Heavy Metal Speciation and Health Risk Assessment of Soil and Jute Mallow (Corchorus Olitorus) Collected From a Farm Settlement in Ikorodu, Lagos, Nigeria

There is an increasing global concern for adverse 
effects of inorganic fertilizer and pesticides applied to agricultural soils. 
This study investigated metal speciation in soil and health risk assessment of 
Jute mallow (Corchorus olitoriuos) 
from a farm settlement in Ikorodu, Lagos State. Soil samples were collected 
according to the set standard procedure, sequentially extracted and analyzed 
for selected heavy metals using standard methods. Results showed that chromium 
(Cr) was associated with reducible fraction (Fe-Mn) for top soil, while cadmium 
(Cd), manganese (Mn), and copper (Cu); lead (Pb) and nickel (Ni); and Cr were 
predominantly bound to carbonate, reducible and residual mineral components 
respectively for sub-soil. The results of Contamination Factor (CF), 
Contamination Degree (CD), Pollution Load Index (PLI) and Geoaccumulation Index 
(Igeo) showed that the soil samples were not polluted for all the 
investigated metals. The Cd level in the soil (13.54 ± 1.21 mg·kg﹣1) and vegetables (0.83 ± 0.05 mg·kg﹣1) were above the USEPA critical permissible limit 
of 3.0 mg·kg﹣1 and 0.1 - 1.2 mg·kg﹣1 respectively. The daily intake of estimated 
selected heavy metals from the vegetable ranged from 8.8 × 10﹣03 to 1.4 × 10﹣02 for adult and 3.8 × 10﹣03 to 1.1 × 10﹣02 for children while the Hazard Quotient (HQ) for adults 
ranged from 5.4 × 10﹣05 to 1.1 × 10﹣01 and that of children ranged from 8.3 × 10﹣03 to 1.4 × 10﹣02. The cancer risk (CR) values of heavy metals in the soil 
ranged from 1.02 × 10﹣11 to 9.90 ×


Introduction
In Nigeria, the rate of increase in human population which, according to the National Population commission, stood at 88.6 million people in 2006, (and is projected to hit 160 million within the next one decade) does not enjoy a corresponding rate of increase in food supply [1] [2]. This thus creates a huge food supply deficit among Nigerians as food demand far outstripped the level of supply, creating an immense pressure on the available food items with the attendant increases in market prices [1]. Availability of food is a major issue in food security and in order to provide for more food, more lands are cultivated and fertilizer and pesticides are increasingly used. Vegetables are part of human diet to take up a lot of essential nutrients and certain trace elements in a short period. In this situation, safety of vegetables is very important [3] [4].
Findings reveal that vegetable farmers use a wide range of pesticides at different levels to reduce losses from pests and diseases. However, despite the contribution of pesticides to agricultural production, pesticides are of major environmental concern. Many pesticides and chemicals are not biodegradable, they bioaccumulate in the food chain and detrimental to human and the ecosystem [5] [6]. The repeated applications of these agrochemicals potentially contributed to the accumulation of heavy metals in agricultural soils as some of these fertilizers and pesticides contain heavy metals such as Cd, Pb, Zn [7]. The accumulation of heavy metals in soil of the study area could either directly endanger the natural soil functions, or indirectly endanger the biosphere by bioaccumulation in the food chain, and ultimately endanger human health [8].
It is commonly acknowledged that total soil heavy-metal concentration alone is not a good measure of bioavailability and is not a very helpful tool to determine the potential risk associated with soil contamination. Therefore, chemical speciation, which plays a vital role in determining the bioavailability of toxic metal in a soil solution, is often used as a predictor of metal bioavailability to soil organisms and plants [9]. The chemical forms of the metal control its bioavailability or mobility. The exchangeable and acid extractable fractions are mobile fractions that are easily bioavailable [10]. This bioavailable metals in the soil provides rough estimate of metal uptake by plants (especially edible plants) and their risk assessment.
Heavily contaminated soils may pose long-term risks to ecosystems and human health [11] [12] [13] via the increased uptake and accumulation of heavy metals in plant tissues [14] [15] [16]. Risk assessment has emerged in recent  [17] [18]. This discipline is becoming increasingly important in modern toxicological and epidemiological practice, both in terms of hazard evaluation as well as at the level of efficient disease control and prevention [19] [20] [21]. Environmental analysis has largely contributed in this direction by careful monitoring of contaminant distribution over space and time [22] [23].
Several studies recently have examined heavy-metal transfer from soil to vegetables [24] [25] [26] [27]. Many studies have also reported that the bioavailability of soil metal to vegetable is controlled by soil properties, soil metal speciation, and plant species [28] [29] [30]. However, not so much has been done on effect of fertilizer and pesticide application on redistribution and bioavailability of heavy metal in soil and vegetable and their possible associated health risk. It is on this background that this study seeks to determine effect of fertilizer and pesticide application on metal speciation in soil and vegetable and to assess the health risk of human exposure to heavy metal from soil and vegetable.

