Minimizing Arsenic Level in Rice Using Biochar in the Vicinity of the Wolfram Mine, North Vietnam

Abstract

Accumulation of heavy metals in agricultural soil can prove toxic to proximal environments. Especially, arsenic (As) is a unique element with distinct physical characteristics and toxicity, and has been classified as a human carcinogen. Arsenic enters agricultural soil and the environment via geogenic or anthropogenic sources under different forms. Although agricultural soil quality, exposure pathways, and human health impacts have been studied in some countries such as Korea, Portugal, Bangladesh, and China, few studies have investigated the soil quality assessment, human health impact, and simple solution to minimize the As level in rice in the Wolfram mine in North Vietnam. To examine the heavy metal contamination in the ecosystem of this mining area, we analyzed six trace elements (As, Cd, Cr, Cu, Pb, and Zn) in the collected paddy soil samples. The analytical results showed that all the soil samples were contaminated by Cd and As. Most of the soil samples were contaminated by As (mean value 50.93 ± 55.44 mg/kg) and Cd (mean value 15.22 ± 9.51 mg/kg), which results of up to 16 and 23 times higher, respectively, compared with the Vietnamese soil quality standard for agriculture (QCVN 03-MT:2015/BTNMT) of 15 mg/kg for As and 1.5 mg/kg for Cd. Contamination factor (CF), enrichment factor (EF), geo-accumulation index (Igeo), principal component analysis (PCA), and hierarchical clustering analysis (HCA) were used to identify the contamination source. The CF, EF, pollution index (PI), and Igeo indicated that this area was extremely polluted by Cd, severely to moderately–heavily polluted by As, and slightly to moderately polluted by other elements such as Cr, Cu, Pb, and Zn. The PCA and HCA results also attribute the source of As, Pb, and Zn contamination and enrichment of Cd, Cr, and Cu in the study area may be from the combination of anthropogenic and geogenic sources. The PI and contamination degree (Cd) values of soil quality indicated that the study area was contaminated with particular reference to Cd and As, and the level of contamination was decreased in the order of Pb > Cr > Cu > Zn. The study area had high potential ecological risk, and the carcinogenic risk value was higher than the acceptable value (1 x 10-6 to 1 x 10-4). The local residents’ health can be potentially affected via dermal adsorption pathway or food consumption when the rice plant grown in the paddy field is the abundant crop in the study area. To investigate the potential metal accumulation and health risk from their daily food consumption, the concentrations of As, Mn, Cu, Zn, Cd, and Pb in the soils and 12 crop species near the Wolfram mine were determined. The results showed that the concentrations of Cd and As in collected samples were higher than the allowable limits (MAL) for agricultural soils in Vietnam (QCVN 03-MT:2015/BTNMT). Kohlrabi, cabbage, vegetable shrinkage, green tea, cassava roots, and ginger accumulated high concentrations (fresh weight) of As and heavy metals. The total hazard index (HI) was 19.59, indicating high non-carcinogenic risk. The incremental lifetime cancer risk (ILCR) was 1.18×10–3, indicating high carcinogenic risk via crop consumption. Rice, green tea, cabbage, mustard green, kohlrabi, and vegetable shrinkage contributed to approximately 90.4% of HI and 96.1% of ILCR, in relation to both non-carcinogenic and carcinogenic risks. We can suggest that these crops should not be cultivated in the study area. Based on the results of soil quality assessment and the potential of health risk, finding a solution that is simple and easy to apply is essential. The present study used different biochar with levels such as 0% (as control), rice husk biochar (RHB) 0.09% & 0.13%, the mixture of sugarcane waste biochar (SCWB) and rice husk biochar (0.1% and 0.05%, respectively), and 0.07% dry husk (DH) to reduce As accumulation in rice plants (Oryza sativa L.) in the field near the Wolfram mine area. The biochar was produced in a funnel to reduce oxygen and under uncontrolled temperature, and those raw materials were bought in the local area. Our results show that the concentrations of As in grain rice were decreased by 0.45% and 9.6% for 0.09% RHB and 0.13% RHB, respectively, 23.1% for the mixture of SCWB and RHB, 32.6% for DH, and the control increased by 31.7% before and after Si-amendments. The results indicated that increasing plant-availability of Si through soil amendment of Si-rich rice residues can decrease As concentration in rice grain from the competition between As (III) and silicic acid transport system in rice. From the FTIR results, the strong competition for metals removal by RHB and DH revealed a competition not only from the ionized phenolic-OH groups but also from the precipitation of metals with CO32- and PO43-. while the removal of metals by SCWB was seen only from hydroxyl-OH groups and CO32-. At the root surface, the phosphorus transport system has a high affinity for phosphorus over As (V). However, phosphorus concentration in plant influences the As transport from roots to shoots. The mycorrhizal association may reduce As uptake via a physiological shift to the mycorrhizal uptake pathway, which has a greater affinity for phosphorus over As(V) than the root epidermal uptake pathway. Exchangeable As content in the soils decreased significantly by the presence of CO32-. It can be useful to identify which vegetable is not suitable for cultivation and to provide the simple solution to minimize As concentration in rice grain for the reduction of the potential health risk.