A Study On The Persistence Of A Combined Amendment (Limestone+Sepiolite) For Remedying Paddy Soil Polluted With Heavy Metals

In order to study the persistence of a combined amendment (LS, limestone+ sepiolite) remedying paddy soil polluted with heavy metals Pb, Cd, Cu and Zn, an in-situ experiment was conducted in paddy soil near a mining area, southern Hunan, in which LS was once applied with ratios of 0,2,4,8 g/kg, and rice was planted for four years consecutively in the year of 2012 (the first season),2013 (the second season),2014 (the third season) and 2015 (the fourth season), respectively. Meanwhile, to validate LS ability on repairing soil contaminated with heavy metals, the additional experiment was conducted when rice was planted in the fourth season, in which each soil plot was separated into two halves and the ratios of 0,1,2,4 g/kg of LS was applied, respectively. The main experiment results indicated that:(1) LS increased significantly soil pH values for all four seasons, and the magnitude of LS enhancing soil pH values was the first season> the second season> the third season> the fourth season. LS decreased obviously exchangeable contents of soil Pb, Cd and Zn for all four seasons, and the decreasing magnitude of exchangeable contents of soil heavy metals was 1.6%-97.7% for Pb,35.5%-80.3% for Cd, and 8.0%-90.5% for Zn, respectively, in the fourth season compared with the control group; however, it increased exchangeable contents of Cu to different degrees. Under the experimental condition, the effects of LS decreasing exchangeable contents of soil heavy metals in four seasons were Pb> Zn> Cd. LS decreased obviously TCLP extractable contents of soil Pb, Cd and Zn for all four seasons, but there was no significant decrease for Cu. The decreasing magnitude of TCLP extractable contents of soil heavy metals was 7.2%-44.6% for Pb, 19.4%-41.9% for Cd, and 7.2%-17.3% for Zn, respectively, in the fourth season. According to decreasing ranges of the TCLP extractable contents of soil heavy metals affected by LS in each season, the effects of LS decreasing TCLP extractable contents of soil heavy metals in all four seasons were Cd> Pb> Zn.(2) LS decreased contents of Pb and Cd in brown rice in the fouth season by 25.0%-74.8% and 15.2%-53.9%, respectively, and the reduction trend increased with increasing LS application. Cu and Zn contents in brown rice did not decrease effectively. The effect of LS reducing contents of Pb, Cd, Cu and Zn in brown rice was Pb> Cd> Cu> Zn for the first season and the second season, but Cd> Pb> Zn> Cu for the third season and Pb>Cd>Zn>Cu for the fourth season. For all four seasons, the total effect of LS reducing heavy metal contents in brown rice was Pb> Cd> Cu> Zn.(3) The effects of LS applying on the translocation factors of Pb and Cd were quite different among different rice organs, from root to straw, from straw to husk, from husk to brown rice, and from root to brown rice. The greatest translocation factors of Pb and Cd were from rice straw to husk, implying that husk was the main organ in rice accumulating heavy metals.(4) The soil pH with additional added LS was significantly higher than that of the soil pH which was not supplemented with LS, it is indicated that LS effectively increased the soil pH values. Compared the contents of Pb, Cd, Cu and Zn in brown rice between supplemented with LS and without additional LS, the contents of heavy metals in brown rice with additional LS in soil were lower than that without additional LS, this further explained that LS decreased heavy metal contents in brown rice effectively.(5) The effect of LS stabilizing soil Pb and Cd emerged gradually with time, and therefore, LS was suitable for remedying soil polluted with Pb and Cd for a relative long time because of its persistence.