Metal Contamination in Field Grown Rice

Da LI,Ben LI,Minyu HU

School of Public Health,Central South University,Changsha 410078,China

With increasing industrialization and urbanization,the contamination of metals is attracting more and more public attention in recent years.There are specific studies conducted in polluted areas concluding that the metal levels in residents’hair and blood are elevated[1-3].Metal contamination in soil and in field grown plants has been found all over the world[4-7].Rice was found to be a significant contributor of residents’intake of cadmium (Cd)[8]and inorganic arsenic (As)[9].Rice is a predominant grain crop and staple food,providing 20 percent of the world’s dietary energy supply[10].As the most important food crop in China,rice consists of 40 percent of the annual crop production[11]in the country;rice intake consists of 59.2 percent of the grain intake for the total population in China[12].

Metals can be taken up by agricultural fields through irrigation with untreated waste water,or through rain drops and aerial deposits onto soil[13].Residential aqueous waste[14],excess use of pesticides,fungicides and fertilizers[15],and soil composition(low pH or small amount of organic matter)[16]are also responsible for causing metal contamination.The interaction between different kinds of metals may also have an impact on their absorption by rice[17].An excess of metals not only has the potential to reduce the yield of rice crops[18],but also causes negative health effects in the pathway where contaminated rice is ingested[19].An excessive amount of lead may cause the malfunction of human central or peripheral nervous systems,with signs of memory deterioration and diminished intellectualcapacity,or disturbance of hemoglobin synthesis;the potential risks of cadmium accumulation include Itai-Itai Disease,kidney damage,corresponding proteinuria,or chronic kidney failure.Cadmium is also related to cancer and hypertension;inorganic arsenic is classified as a known human carcinogen by the U.S.ATSDR[20],U.S.EPA[21],and the IARC[22].Inorganic arsenic may lead to the disturbance of the cardiovascular and nervous system[23].

Studies provide evidence of metal contamination in rice.Research found that the Cadmium (Cd)concentration in rice is 0.02-1.06 mg/kg in the Jinzu river basin,and 0.11-0.67 mg/kg in the Kakehashi river basin[24];rice con-tains above 0.1 μg/g of arsenic(As)in the area of West Bengal[25].However,one of the studies from China found that the concentration of lead(Pb)and Cd in rice can be as high as 0.8 and 6.99 mg/kg respectively[13];the concentration of As can be as high as 0.5-7.5 mg/kg[8].In the present study,the research selected rice from one test field in Hunan province and detected the concentration of metals(Pb,Cd,As)in it,to assess and understand the current status of metal contamination in rice,in order to come up with a scientific basis for further research.

Materials and Methods

Sources and sample collection

In one test field,the research randomly selected 15 adjacent fields(every field is about one Mu,which is roughly equivalent to 0.067 hectare)and divided them into five groups(Group A,B,C,D,E;every group consists of three fields).Group A was grown with water and the other four groups were grown with the same fertilizer.The research randomly selected two samples in every field(30 samples in all).All samples are Y399 hybrid rice harvested in Aug.2011.

Experimental methodology

All concentrations were determined according to methodology set bythe Ministry of Health of the People’s Republic of China,as are denoted as GB/T5009-2003.The concentration of Pb was determined by FAAS(Flame Atomic Absorption Spectrometry)according to the third method in GB/T5009.12-2003 Determination of lead in food;the concentration of Cd was determined by AAS(Atomic Absorption Spectrometry)according to the second method in GB/T5009.15-2003 Determination of cadmium in food;and the concentration of As was detected using silver diethyldithiocarbamate method according to the second method in GB/T5009.11-2003 Determination oftotal arsenicand abio-arsenic in food.

Evaluation Methodology and Standards

According to GB2762-2005 Maximum Levels ofContaminants in Foods,the limits of concentration of Pb,Cd and inorganic As in rice are 0.2 mg/kg,0.2 mg/kg and 0.15 mg/kg respectively;based on a paperby Heikens from the FAO,the research converted the results of total As measured by silver diethyldithiocarbamate method to inorganic As (the conversion coefficient is 50%),which can be used for the As calculation and evaluation.

It evaluated the contamination in rice samples of three types of metals,Pb,Cd,and As,using the overall pollution score (OPS)from a paper by Leeet al.The equations are:OPS=ΣPi;Pi=Ci/Si.Pis defined as the pollution index of every single type of metal.;C(mg/kg)is defined as the measured concentration of every single type of metal;S(mg/kg)represents the concentration limit of a specific type of metal in GB2762-2005 Maximum Levels of Contaminants in Foods.The OPS evaluation standard has four levels:not problematic(OPS≤3),slightly multi-contaminated(3＜OPS≤6),moderately to highly contaminated (6＜OPS≤30),and extremely contaminated(OPS＞30).

Statistical analysis

All data were statistically analyzed using SPSS18.0.Data that is normally distributed was analyzed by one-way ANOVA comparing means between groups,with a LSD-ttest for pairwise comparison;data that was not normally distributed was analyzed by a Rank Sum Test comparing the variation between groups.The significance level was 0.05.

Quality Control

All glass and plastic ware in the experiment were previously scrubbed with detergent,soaked in HNO3(10%)for 24 hours,rinsed thoroughly once with deionized water and three times with quartz distilled water.Then all ware was ready for use after air drying.

