Transfer of Nutrient and Harmful Elements from Soil to Rice and Health Risk Assessments for the Vietnamese Population
Thuy Phuong Nguyen
Doctoral thesis
Date of Examination:
2019-09-16
Date of issue:
2019-10-11
Advisor:
Ruppert, Hans Prof. Dr.
Referee:
Ruppert, Hans Prof. Dr.
Referee:
Dittert, Klaus Prof. Dr.
Persistent Address: http://hdl.handle.net/21.11130/00-1735-0000-0005-1270-8
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Abstract
English
“Transfer of nutrient and harmful elements from soil into rice and health risk assessments for the Vietnamese population” Main and trace element concentrations in paddy soils and corresponding rice plants collected along some transnational-river systems in Vietnam including Red River in the north and Mekong River in the south, and Huong River in the center were investigated to provide an insight into paddy soil characteristics and the element transfers into rice plant, and to assess chronic health risks by potentially harmful elements through rice consumption. The studied paddy soils have similar parent materials consisting of alluvial sediments deposited by inundation and irrigation. Most of the soils are not or only slightly affected by heavy metal(loid)s contamination from anthropogenic activities, except for Cd enrichment by the use of phosphate fertilizer. Elevated arsenic concentrations exceeding the allowable limit of agricultural soil (15 mg kg-1), are found mostly in the northern and the central paddy soils (80%), but at fewer soils in the south (11%). These high concentrations are the result of natural processes related to redox reactions of As-rich sulfide and Fe-oxides/hydroxide phases. Some specific sites close to the river bank and near a fertilizer and chemical factory show a strong enrichment of heavy metals, resulting from industrial wastewater application. Due to similar concentrations of elements in the parent material not polluted by human activities, many trace elements show extremely sharp correlations with each other, which can be arranged into groups. The correlations are mainly caused by variable concentrations of dilution by quartz, bio-opal and organic matter in the soil. These correlations can be used to assess if a soil sample is polluted by a certain element. Soil parameters play an important role in the transferability of elements from soil to rice plant causing a large spread of transfer factors. Although the background concentrations of elements in areas are hardly distinguishable, differences in soil pH-value, the content of organic matter, Fe- and Mn-oxides/hydroxides and clay minerals as well as the fertilizer input are the main reasons for contrasting element concentrations in the rice grains of the three research areas. Particularly, the translocation of the potentially harmful elements As, Cd, and Mn shows intermediate to high transfer factors. In general, concentrations of most elements in the plant parts decrease in the order: shoot >> husk > grain. Exceptions are Ni, Mg, Zn, S, Cu, Mo, and P, which are more easily transported to the grains due to their electrostatic repulsion at the negative cell wall charges or to their formation of soluble organic complexes. Health risks of harmful elements intake from rice consumption are estimated by applying four approaches: (1) Tolerable Upper Intake Level (UL) of total daily consumption – data available for As, Cd, Co, Mn, Mo, Ni, Pb, Sb, and U; (2) allowable Maximum Concentration (MC) of rice - data for As, Cd, and Pb; (3) non-cancer risks (HI) - data for As, Cd, Co, Cu, Mn, Mo, Ni, and Pb; and (4) cancer risk (∑ILCR) - data for As and Pb. The mentioned elements are the riskiest pollutants in rice for the Vietnamese population. For the UL-guidelines, 14% of the total studied samples cause health risks of exposure to As, 32% to Cd, and 21% to Pb. In comparison with the MC-values, 4%, 3%, and 19% of the rice samples exceed these levels of As, Cd, and Pb respectively. Results of non-cancer risks and cancer risks exhibit that all rice grains have the HI-values and ∑ILCR-values surpassing their safe and acceptable thresholds. Manganese occupies the highest portion of HI-index and As is the most potential oral carcinogenic factor. People in the three studied areas are facing the different levels of intoxication risk by these elements. People living in the Red River and Huong River area suffer from higher hazard of As and Cd, but not of Pb. People in the Mekong area are additionally exposed to Pb in rice, 10-times more than in the other areas. To mitigate these health risks, controlling the soil pH-value is a simple way that needs to be considered first.
Keywords: Transfer; Nutrient and harmful elements; Rice; Paddy soil; Health risk assessments; Vietnamese population
