sample was thousands of years, West Africans have completely digested, at near dryness, 20ml cultivated it across the dry savannas.Indeed, it double distilled water was carefully added was once their major food.Even though few through the side and allowed to cool, filtered and other people have ever heard of it, this crop still the filtrate made up to 100ml in a standard flask.remains important in areas scattered from Cape The dried plant samples were each ground and Verde to Lake Chad.Therefore, this study aimed weighed (0.20g) into a beaker, 15ml freshly at investigating the phytoremediation ability of weighed (0.20g) into a beaker, 15ml freshly Duncan Multiple range test.prepared aqua-regia was added.The beaker was allowed to stand for 24hrs after which it was Results heated to near dryness.20ml double distilled The properties of the soil used for the water was then added and heated until a clearer experiment are stated in Table 1.The soil used digested was obtained; this was then allowed to was sandy loam, alkaline with exchangeable cool and filtered.The filtrate was made up to acidity of 2.5mg/k and low organic compound of 50ml in a standard flask.9.0g/Kg (Table 1).The values of heavy metals Quantitative data were subjected to one such as Cd, Fe, Zn, Pb and Cu ranged from 3.00way analysis of variance at P=0.05 and 45.00.differences among means were separated using Table 1: Soil Physio-Chemical Properties of the Automobile Mechanic Soil AUMS and CNT.However for Arsenic, accession Plant Growth DID accumulated higher than the other Fonio were cultivated on the automobile accessions in the shoot.Accession JAK mechanic soil for four weeks.The plants showed accumulated 22% Pb higher than DID and JIW.symptoms of stunted growth for the period of JAK also absorbed 50% of Cd higher than JIW exposure.There was an increase in plant height and 37.5% greater than DID.Copper over the four weeks for both AUMS and CNT.accumulated in the shoot of JAK was 46.78% Plants cultivated on AUMS had no significant higher than accession DID and 36.67%greater differences in plant height at 2 weeks after than JIW.Iron (Fe) uptake in the shoot of planting (WAP) and 4 WAP for the three accession JAK is significantly higher than other accessions.Accession JAK had the highest plant accessions.Shoots of accession JAK absorbed height of 1.6 cm and 25.2 cm at 4WAP for AUMS Zinc (Zn) of about 20.3% greater than DID and and CNT respectively (Figure 1).13% higher than JIW.Accession JIW significantly accumulated Cu and Zn at the shoot Heavy Metals Accumulation than DID accession.Accumulation of Cd, Pb and The concentration of heavy-metals found Fe at the shoots of Fonio ranged as follows: in the shoot of fonio accessions ranges from 0.5 JAK>DID>JIW (Table 2).mg/l -27.25mg/l.Accession JAK absorbed Pb, Cd, Cu, Zn and Fe higher than other accessions in   Heavy metals were highly concentrated concentration at the root of DID.The absorption at root of accession JAK than any other level of heavy-metals at the root of Fonio ranged accessions in both AUTS and CNT.JAK absorbed as follows: JAK>DID>JIW (Table 3).above 50% Cd and Pb at the root more than JIW The bioaccumulation factors of accession and JAK accumulated 21.4% Cd and 31% Pb JAK at the root and shoot were significantly higher than accession DID.There is no higher than other accessions.Bio-concentration significant difference in absorption of Cu at the factor of Cd and Pb at the root and shoot of JIW root of JAK and DID whereas accession JAK was twice that in JAK.There were no significant absorbed 41% Cu higher than accession JIW at differences in bio-concentration factors of Cu, Zn the root.Zinc concentration at the root of JAK and Fe at shoot level among the accessions.plant on AUMS is 44% greater than zinc at the However, roots of JAK accounted for below 40% root of JIW and 34.65% higher than zinc bio-concentration higher than JIW (Table 4).
As shown in Table 5, the transfer factor of factors between 0.00 -0.65 whereas, JIW had its all the accessions ranged from 0.00 -0.90.
transfer factor of Zn to be 0.91.Accessions JAK and DID had their transfer

Discussion
Consequently, the higher heavy metal The accumulation and concentration of concentrations in the roots of the fonio these heavy metals in the roots of the three accessions studied could be as a result of the accessions studied is a further indication that large surface area of their fibrous root systems they are good candidates for phyto-extraction.
being monocots.This is supported by USEPA (2000) that heavy As reported by Oloyede et al. ( 2013), biometals such as Pb, Cd, Cu, Ni, Zn, and Cr are concentration factor (BF) defined as the primarily retained within the roots and they can concentration ratio of heavy metal in the plant to only be removed by rhizo-filtration.According to that in the soil is used to measure the Jadia and Fulekar (2009), rhizo-filtration is effectiveness of a plant in concentrating heavy beneficial in disallowing heavy-metals from metal into its biomass.The highest heavy metals being translocated to the shoots.Therefore, the bio-concentration factors observed in both roots Fonio accessions studied, being non-and shoots of accessions JAK compared to the hyperaccumulators could be used for this other accessions shows that it was more purpose.Raskin and Ensley (2000) reported that effective in accumulating heavy metals in its terrestrial plants with fibrous and longer root biomass.Also the highest bio-concentration systems are good candidates of rhizo-filtration factors which are observed in the roots than because of increase in the amount of root area.
shoots of all the accessions means that fonio is shoots of all the accessions means that fonio is P., Gruber, W. and Verbruygen, N. ( 2003).

Figure 1 :
Figure 1: Plant Height (cm) of Digitaria exilis Cultivated on Mechanic Automobile Soil better in accumulating heavy metals to its roots G e n e t i c b a s i s o f C d t o l e ra n c e a n d than shoots.This means that these fonio hyperaccumulation in Arabidopsis halleri.Plant & accessions can effectively serve two purposes soil 249: 9 -18.i.e. as food crops and as remediators of heavy metal contaminated sites.This is because the Danh, L. T., Truong, P., Mammucari, R., Tran, T. bulk of the heavy metals are concentrated in the and Foster, N. (2009).Vétiver grass, Vetiveria roots of these plants and not the shoot which is zizanioides: A choice plant for phytoremediation consumed by humans. of heavy metals and organic wastes.Int.J. Transfer factor (TF) which is the heavy Phytoremed.11(8): 664-691.metal concentration ratio between shoot and root biomass and is used to measure the Ghosh, M. and Singh, S. P. (2005).A review on effectiveness of a plant in translocating heavy phytoremediation of heavy metals and utilization metal from root to the shoot.If the transfer of it's by products.App.Ecol.& Environ.Res.factor is below 1 in any of the treatments, it 3(1): 1-18.means that the plant is poor in translocating that particular heavy metal to its shoots in that Glick, B. R. (2003).Phytoremediation synergistic treatment (Oloyede et.al., 2013).The lower use of plants and bacteria to clean up the transfer factors observed in accessions JAK and environment.Biotechnol.Adv.21: 383-393.DID indicate that they could not transfer the heavy metals effectively to their shoots.This is Jadia, C. D. and Fulekar, M. H. (2008).unlike of accession JIW whose transfer factor is Phytoremediation: The application of much higher in Zn.Hence JIW could translocate vermicompost to remove zinc, cadmium, copper, Zn more effectively to the shoot.nickel and lead by sunflower plant.Environ.Eng.& Manag.J. 7(5): 547-558.Conclusion

Table 2 :
Absorption Concentrations (mg/l) of Heavy-Metals in the Shoot