Accuracy of Experiment.①The research made a regression analysis for the fitting curves of Pb,Cd and As standard solutions(The regression coefficients,r,are 0.995,0.998,0.999,respectively.)②During the sample measurements,if the result was not on the standard curves,the research remeasured it after dilution using quartz distilled water.

Precision of Experiment.①One sample was randomly selected and digested.The parallel determination of the digested solution was done six times,with the Relative Standard Deviation(RSD)not exceeding 10%.If theRSDwas above 10%,re-sampling was done until theRSDwas within the limit.In order to identify suspicious values,the research used aQtest,with Q0.90＞0.94 to be excluded.② The research added standard material(10 μg of Pb,1 μg of Cd and 10 μg As)to 5 g of samples and treated them using the same method of digestion and measurement used previously on the samples.The recoveries of Pb,Cd and As were 97%,95%and 98%,respectively,applying the formula:Recovery%=100X.

Results

General review

The average concentration of the Pb,Cd,and As in the 30 samples are respectively:(0.62±0.47),(0.45±0.09),and (0.29±0.16)mg/kg,with the above-standard rates of 83.3%,100%and 36.7%(Table 1).

Measured concentration of Pb,Cd and As in samples

The concentrations of Pb in all groups of rice have no statistically significant difference

(H=4.228·CP=0.376)(not shown);the concentrations of Cd have a statistically significant difference (F=3.160;P＜0.05),with the mean concentration of Group A lower than others’(Table 2);the concentrations of As have no statistically significant difference (H=3.568;P=0.468)(not shown).The interquartile range (IQR)of Pb concentration in five groups are 0.85,0.84,0.78,0.64 and 0.29,respectively(not shown),which is relatively higher than the other two metals.

Table 1 Results of the Detection of Pb,Cd,As in All Samples（mg/kg,n=30）

Table 2 Results of the Detection of Pb,Cd,As in the Five Groups(mg/kg,n=6)

Table 3 Pollution Index of Pb,Cd,As in the Five Groups

The calculation of pollution index

The OPSs of Group B and Group C exceed 6,while the OPSs of Group A,D and E are not above 6.The pollution index of Pb constitutes the largest percentage of OPS,which is 60%,while As constitutes the smallest percentage,which is 14.3%(Table 3).

Discussion

The research compared the average concentration of the Pb,Cd,and As with the GB standards,resulting in the above-standard rates of 83.3%,100%and 36.7%,respectively.This demonstrates a metal contamination of rice in this test field.Evidence of metal contamination has been documented in other papers.In 2002,the sampling inspection for rice in markets showed that the above standard rates of lead,cadmium and arsenic are respectively 28.4%,10.3%and 2.8%[26];a study measured the content of 70 samples of rice,which are randomly selected from the markets and polluted farms from 4 provinces of the Southern China,and found out that the over standard rate of cadmium is 70%[27];similar statistics show that the over standard rates of lead,cadmium and arsenic in polluted area can be as high as 100%,50%and 50%[28].

Compared to other groups,the Cd concentration in water group is relatively low,which may be related to factors such as soil situation,environment,fertilizer or irrigated water.It is noteworthy that though the Cd concentration in the water group is relatively low,the Cd concentrations in all samples are above the permissible limit,which may result from the easier translocation of Cd from soil to plants[29].

As the interquartile range(IQR)of Pb is relatively higher than the other two metals,the degree of variation is relatively higher.This means that the Pb concentration in rice may be easier to vary from each other,even if they are treated with the same thing and in the same place,which has for example been discussed in a study by Leiet al.[29].One of the possible explanation for this phenomenon is that Pb translocation is more sensible to soil and otherenvironmentalelements.Further research is needed for confirmation.

The results of the OPS calculation indicated that this test field has been metal contaminated (slight to moderate or high contamination),coincided with the results of GB evaluation.Though further work is needed to find the contributor of metal contamination in this test field,the most likely reason for the exceeded content of metals in rice is that the soil in the test field has been metal polluted.This was discussed by a study on Chenzhou,Hunan.The paper showed that the concentration of lead,cadmium and arsenic in soilreach respectively 321.11-1088.30,2.70-7.57and192.49-709.29 mg/kg(According to GB15618-1995 Environmental quality standard for soils,the criterions corresponding to local soil pH are respectively 300,0.6 and 30 mg/kg.)The maximum concentrations of the same three metals in rice are respectively 0.80,6.99 and 0.49 mg/kg[13].

In particular,our study calculated the overall pollution score by adding the pollution indexes of Pb,Cd and As,finding that the pollution index of Pb constitutes the largest part (60%),while Cd and As constitute a smaller part(14.3%).This concludes that Pb is the major reason for the metal contamination in the study field.

Conclusion

In our study,excessive levels of Pb,Cd and As were observed in the rice in the test field;OPS calculation indicated that the metal contamination level of the test field is slight or moderately high,to which Pb made the most contribution;the Cd concentrations from all samples exceeded standard limits.Metalcontamination in rice needs improvement in monitoring.The Joint FAO/WHO Expert Committee on Food Additives (JECFA)[30]proposed‘Evaluation of Certain Food Additives’and worked on the evaluation of metal contamination.The Chinese government has raised deep concerns about metal contamination:the State Council promulgated‘The 12thFive-Year Plan for Metals Pollution Control’in April,2011,mandating the development of a‘relatively complete metal protectiontreatment system,accident-emergency system andenvironmental-health evaluation system’by 2015.We are looking forward to more developments in the future.

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