Study Area
This study was conducted in a farm settlement in Ikorodu, Lagos state, Nigeria.
Ikorodu Farm settlement is on the designated areas for farming in Lagos State. It is located in Ikorodu, its geographical coordinates falls within 6˚40'0'' North, 3˚

Sample Collection
Soil samples were collected randomly at 0 -15 and 15 -30 cm depth after land clearing, before the application of fertilizer and pesticide. The sample was collected at the uphill, hill wash and valley bottom. Two (2) weeks later after fertilizer application, pesticide application soil sample was collected at the same depth. Soil and vegetable samples were collected 4 weeks after pesticide application (Table 1).

Physicochemical Analysis of Soil Samples
Soil pH was determined using pH meter at a ratio of 1:2.5 soil/water according to the procedure described by [31]. The soil moisture content was determined according to the procedure outlined by [32] while the cation exchange capacities of the soil samples were determined by ammonium saturation method. Organic carbon and the organic matter were determined according to the procedure Journal of Agricultural Chemistry and Environment

Sequential Extraction of Heavy Metals
A large number of sequential extraction methods have been reported, many of which are variant on Tessier procedure [35] in which the exchangeable metals and those nominally associated with different reagents. In defining the desired partitioning of metals, care was taken to choose fractions likely to be affected by various environmental conditions; the five fractions (Exchangeable, Carbonate bound, Reducible, Oxidizable, Residual) were selected in this study and this method was developed by [35] and modified by [36].
The sequential extractions were carried out on 1 g of soil, in 85 ml polypropylene centrifuge tube to simplify centrifuge-washing of the residue after each ex-

Vegetable Digestion
Digestion of the vegetable was carried out according to the method described by [37] with some modifications. Five milliliter (5 ml) of concentrated nitric acid and five ml of hydrogen peroxide were added to one gram of the vegetable sample and heated on a hot plate at a temperature of 60˚C to near dryness. Ten milliliter (10 ml) of deionized water was then added to the mixture and then filtered. The digested sample was made up to 100 ml and stored for analysis.

Chemical Analysis
The concentration of Pb, Cd, Cu, Zn and Fe present in the soil extracts and vegetable digest were assayed using AAS (Buck Scientific Model 200 A) with air acetylene flame. The calibration of AAS was done using multi-elemental solution prepared by serial dilution of 20, 10, 5, 3, 2 and 1 ppm with r 2 value above 0.9 before the analysis of the samples. As samples were aspirated into the flame, the heavy metals of interest present in the sample absorbed some of the light from the hollow cathode lamp reducing the intensity of the light transmitted. The computer data system of the machine converted the intensity of light into the absorbance which was directly proportional to the concentration of the heavy metals present in the sample. The metal concentration in mg/l was converted to mg/kg using the following equation:

Data Analysis
Descriptive (Mean and Standard deviation) and inferential (ANOVA) statistics were used. Duncan was used to compare means using statistical analysis system (SAS). The data were expressed as mean ± standard deviation. The significant differences between groups were compared to find out the major biogeochemical processes controlling the distribution and partitioning of metals.

