Investigation of Cadmium Content in Rice in Heilongjiang Province and Health Risk Assessment


 We investigated the cadmium content in soils and rice in Cha Hayang, Wuchang, Fangzheng, Xiangshui, and Jiansanjiang areas of Heilongjiang Province, and characterized the effect of rice intake on human health. The samples were analyzed by ICP-MS, and the cadmium transfer in soil-rice system was modeled by the Nemero comprehensive pollution index method. The health risk assessment model was used to study the status of cadmium pollution in rice and its health risk assessment for adults and children. The results showed that the average contents of cadmium in rice were 0.003 (Cha Hayang), 0.016 (Wuchang), 0.006 (Fangzheng), 0.006 (Xiangshui), and 0.005 (Jiansanjiang) mg kg-1. The prediction model developed in this study, including the total heavy metals and pH value of the soil, effectively described the transfer of cadmium in the soil-rice system of Wuchang, Chahayang and Xiangshui paddy fields (with R2 between 0.256 and 0.468). The pollution index of the study area was less than 1. The comprehensive pollution index was 0.037<1, sugesting no pollution, and the comprehensive pollution index was between 0.059 and 0.158. The health risk index of carcinogenic heavy metal cadmium to adults and children in Cyang and JianSanjiang areas was lower than that recommended by USEPA (1 × 10-4), suggesting no risk of cancer. However, the mean values in Wuchang, Fang Zheng and Xiangshui were higher than the maximum acceptable risk recommended by USEPA, suggesting a risk of cancer.


Introduction
As a country with a large population, agriculture in China is of primary importance. Most people in the country use rice as their staple food [10]. With the development of industrialization, the pollution of soil with heavy metals, especially cadmium, is particularly prominent. Scholars have done a lot of research on the potential risks of heavy metals to human health. However, this research concentrated mostly on mining areas [11,12], sewage irrigation [13,14] and waste treatment plants [15,16], with the research subjects focusing on dust reduction [17,18], corn [19,20] , vegetables [21,22], drinking water [23,24], rice [25,26], etc. These studies have carried out in-depth research on the accumulation characteristics, physicochemical properties, occurrence forms and physiological and biochemical effects of contamination by heavy metals, laying a foundation for further research on accumulation of heavy metals in crops and their impact on human health. However, most of these studies were the pot experiments conducted under controlled conditions; there are only few studies on the accumulation characteristics of heavy metals in rice grown in the natural environment [27].
In this paper, we study the content of cadmium in soil and rice as well as a risk of harm to human health.
We chose Heilong Jiang province as the big grain-producing area to collect soil and rice for detection of cadmium pollution situation. We selected ve areas to assess cadmium intake by the population via eating rice . We also conducted risk assessment regarding potential health effects. By analyzing the pollution situation, we can emphasize sustainable development that pays attention not only to the gross national product, but also to environmental protection and human health.

Research Method
Overview of the study area Heilongjiang Province is located in the rice-growing area with the northernmost latitude. It has large temperature difference between day and night, fertile soil, excellent water quality and low pollution. It is conducive to the development of rice production. Heilongjiang rice production is now entering a new era of high quality, high e ciency and professionalism. Due to stable production, excellent rice quality and high commodity rate, Heilongjiang rice has become an important high-quality glutinous rice production base in China, and its products are shipped to all parts of the country. In 2015, rice planting area in Heilongjiang Province accounted for 17.0% of the country's rice planting area, and represented 69.25% of the rice planting area in the three northeastern provinces. Rice production in Heilongjiang accounts for 16.3% of the national rice production, and 67.6% of the rice production in the three northeastern provinces, playing an important role in the rice market.

Sample source
The rice and corresponding soil samples used in this study were from the rice producing areas in Heilongjiang province, including 22 from Chahayang 123°56 ' E-124°20 ' E, 48°05 ' N-48°30 ' N region, 22 from

Sample testing
The determination of the content of Cd in rice samples was carried out according to the method speci ed in Chinese National Standard GB 5009.286-2016 "Determination of Multi-Elements in Foods".

