Appraisal of chromium and cobalt contents of vegetables grown in soil irrigated with sewage water : A risk for consumers ’ health

The aim of the present research was to determine the concentrations of chromium (Cr) and cobalt (Co) in soil, vegetables grown at the areas which are usually irrigated with canal water (CW) and sewage water (SW). The samples were analysed through Atomic Absorption Spectrophotometer. The mean concentrations of Cr and Co in soil samples varied from 0.9 to 1.80 mg/kg and 0.575 to 1.10 mg/kg, respectively. The highest value of Cr was observed in Solanum lycopersicum irrigated with SWI and the lowest in Brassica rapa irrigated with CW. While in case of cobalt, the highest value was found in Solanum lycopersicum irrigated with SW and the lowest was noticed in Capsicum baccatum irrigated with CW. In all vegetables, the daily intake of metal values for Cr


Introduction
Trace metals are potentially toxic and even lesser quantities of these metals in organisms are harmful as they are utilized in industrial processes [1].Trace metals could be collected in the food stuff consumed by humans and other living beings The main objective of the present research, therefore, was to determine the concentration of chromium and cobalt in vegetables grown at the areas which were usually irrigated with canal water and wastewater.The soil was also observed for its metal content at two sites i.e. site I, where canal water was used for irrigation and site II, where industrial wastewater was used.Moreover, fitness of these vegetables for human consumption was evaluated.

Study area
The present research was performed in an urban area of Sargodha, Punjab, Pakistan.This city has tolerable winter season and warm temperature in months from May to July.The temperature differs from 25 to 50 º C in the summer.The main production of this city is citrus fruit.Areas irrigated with sewage and canal water in Sargodha were selected for this study.The land used in agriculture is saline in nature.Mostly the areas surrounding Sargodha are irrigated with the industrial wastewater.Two sites were selected for study i.e. site I, where canal water was used for irrigation and site II, where industrial sewage water was used.

Wet digestion process
The procedure for sample arrangement and preparation involved the wet digestion process.This process involved following steps: Firstly, complete digestion of samples was done by using acid and hydrogen per oxide.After digestion, the samples were diluted with distilled water and filtered through Whatman filter paper.Wet digestion breaks down organic components found in plant tissue into CO2 by strong reducing agents.After the wet digestion process, a colourless transparent and clear solution is obtained.Firstly, soil was weighed, and 1 g sample was added in flask with 20 ml of nitric acid and 10ml of hydrogen per oxide to be eventually placed on hot plate for heating till boiling.After removing the sample from hot plate, the sample was allowed to cool down.Samples were then filtered and put in clean plastic bottles till futher use after making volume upto 50 ml by adding distilled water.For plants, the parts were first converted into powdered form and then dried completely in oven for 24 h.The samples were then added to the digestion flask with hydrochloric acid and nitric acid in 1:3 ratio.Hotplate boiling was done till achieving transparent solution which was allowed to cool.The volume of the solution was made 50 mL ater filtration through Whatman filter paper.The bottles were labelled and stored till further use.

Chromium and cobalt analysis
The analysis of heavy metals requires the formulation of specific standard solution for the heavy metals that are under research.Before starting the analysis of Cr and Co, the formulation of the standard solution was done.After formulation of specific standard solution, the heavy metal analysis done with atomic absorption spectrophotometer (AAS-6300 Shimadzu Japan).

Preparation of the standard solution Statistical analysis
Among different plants (crops) and soil samples, the degree of variation was measured by using SPSS 22 (Statistical Package for Social Sciences).To measure the mean concentration values for soil and food crop samples, one-way ANOVA was conducted.

Daily intake of metals (DIM)
The normal value of DIM is 0.242 kg with standard human mean weight of 55.9 kg.It is defined as the amount calculated to the intake of trace metals orally which was obtained by formula given by Sajjad et al.The concentration of a substance in the tissue of organism is called bio-concentration factor (BCF).It is also known as enrichment factor or biomagnification factor.It is measured by the following formula: BCF = Vegetable metals/Soil metals

Chromium and cobalt concentrations in soil samples
The mean Cr concentration in soil varied from 0.9 to 1.80 mg/kg.Higher Cr values in soil were observed during sewage water irrigation and lower values were found in canal water irrigation.The ANOVA results showed that the irrigation water had significant effect (p≤0.05) on the Cr concentration in soil (Table 1).The mean Co concentration in soil ranged from 0.575 to 1.10 mg/kg.Higher Co values in soil were observed during sewage water irrigation.The ANOVA results showed that the irrigation water had significant effect (p≤0.05) on the Co concentration in soil (Table 1).Soil serves as the most central component in agricultural environment and contents of heavy metals and various other minerals in soil determine the accretion of heavy metal in plant body.In the present study, the mean Cr concentration in soil ranged between 0.9-1.80 mg/kg.The Cr levels in the soil samples investigated in the present research were lower than the maximum permissible limit of Cr (