Potential Human Health Risk of Metals in the Study Sites
The health risk assessment model used in this study to calculate the exposure risk to children and Adults from heavy metals in soil is based on those models developed by [38] and the Dutch National Institute of Public Health and Environmental Protection [39] which defines guidelines or screening levels of con-Journal of Agricultural Chemistry and Environment taminants in soils in urban exposured scenarios. Human exposure to heavy metals in soil can occur via the following three main paths: a) direct ingestion of substrate dust particles (CDI ing ); b) inhalation of suspended dust particles through mouth and nose (CDI inh ); c) dermal absorption of heavy metals in particles adhered to exposed skin (CDI dermal ). The dose received through each of the three paths was calculated using the following Equations (2) where CDI (mg•kg −1 •day -1 ) is the Chemical daily intake through ingestion (CDI ing ), inhalation (CDI inh ), dermal contact (CDI dermal ) R ing is the ingestion rate at 200 mg•day −1 for children (1-6 years) and 100 mg•day −1 for adults [43]; R inh is the inhalation rate at 7.6 m 3 •day −1 for children and 20 m 3 •day −1 for adults [42]. Exposure frequency (F exp ) in this study was 180 day•years −1 [44] while exposure duration (T exp ), in this study, was 6 years for children and 24 years for adults [45]. Average body weight (ABW) was 15 kg for children and 70 kg for adults [43] while PEF is the particle emission factor taken to be 1.36 × 10 9 m 3 •kg −1 for both children and adults [46]. Skin surface area (A skin ) was 2800 cm 2 for children and 3300 cm 2 for adults [46] and SAF is the skin adherence factor given as 0.2 mg cm 2 •h −1 for children and 0.07 mg•cm 2 •h −1 for adults [47]. Dermal absorption factor (DAF) (unitless) was 0.001 for both children and adults [48]. T avg is the average time [fornon-carcinogens T avg = 365 9 Texp; for carcinogens T avg = 70 × 365 = 25,550 days [43]. C UCL (exposure-point upper confident limit content [mg•kg −1 ]) which is the upper limit of the 95% confidence interval for the mean was calculated using equation v [43].
where X is the arithmetic mean, s is the standard deviation and n is the number of samples. In this study, quantified risk or hazard indexes for both carcinogenic and non-carcinogenic effects were applied to each exposure pathway in the  [49]. If the value of HI (cancer risk) falls within the range of threshold values (10 −4 -10 −6 ), the cancer risk is acceptable [50]. Therefore, hazard index methods and cancer risk methods were used to assess the human exposure to heavy metals in the study area.

Potential Daily Intake of Vegetable at the Study Site
The methodology for the estimation of non-carcinogenic risks was provided in the USEPA Region III's Risk-based Concentration Table [51]. The non-carcinogenic risk for each individual metal through vegetables consumption were assessed by the target hazard quotient (THQ) (US Environmental Protection Agency [43], which is the ratio of a single substance exposure level over a specified time period (e.g., sub-chronic) to a reference dose (RfD) for that substance derived from a similar exposure period. The equation used for estimating the target hazard quotient is as follows: where C (metal conc) = heavy metal concentration in vegetable (mg•kg −1 ); C (factor) = conversion factor (0.085); In order to assess the overall potential for non-carcinogenic effects from more than one heavy metal, a hazard index (HI) has been formulated based on the where, THQ is the target hazard quotient; EFr is the exposure frequency (365 days/year); ED is the exposure duration (70 years); C is the metal concentration in foods (mg•kg −1 •fw); RfD is the oral reference dose (mg•kg −1 •day −1 ); AT is the averaging time for non-carcinogens (365 days/year × number of exposure years).
The oral reference doses were based on 1.5, 0.02, 0.04, 0.0003, 0.0005, and 0.004 mg•kg −1 •day −1 for Cr, Ni, Cu, As, Cd, and Pb, respectively [51] [52]. If the THQ is less than 1, the exposed population is unlikely to experience obvious adverse effects. If the THQ is equal to or higher than 1, there is a potential health risk and related interventions and protective measurements should be taken.