Risk assessment of heavy metal pollution in rice grains
The limit of Cd in rice grains was based on the limits of 8 elements including lead, chromium, cadmium, mercury, selenium, arsenic, copper and zinc in grain (including cereals, beans and potatoes) and products (NY861-2004). The single factor pollution index method and the Nemero comprehensive pollution index method were used to evaluate the heavy metal content in crops [28].
Single factor pollution index: See formula 1 in the supplementary les.
In the formula, Pcd is the comprehensive pollution index of heavy metals in crop grains; Ccd is the average value of single metal pollution index of heavy metals, and Scd is the maximum value of one-way pollution index of heavy metal Cd.

Nemero Integrated Pollution Index:
See formula 2 in the supplementary les.

Rice health risk assessment
In order to evaluate the health risks of rice in the diet of adults and children in the study area, the US Environmental Protection Agency (USEPA) recommended health risk assessment model was used [29]. The model used in this study was the carcinogenic risk model.

Results And Analysis
Contents of cadmium in soil and rice in Heilongjiang region The cadmium content in 110 soil samples collected is shown in Table 2-4, between 0.061~0.225mg/kg, the average value was 0.122 mg/kg, and the coe cient of variation was 0.280% <11%. Within the range of the coe cient of variation allowed, and according to the Chinese national standard GB 15618-2018 (soil environmental quality -agricultural land risk control standards), the cadmium content in the soil in the ve regions did not exceed the acceptable range. Hence, the risk of cadmium pollution in agricultural land in the study area was low. The content of cadmium in the soil was in the order: Chahayang> Xiangshui> Wuchang> Fangzheng> Jiansanjiang.
The content of cadmium in brown rice and polished rice in 110 rice samples collected was between 0.0003-0.0610 mg/kg and 3.96×10-6 to 0.056, with the average values of 0.007 mg/kg and 0.004 mg/kg, respectively. The coe cients of variation were 1.470% (brown rice) and 2.009% (polished rice), both within the range allowed by GBT27404-2008 "Laboratory Quality Control Speci cation Food Physical and Chemical Testing".In addition, according to GB 2762-2017 "Food Safety National Standards for Contaminants in Foods" the standard content of cadmium is below 0.2 mg/kg. None of the 110 samples of rice samples determined in this experiment exceeded this limit. The content of cadmium in brown rice ranged in the order: Chahayang> Xiangshui> Fangzheng> Jiansanjiang> Wuchang. The order of cadmium content in polished rice was consistent with that in brown rice.
The average value of cadmium in soil in this study was 0.122 mg/kg, which does not exceed the limit speci ed in the China's soil environmental quality standard. It is higher than the content of cadmium in soil of Heilongjiang area (0.096mg/kg) reported in 2012 [34]. The content of cadmium in the soil of Wuchang area was 0.124, which was lower than the cadmium content in the soil of Harbin area reported by Wang (2011) [35]. The cadmium content was much higher in Chahayang than the other four regions, but the cadmium content in the soil was not much different in the ve regions.
Studies have shown that the absorption and accumulation of heavy metals in rice is greatly affected by genetic background, cultivar type and heavy metal interaction [36] (Lin 2018). The varieties with high Se accumulation showed a tendency to inhibit the accumulation of heavy metal Cd [37] (Li 2003). In addition, some studies found that the cadmium content in rice was decreased by the optimal zinc content in soil [38] ( Zhang 2015).
The single rice variety (rice ower) and soil background in Wuchang area are the main factors leading to the difference between Wuchang and other regions. The cadmium content in rice in the two areas of Jianshuanjiang is also different by the cadmium content in paddy soil. In the process of absorption and accumulation of heavy metals, rice is not only affected by heavy metal content in soil, but also affected by other factors. Such as rice varieties, soil microbial content, precipitation, air quality and so on.

