Effects of calcinated dolomite on the amendment of acid soil and release kinetics of Ca-Mg

Abstract: Widespread soil acidification in China has posed a series of severe problems on agriculture ecosystems. In previous control measures, the use of lime can lead to soil hardening, whereas, industrial alkaline wastes inevitably face the risk of secondary pollution. Recently, cheap dolomite can be expected to serve as a soil functional conditioner, particularly for providing adequate amounts of calcium and magnesium nutrients for crops. This paper aims to investigate the effects of particle size and calcination time on dolomite alkalinity, microstructure, and release kinetic characteristics of calcium and magnesium during a low temperature calcination process. Two concepts were proposed, the capacity of alkalinity and short-term cumulative alkalinity, in order to evaluate the acid treatment of calcined dolomite. The results showed that after the calcination of dolomite with a particle size less than 3mm, the capacity of alkalinity increased by 78%, and the short-term cumulative alkalinity increased by 1 100 times, whereas the calcination time was extended from 1 hour to 3 hours without significantly increasing the total alkalinity. After calcining dolomite with a particle size of 0.15 to 0.25 mm, the release rate of calcium and magnesium reached the maximum after 3-hour calcination. However, the calcined dolomite with a particle size of 1 to 3 mm indicated a stable release rate of calcium and magnesium after the calcination for 1 to 3 hours, and the maximum release rate occurred after the calcination for 2 h. After 26 days incubation, the cumulative release rate of calcium in the calcined dolomite was over 87%, and the cumulative release rate of magnesium was less than 9%. Release curves of calcium and magnesium from calcined dolomite can be fitted using parabolic, first order kinetics, Richards and Bertallanffy equations. The modified Bertallanffy equation can be the most suitable to describe the release change of CaO amount from calcined dolomite over time, where r = 0.990-0.999, SE=0.47-1.53. Richards and the modified Bertallanffy equations can be used to quantitatively describe the MgO release pattern of dolomite, or predict the amount of magnesium supplied by dolomite. The goodness of fit test of the Richards equation was r=0.959-0.985, SE=0.14-0.20, while that of the modified Bertallanffy equation was r=0.980-0.993, SE=0.13-0.19. It infers that the calcined dolomite can serve as high capacity and intensity of alkaline, total and short-term cumulative alkalinity, while the calcination process can promote the release or supply of calcium and magnesium, where the release rate can be predicted by using Richards and modified Bertallanffy equations. In the calcinated dolomite with a large particle size of 1 to 3 mm, 1 hour can be an economical calcination time to increase its capacity of alkalinity, whereas the optimum is 2 hours calcination process to promote rapid cracking, short-term cumulative alkalinity, effective calcium and magnesium. The research results can provide a theoretical basis for calcinated dolomite translating to serve as soil conditioner and its manufacturing process.