Results and Discussion
The descriptive statistics result of the physico-chemical properties at the study location is presented in Table 2 and Table 3 while the result of metal speciation of the soil for both top and sub soil for six elements and for all soil treatment is Journal of Agricultural Chemistry and Environment shown graphically in Figure 2.
The result revealed that the total extractable cadmium levels in the study area were above the critical permission of 3.0 mg•kg −1 for agricultural soil [53] [40].
Cadmium was found to be mostly associated with the residual fraction in the top soil and carbonate fraction in the sub soil with percentage range of 34.7% to 44.9% and 33.6% to 46.2% respectively ( Figure 2 (Figure 2).
These extractable metals may find their way through the food chain into human body. Cadmium in the body is known to affect several enzymes [56]. It is believed that the renal damage results in proteinuriais, the result of Cd adversely affecting enzymes responsible for reabsorption of proteins in kidney tubules [57].   The total extractable Chromium in the soil was below 750 mg•Kg −1 limit permissible by [58] and [59]  The concentrations of total extractible Cu in the farmland were all below the toxic limit of 250 mg•kg −1 set by [64] for agricultural lands indicating that the soil is not polluted with Cu. The results also indicated that majority of Cu in the top soil was associated with the residual fraction (i.e. bound to silicates and detrital materials) having a percentage range of 25.7% to 42.6% (Figure 2) which is similar to the reports of [65] and [36]. The result is at variant to [66] and [67]  On the other hand, appreciably high amount of Cadmium found in carbonate fraction in the sub soil suggested that cadmium is potentially available to some extent in these soils because metals in this fraction are usually thought to be readily available for plants uptake. By these criteria, cadmium must be considered quite mobile and biologically available in the soil samples. The availability of this metal in the sequentially extracted fractions in the sub soil shows the order; Carbonate (33.6% to 46.2%) > Oxidizable (24.7% to 29.9%) Reducible (3.8% to 33.7%) > Residual (7.0% to 20.5%) > Exchangeable (1.22% to 1.75%) > Oxidizable (0.74% to 1.81%) (Figure 2). Cadmium reduces the activity of delta aminolevulinic acid synthetase, arylsulfatase, alcohol dehydrogenase, and lipoamidedehydrogenase, whereas it enhances the activity of delta aminolevulinic acid dehydratase, pyruvate dehydrogenase, and pyruvate decarboxylase [68].
Lead was found to be mostly associated with the reducible fraction in the sub soil with percentage range of 15.8% to 55.4% respectively ( Figure 2). Pb in reducible fraction constituted more than 32% of total concentration for all samples in the sub soil which is consistent with work of some researchers that showed reducible fraction of Pb to be the most important compound form in soil [60] [69] [70] [71]. Fe-Mn oxides are important scavengers of heavy metals in soils (Ping et al., 2008). This is because Pb element exists as Pb (II) in the earth's surface, which could intensely absorb on the surface of Al, Fe, Mn oxides, and silica. The potential bioavailability of the metal in the sub soil is in the following order: Reducible (15.8% to 55.4%) > Carbonate (14% to 55.4%) > Oxidizable (13.9% to 28%) > Residual (13.5% to 28%) > exchangeable (1.13% to 3%) for sub soil respectively ( Figure 2).
The Chromium content was strongly associated with the residual fractions in the sub soil with percentage range of 20.8% to 39.9% respectively (Figure 2).
High concentration of residual Cr in the sub soil could probably be envisage because through a series of reactions in soils, Cr could be easily transformed into insoluble hydroxide precipitates, which stayed in the residue [71] (Figure 2).
Most of the Nickel was found in the reducible fractions in the sub soil with percentage range 6.3% to 53% respectively (Figure 2). High concentration of Ni in the reducible fraction for both sub soil indicated that Ni was in active speciation [15], which suggests that Ni in these mediums has strong activity and bio-  [62] and so on.
The Muller Index of Geoaccumulation, Igeo indicating the level of contamination found in various soils, is widely recognized in Europe. I geo consist of seven grading ranging from unpolluted to very seriously pollute. Grade 6 indicates a 64-fold enrichment over the background values [75].
The result from the study shows that the soil was not contaminated for all the metal analyzed in the soil (Table 3). Contamination factor (CF) is used to illustrate the contamination of particular toxic substance at a given site [76].  (Table 4).
Higher PLI values in soil demonstrated substantial anthropogenic impacts on the soil quality whereas lower PLI values pointed to no considerable anthropogenic activities. This investigation strengthened employment of CD and PLI as effective instruments for assessing the environmental geochemistry of soil and could be used individually or in combination as they closely complemented each other. Furthermore, they easily convey information to the public and policy makers to ascertain the contamination load of the soil to take necessary remedial measures.
Heavy-metal exposure has potential and serious health risk to [77] Thus, in this study, the health risk of heavy metal exposure to humans was one of the main focus issues. The results of the carcinogenic and non-carcinogenic risk assessment for children and adults using the summation of mobile fractions are presented in (Table 5). For the non-cancer effects for adults; dermal exposure to Cd (4.28E+06) and ingestion route to Cr (1.01E−02) are the major exposure routes. The non-cancer distribution pattern for both ingestion and dermal routes was: Cd > Cr > Ni > Mn > Pb > Cu. Additionally, total exposure Hazard Index (HI) from ingestion, dermal contact, and inhalation for Cd, Pb and Mn were greater for children than for adults. Children are more susceptible to a given dose of toxin and are likely to inadvertently ingest significant quantities of metals because of their hand-to-mouth behavior, which has been widely regarded as a key metal exposure pathway for children [78].
Hazard Index (HI) values for analyzed elements to both adults and children decrease in the order of Mn > Cd > Cr > Ni > Pb > Cu. The HI summation for the sites using Mobile Fractions (F1 + F2 + F3 + F4) showed that Manganese poses a higher risk of non-cancer effects among the studied elements while Copper poses the lowest ( Table 5). The sum of hazard index (ΣHI) for all of the metals and all routes for both adult and children is 6.15E+15. This HI value is > 1 and this is an indication that the soil poses non-cancer threat to human. The range reported for the non-cancer effects in the present study is at variance to the values reported by some authors [79] [80] [81], but also in agreement with [82]. Among the carcinogenic metals, only Cd, Pb, Cr and Ni are analyzed, and carcinogenic risk was assessed from calculated daily dose (1.32E−08) were lower than the 10 −5 risk factor acceptable by some authorities [83].
O. M. Makanjuola et al.  The estimated daily intake of metals from the Jute Mallow and the hazard quotient and hazard index for adult and children for were presented in (Table 6).
Cadmium is a dangerous element because it can be absorbed via the alimentary tract; penetrate through placenta during pregnancy, and damage membranes and DNA [57]. Once in the human body, it may remain in the metabolism from 16 to 33 years and is connected to several health problems, such as renal damages and abnormal urinary excretion of proteins. Decrease in bone calcium concentrations and increase of urinary excretion of calcium have also been attributed to exposure to Cd, eventually causing death. It also affects reproduction and endocrine systems of women [84]. Vegetables may contribute to about 70% of Cd intake by humans, varying according to the level of consumption [85].
The daily intake of Cd for adult ranged between (0.023 and 0.026 mg•day −1 • person −1 ) and children ranged between (0.016 and 0.018 mg•day −1 •person −1 ) ( Table   6) with RfD, established to 0.001 mg•kg -1 of body weight per day, equivalent to 0.07 mg per day for a 70 kg adult [86]. The daily intake was higher than the tolerable daily intake (TDI) for the jute mallow. The toxic effects of Pb focus on several organs, such as liver, kidneys, spleen and lung, causing a variety of biochemical defects. The nervous system of infants and children is particularly affected by the toxicity of this Heavy metal. Adults exposed accidentally to excessive levels of Pb exhibit neuropathology. There is association between Pb in human body and the increase of blood pressure in adults [87]. Although Pb effects are more relevant for children, calculations for risk assessment were made for adults and children. The daily intake of Pb ranged from (0.020 -0.038 mg•day −1 •person −1 ) for adult and (0.014 -0.026 mg•day −1 •person −1 ) for children (Table 6)  Cr is an important element for the insulin activity and DNA transcription.
However, an intake below 0.02 mg per day could reduce cellular responses to insulin [89]. The daily intake, ranged from 0.012 to 0.023 mg•day −1 •person −1 for adult and 0.079 to 0.099 mg•day −1 •person −1 for children estimated, was lower than  [86]. The daily intake was below the TDI of 1.4 mg per day.
Although the HQ-based risk assessment method does not provide a quantitative estimate for the probability of an exposed population experiencing a reverse health effect, it indeed provides an indication of the risk level due to exposure to pollutants [92]. Many researchers consider the risk estimation method reliable  [96]. When the hazard index exceeds 1.0, there is concern for potential health effects [95].