Analysis of the signi cance of differences between different regions
The variance analysis of ve areas of polished rice, brown rice and soil was carried out by SPSS statistics 12.0. The results are shown in the gure below.
It can be seen from Fig. 1 (a) that the content of cadmium in soils in Wuchang, Chahayang and Jiansanjiang was signi cantly different. In addition, there was a signi cant difference in the content of cadmium in soils in Chahayang and Fangzheng areas and also in soils in Fangzheng and Xiangshui areas and between Xiangshui and Jiansanjiang. The differences between Wuchang and Fangzheng and between Chahayang and Xiangshui were not signi cant.
It can be seen from Fig. 1 (b) that the content of cadmium in brown rice was signi cantly different between Wuchang and Chahayang, Fangzheng, Xiangshui, and Jiansanjiang as well as between Chahayang and Fangzheng, Xiangshui and Jiansanjiang. There was no signi cant difference between Xiangshui and Jiansanjiang in the content of cadmium in brown rice.
It can be seen from Fig. 1 (c).There were signi cant differences in the content of cadmium in polished rice between Chahayang and Wuchang, Fangzheng, Xiangshui, and Jiansanjiang. The differences in cadmium content in polished rice between Wuchang and Fangzheng, Xiangshui and Jiansanjiang were not signi cant.
In summary, the differences in cadmium content in soil, brown rice and polished rice in the study area were not consistent. The rice varieties have an effect on the absorption and accumulation of cadmium by rice, and some agricultural factors such as pesticides, fertilization and irrigation may also greatly affect absorption of cadmium by rice. Some studies have shown that natural conditions such as precipitation and CO2 concentration [39] also have an impact on absorption and accumulation of cadmium in rice.

