Mathematical model for heavy metal phytotoxicity and accumulation in phytoremediation using Helianthus annuus L.

Abstract

Ecological rehabilitation of the contaminated soils in the industrial, agricultural, and urban territories is a great challenge in recent decades due to anthropogenic activities. Heavy metal contamination of the land surface and groundwater is a serious environmental problem that limits crop production and bioaccumulates in the food chain, threatening human health. Phytoremediation is an in situ technology employed by plants to remediate sites contaminated with toxic metals and hazardous organics in a cost effective and environmental friendly method. Since phytoremediation involves growing plants in a contaminated matrix, various types of plants have been evaluated. Among the various types of plants, sunflowers (Helianthus annuus L.) have been commonly used in numerous phytoremediation studies due to their high tolerance to heavy metals and their ability to use the seeds as raw materials for biodiesel production. However, no mathematical model has quantitatively described the relative effect of the concentrations of heavy metals (in the aqueous phase or in soils) on the sunflower seeds early growth. In addition, there is no simple mathematical model to estimate the heavy metal accumulation in sunflower from contaminated soils. The main objective of this study was to assess effectiveness of sunflowers in accumulating heavy metals (i.e., Cd, Ni, Pb, and Zn) from contaminated soils. The objectives of this study were as follows: (1) to investigate the effects of heavy metal phytotoxicty on sunflower seeds in the aqueous phase and soil using SVI model

(2) to estimate the heavy metal accumulation in sunflower from contaminated soils by introducing sigmoid model. In order to quantitatively describe the relative effect of the concentrations of heavy metals (i.e., Cd, Ni, Pb, and Zn) on the germination and seedling growth of plants, seed germination tests were conducted using sunflower seeds treated with various heavy metals, and a mathematical model to estimate the seedling vigor index (SVI) was proposed. From the results of germination tests, a decrease in the seed germination percentage with an increase in the heavy metal concentration was observed, and the inhibitions of seedling growth were clearly detectable above certain critical concentrations (e.g., 50 mg-Cd/l, 50 mg-Ni/l, 100 mg-Zn/l, and 1,000 mg-Pb/l). According to both the IC50 and a1 values, the resulting order of phytotoxicity of heavy metals on sunflower seed germination was Cd > Ni > Zn > Pb. The SVI estimation model developed in this study significantly explained the relationship between the heavy metal concentration and the SVI values. Thus, SVI values for a certain plant can be, a priori, obtained, with the heavy metal concentrations in the aqueous phase without performing the germination tests. Phytoremediation are mainly applied in soil rather than aqueous phase. Therefore, it is crucial to determine the heavy metal phytotoxicity in soils. Three cases were performed independently where sunflower seeds were tested in loam, sandy loam, and silt loam to determine SVI model. The effect of increasing concentrations of heavy metals (i.e., Cd, Ni, Pb, and Zn) on the germination and seedling growth of sunflower seeds was examined. Linear function well described the relationship between rates of seed germination, length of shoot and root with logCHM in soil. The logarithm transformation SVI model was proposed and shown to give acceptable estimation of heavy metal phytotoxicity on plants seedling growth in different soil texture. A mathematical approach was developed and investigated to describe the contaminant movement from soil to plants. Pot experiments were carried out with sunflowers and target heavy metals, i.e., Cd, Ni, Pb, and Zn. The results presented has showed the heavy metal concentrations in sunflower increased rapidly during the establishment stage and followed by no statistically significant difference (p > 0.05) observed on heavy metal concentrations in sunflower after 30th day. Statistical analysis revealed that heavy metal accumulation in plants is dependent on the biomass production but there was no significant difference between contaminated soils and control soils. Sigmoid function, a common model to describe the plant growth could fit the data of plant biomass well (R2 > 0.94). The masses of heavy metal uptake by sunflower were estimated on the basis of sigmoid function of biomass, the estimated results were in basic agreement with the experimental data in this study. Heavy metal phytotoxicity and performance of phytoremediation to remediate contaminated sites can be determined with SVI model and sigmoid model, respectively. Although the coefficients of SVI estimation model and sigmoid model vary from different species of plants, the empirical estimation models derived from theoretical background can be utilized for other species of plants.