Conclusion
Metal speciation and health risk assessment of soil and vegetable collected from Ikorodu farm settlemen of Lagos had been carried out. The results of speciation analysis showed that Pb and Ni, Cd, Mn and Cu were predominantly associated with exchangeable and residual fractions respectively, while Cr was associated with reducible fractions for top soil. Also, for sub soil; Cd, Mn and Cu, Pb and Ni, and Cr were predominantly bound to carbonate, Fe-Mn oxide and Silicate mineral components of the soil respectively. A comparison of the result of total extractible metals with standard set by USEPA reveals that Pb, Cr, Ni, Mn and Cu were below the threshold limit in the vegetable farmland while Cd level was above the critical permissible limit of 3.0 mg/kg for agricultural soil and therefore portend a health risk. The result also shows that the plant species investigated (Corchorus olitorius) accumulate all metals below the threshold limit suggested by USEPA (1986) except for Cd and Pb which exhibit higher concentration above the limit of 0.3 mg•kg -1 and 5 mg•kg -1 . The concentration of total extractible metals in farmland was also compared to some other soil around the world, even though each area has its own geochemical characteristics. The investigation revealed Cd as the biggest contaminant and chief cause of concern. Four soil quality indices namely; contamination factors (CF), contamination degree (CD), pollution load index (PLI) and geo-accumulation index (Igeo) were further used to determine degree of anthropogenic influence on the soil quality.