Soil-rice system migration model of Cd element
The absorption and accumulation of heavy metals in rice is not only affected by the total metal content in the soil, but also by the physical, chemical and biological properties of the soil. Many researchers have studied the factors affecting the absorption of metal elements in rice, including soil pH [ Table 5 shows the physical and chemical properties of soils in the study area.
The present paper studied the factors affecting absorption of cadmium in the "soil-rice system" and proposed the best tting model for predicting the content of cadmium in rice. A multivariate regression model of arsenic content in rice was established by using soil pH and organic matter. The multiple regression equation was shown in Table 6. There was a signi cant negative correlation between the content of cadmium in rice and the concentration of cadmium in soil (P<0.05). The content of cadmium in Chahayang rice was positively correlated with soil pH (P<0.05), and it was signi cantly related to soil cadmium concentration The negative correlation (P<0.05); the content of cadmium in Xiangshui rice was signi cantly positively correlated with the concentration of cadmium in soil (P<0.05). The partial coe cient between cadmium content and soil pH in Jiansanjiang rice was not signi cant. Therefore, the best prediction model for the Jiansanjiang area was based on the concentration of cadmium in soil.
The content of cadmium in rice in Chahayang, Wuchang and Xiangshui areas could be predicted well by the concentration of cadmium in soil. The pH value of soil could predict the content of arsenic in rice in Chahayang area. However, in the established regression model, the content of cadmium in rice in Fangzheng and Jiansanjiang areas was not signi cantly correlated with soil cadmium content, pH and organic matter. Therefore, the prediction models of these two regions have yet to be worked out. Dudka [49] etal. (1996) reported that the relationship between heavy metals in rice and soil could be described by three models: linear (constant distribution model), plateau model (saturated) and Langmuir model. The metal adsorption also followed a linear model in the range of low metal concentrations in the soil.
In the present study, soil samples collected from paddy elds contained relatively low levels of cadmium. By tting and comparing the three models, it was found that the linear model was the best tting model. Therefore, the linear model is used for tting. The R2 value of the tted model was between 0.256 and 0.468 ( Table 5). The D-W index was close to 2, the autocorrelation of the independent variables was not obvious, and the model design was good . Dudka etal.[49] reported R2 values of 0.94 and 0.92 for the correlation between, respectively, Cd and Zn contents in barley grains and Cd and Zn contents in soil. McBride [50] found similar correlation coe cients. In the present study, the correlation coe cient was lower than the correlation coe cients (<0.9) of the previous studies. These above-mentioned studies were carried out in pots or small experimental eld; so, the soil properties changed little, if at all, during the modeling process. Hence, the model established under these conditions had a higher degree of t. In the present study, the rice and soil samples were collected under natural eld conditions. Paddy soils are a complex system. In addition to the variability in total metal content and pH value of soil, other soil properties may also affect the availability of heavy metals, potentially weakening the model t between metal accumulation by rice and the soil metal content and pH.
Evaluation of cadmium pollution in rice grains and health risk assessment of intake According to formulae (1) and (2), the results of heavy metal pollution assessment in the study area were obtained (Tables7-8). The single factor pollution index evaluation was less than 1 ( Table 7), indicating that the ve areas of the study were not polluted by Cd (the proportion of pollution-free soil in each region was 100%). The comprehensive rice pollution index in the study area was 0.153 (Table 8), which was non-polluting; the comprehensive pollution index of each region was in the order Chahayang> Fangzheng> Xiangshui> Jiansanjiang> Wuchang.
Due to the different geographical locations, and the differences in economic development level and industrial structure distribution, there are expected differences in the content of Cd in rice in different regions. The areas studied in this paper represent the geographically-protected rice products in Heilongjiang Province. The results reported in this paper did not exceed the values speci ed in the health risk index, and 100% of rice in the ve regions was non-contaminated. This is despite extensive economic development in the ve regions studied in recent years, including the construction of farm towns, usage of pesticides and fertilizers, agricultural irrigation, and automobile exhaust gases, representing the main sources of heavy metals.
According to the results of heavy metal carcinogenic risk assessment (Table 9), the average intake (ADD) of Cd in adults and children was lower than the reference exposure dose (RfD). Hence, the individual health risk index was less than 1, indicating the amounts of daily intake of cadmium would not be considered a health risk to humans. The order of rice intake in uencing the health risks to adults and and Xiangshui were higher than the USEPA recommendation. The maximum acceptable level was 1×10-4,and there is a risk of cancer; The risk index values of both Wuchang and Jiansanjiang were lower than the maximum acceptable level recommended by USEPA (1×10-4). The health risk of Fangzheng and Xiangshui was on the edge of the acceptable risk range for humans, and the risk of cancer was low.
There was a certain deviation between the Cd carcinogenic risk index values and the evaluation results of rice single factor pollution in the study area. In the single factor pollution assessment, the pollution index of the Fangzheng area was higher than that of Xiangshui, and the opposite was true in the cancer risk assessment, which is mainly related to the original level of heavy metals. Cadmium is a harmful element upon accumulation. It has obvious toxic effects on human nervous and reproductive systems, and is a heavy metal element with a high carcinogenic risk.

