Heavy metals are regarded as toxic trace elements in the environment. Heavy metal pollution in soil or rice grains is of increasing concern. In this study, 101 pairs of soil and rice samples were collected from the major rice-producing areas along the Yangtze River in China. The soil properties and heavy metal (i.e., Cd, Hg, Pb and Cr) concentrations in the soil and rice grains were analyzed to evaluate the heavy metal accumulation characteristics of the soil-rice systems. The results showed that the Cd, Hg, Pb and Cr concentrations in the soil ranged from 0.10 to 4.64, 0.01 to 1.46, 7.64 to 127.56, and 13.52 to 231.02 mg·kg−1, respectively. Approximately 37%, 16%, 60% and 70% of the rice grain samples were polluted by Cd, Hg, Pb, and Cr, respectively. The degree of heavy metal contamination in the soil-rice systems exhibited a regional variation. The interactions among the heavy metal elements may also influence the migration and accumulation of heavy metals in soil or paddy rice. The accumulation of heavy metals in soil and rice grains is related to a certain extent to the pH and soil organic matter (SOM). This study provides useful information regarding heavy metal accumulation in soil to support the safe production of rice in China. The findings from this study also provide a robust scientific basis for risk assessments regarding ecological protection and food safety.
This paper proposed an improved sensorless control strategy for ship electric propulsion which bases on I/f control and back electromotive force integral (EMF), suppressing the speed fluctuation during algorithm switching, improving the flux estimation accuracy of internal permanent magnet synchronous motors (IPMSMs) and achieving the stable operation of the motor. Firstly, this strategy was adopted for controlling the stator current during the switching process to improve speed fluctuation. Then, a simple flux estimation method is presented by rewriting the mathematical model of IPMSM. To achieve the result which overcomes the problem of flux estimation and current measurement errors and improved LPF was developed which solved the phase delay and DC-offset problem of flux estimation. Finally, the propeller load platform is under construction to verify the effectiveness of the control strategy. Experimental results show that the system based on this method works smoothly and it has better speed tracking performance.INDEX TERMS I/f control, IPMSM, switching strategy, back EMF, sensorless control algorithm.
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