[12].
The results revealed that HRI of Cr and Co was less than 1 in each vegetable which was below the permissible limit.To determine the health risk associated with heavy metal contamination of plants grown locally, estimated exposure and risk index were calculated.In all vegetables, the HRI values for Cr and Co were higher in sewage water irrigation compared to canal water irrigation (Table 3).In the present study, the HRI values of Cr and Co fell under the safe limit of HRI and were considered fit for human consumption.
[2].Wastewaters have higher concentration of potentially toxic metals [3, 4].The infectivity of vegetables by these metals could be because of excessive use of sewage water in irrigation [5, 6].Extended ingestion of these heavy metals by consuming contaminated vegetables could be lethal to humans and organisms resulting in many biological disorders [7, 8].The levels of trace metals are increasing day by day in ecosystems and cause uneasiness among people because of occurrence of trace metal remains [9, 10].The community has become fearful and worried regarding the safety of vegetables because the food items having heavy metals are not considered appropriate for consumption.Hazards related to utilization of contaminated food items have increased manifolds [11, 12].Pollution by trace metals is owed to usage of industrial wastewater, automobiles, organic substances used for enhancing plant growth, pesticides and harvesting processes [13, 14].The food components absorb trace metals from land polluted with wastewater [15, 16].By observing the possible hazards of trace metals, their constant life and the regular use of contaminated vegetables, the matter of concern is to examine the toxic levels in food items to protect humans from health hazards [17, 18].
[19].DIM = (Concentrations of metal) x Daily consumption of vegetable (kg per person)/Average body weight of a person Pollution load index (PLI) The presence of heavy metals in soil was assessed by this factor.By relating the amount of trace element in the polluted soil under consideration with reference to the amount of the same trace element as mentioned by the reference value of that element in soil, PLI is measured [20].It is calculated by following formula: PLI= metal concentrations in soils/metal concentration taken as reference Health risk index (HRI) Health risk index is calculated relative to DIM value and relative dose (RfD).The formula described by Cui et al. [20] is used to measure the relative measurement of HRI.HRI= DIM/ food oral reference dose for the metal Bio-concentration factor (BCF)

Table 1 )
. Higher Cr values were observed in S. lycopersicum irrigated with sewage water and lower Cr contents were noticed in B. rapa irrigated with canal water.According to the ANOVA results the irrigation water had significant effect (p≤0.05) on the Cr concentrations in R. sativus, B. rapa, Z. officinale, C. frutescens, C. annuum and C. longa whereas non-significant effect (p>0.05) was noticed in C. baccatum and S. lycopersicum (Table 1).The Cr levels in the vegetable samples investigated in the present research were lower than the maximum permissible limit of 50 mg/kg reported by Chiroma et al. [25].The investigated values of Cr were also lower than the study conducted by Nagajyoti et al. [26] who found Cr concentration in five leafy vegetables ranging from 0.89 to 1.08 (μg/g) except for S. lycopersicum.Haiyan and Stuanes [22] reported that the Cr content in the vegetables grown in controlled area ranged from 0.4 to 2.7 (μg/g of dw).

Table 1 )
. Higher Co values were observed in S. lycopersicum irrigated with sewage water and lower Co contents were noticed in C. baccatum irrigated with canal water.According to the ANOVA results, the irrigation water had significant effect (p≤0.05) on the Co contents in R. sativus, Z. officinale, C. frutescens, C. annuum, S. lycopersicum and C. longa while nonsignificant effect (p>0.05) was observed in B. rapa and C. baccatum (Table 1).Cobalt is a basic component of vitamin-B12.It is essential in synthesis of red blood cells and prevents anemia.Too much intake of cobalt may cause overproduction of red blood cells [30].

32]. Pollution load index
The present study results were relatively identical to the observations of Zhuang et al. [Values of PLI for Cr and Co of two study sites are given in Table 4.The PLI values of Cr in soil samples were 0.0140 and 0.0280 for CWI and SWI treatments, respectively.On the other hand, the PLI values of Co for soil samples were 0.1099 and 0.2103 for CWI and SWI treatments, respectively.Contamination level in soil can be explored using PLI.This index provides a simple and comparative means for assessing the quality of different combinations of water irrigation.As described by Tomlinson et al. [33], a value of zero indicates no risk, whereas a value of one and values above one would indicate progressive deterioration of the site irrigated with this water quality.