Discussion
According to the "National Soil Pollution Status Survey Bulletin" published by the Ministry of Environmental Protection of China and the Ministry of Land and Resources of China, out of the total soil area surveyed in the country (6.3 million km2) 16.1% (1.08 million km2) had pollution exceeding the acceptable standards. On May 28, 2016, the "Soil Pollution Prevention and Control Action Plan" issued by the State Council of China clearly requires the provision of soil environmental management for agricultural land based on soil pollution prevention, investigation and monitoring, classi cation management, supervision and management, etc., to prevent pollution, control agricultural use, and guarantee the quality and safety of agricultural products. This paper theoretically evaluated the heavy metal content in the geographically-protected products of the ve major rice-producing areas in Heilongjiang Province, and analyzed the health risks brought about by eating local rice, which has practical signi cance. According to the research results (Table 4), the average content of cadmium in rice in the study area was in the order of Chahayang> Xiangshui> Fangzheng> Jiansanjiang> Wuchang. The content of cadmium in rice in Chahayang area was the highest in the study area. The average cadmium content did not exceed the national food hygiene standard limit, and was much lower than the cadmium content in rice in Hunan Province [35] and Taihu Lake, Jiangsu Province [34]. Given that the rice produced in the study area is China National Geographic Protection Mark rice, the production area is located in the Songnen Plain and Sanjiang Plain in Northeast China, and is little affected by human activities (urban, mining and metallurgical activities). Therefore, the cadmium in rice in the study area was within the non-pollution standard (with the compliance rate of 100%). However, the difference between the content of cadmium across China and in the study area was not consistent. This is because the rice varieties planted in the study area were different, and the pesticides applied, irrigation water quality and rainfall were also inconsistent. The extent to which rice absorbs and accumulates cadmium from the soil would be expected to differ among various regions, in uencing the modeling of soil-rice systems in different regions. The modeling results were shown in Table 6. There were signi cant differences in the cadmium content in rice and soil in Wuchang, Chahayang and Xiangshui areas. The cadmium content in rice in Fangzheng and Jiansanjiang areas was greatly affected by other factors, and the model t was low. The better-tting model remains to be determined.
The average daily intake (ADD) of cadmium in adults and children in the study area was lower than the reference exposure dose (RfD). The cancer risk index in Wuchang and Jiansanjiang was within the maximum acceptable risk level recommended by USEPA. The carcinogenic risk index of both Fangzheng and Xiangshui was close to the maximum acceptable risk level recommended by USEPA, with the cancer risk still being low. However, the carcinogenic risk index in Chahayang was much higher than the maximum acceptable level recommended by USEPA, suggesting a high risk of causing cancer, which was similar to the results of cancer risk assessment in Zhejiang Province [37] and Zhejiang area [51]. According to the research results (Tables 5-7), the cadmium pollution existed in the Chahayang area, and the corresponding carcinogenic risk exceeded the acceptable level for humans, which should be paid attention to by the relevant departments.
Due to the international nature of the experimental parameters and the regional nature of the study area, as well as the differences in human quality status under different living conditions, the results of this study have certain limitations and one-sidedness.In addition, since the impact of rice varieties is not considered in the research process, in the subsequent research, the migration in the "soil-plant-human body" system should be established on the basis of comprehensive consideration of various current impact factors. Experimental studies on transformation, and bioavailability, with a view to providing data references for soil conservation and food security in agricultural land.

Conclusion
1 The average content of cadmium in brown rice in the study area was (in mg·kg-1) 0.003 (Wuchang), 0.016 (Chahayang), 0.006 (Fangzheng), 0.006 (Xiangshui), and 0.005 (Jiansanjiang). The cadmium content in brown rice in the ve regions did not exceed the cadmium content speci ed in the National Food Hygiene Standard of China. The difference in cadmium content between brown rice, polished rice and soil was inconsistent. The prediction models developed in this study, including the total cadmium content and pH of the soils, could signi cantly describe the transfer of cadmium in the soil-rice system of Chahayang, Wuchang and Xiangtian paddy elds, with R2 values ranging from 0.256 to 0.468.
2 The pollution index of the entire study area was less than 1.The comprehensive pollution index was 0.037<1, which belongs to the non-polluting category. The comprehensive pollution index of each region was in the order of Chahayang> Fangzheng> Xiangshui> Jiansanjiang> Wuchang, and ranged between 0.059 and 0.158. The average daily intake of cancer-causing cadmium (ADD) for adults and children was lower than the reference dose (RfD). The average risk of carcinogenic heavy metal Cd for adults and children's in Chahayang, Fangzheng and Xiangshui was higher than the maximum acceptable risk level of 1×10-4 recommended by USEPA, suggesting a real cancer risk.

Availability of data and materials
It is hereby declared that the source of this experimental material is reliable and the measurement data is accurate.

Supplementary Files
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