| Cadmium (Cd) is a heavy metal with strong toxicity, and it’s easy to enter the bodythrough the food chain so that have impacts on health. Cd is one of the global pollutants andCd pollution in soil has become a hot issue of scholars at home and abroad. Chengdu Plainhas high population density, fast industrialization and urbanization, high intensive utilizationintensity of farmland. The typical sampling, field testing and laboratory analysis were used.The spatial and temporal distribution of Cd content and causes of pollution in soil, migrationand accumulation characteristics of Cd in Farmland soil-rice system, and pollution preventioncontrol technology were studied in this paper. The main results were as follows:1. On the basis of the analysis to spatial and temporal distribution characteristics of farmlandsoil Cd in Chengdu Plain economic area, pollution evaluation of Cd and it’s the influencefactors of spatial distribution were carried out in a typical industrial peri-urban region(Jingyang district, Deyang city). The effect of Cd adsorption buffer characteristics toexogenous Cd contamination and the main causes of pollution were cleared.(1) The content of soil Cd was in a mild to moderate pollution level of Chengdu Plaineconomic area, while the spatial variation was large and the time cumulative effect is apparent.The contents of Cd were0.27mg·kg-1and0.38mg·kg-1in2002and2012, respectively, whichwere2.25times and3.17times separately of geochemical background values in this area.(2) The content and variation coefficient were relatively high in Jingyang district,Deyang city. The geo-accumulation index(Igeo) and potential ecological risk(Er),which werestrongly influenced by geological background(Cd content of “old wall soil†in1960s was0.29mg·kg-1) and human activities, were in a medium range and mid to heavy range,respectively. Tianyuan, Bajiaojing, Bolong, Huangxu and Xiaoquan were the seriouspollution regions, and both of them were industrial development zone in Jingyang district.The content of farmland soil (vegetable, paddy field) was comparatively high andsignificantly higher than local soil background value (Cd content of “old wall soil†in1960swas0.29mg·kg-1). The content of soil developed by Holocene series (Grey alluvium)Sediment system (Chengdu clay) was higher. The Cd content in paddy field was higher thanthat in dry land, and it was the highest (0.49mg·kg-1) in gray brown alluvial field. The resultsof main control factors showed that the pollution contribution size order to farmland soil Cd was: distance to River (Shiting River)> distance to Highway (DE A Highway)> Soiltypes>distance to Chemical plant> Parent material. The notable affection horizontal distanceof river, highway and chemical plant were200m,400m,100m.(3) The representive soil in study area (gray brown alluvial field) had a large adsorptionbuffer capacity to exogenous Cd, but it also existe some difference by the concentration ofCd2+400mg·L-1of exogenous Cd was the critical concentration for the significant changes ofadsorption-desorption characteristics with the static adsorption and desorption conditions.With the dynamic adsorption equilibrium condition, the time of soil column adsorptionreaching saturation was long under the treatment of100mg·L-1. The Cd adsorption amountincreased rapidly and the adsorption reached saturation when the soil column leaching wasabout60penetration volumes (V/V0) under the treatment of800mg·L-1Cd. It suggested thatthe soil had an obvious absorption and cushioning effect under the light-mid pollution level ofexogenous Cd, while the effect would weaken and easily achieve balance to produce Cdpollution under the heavy polluting. The capacity of soil column adsorption buffer exogenousCd2+had a close relationship with the pH of leachate. The pH of leachate presented thecharacteristic that increased at first but declined rapidly in a short-term (0-15d). Thedisassociation of Ca2+enhanced with the absorption amount of soil column increasing.2. The Cd absorption accumulation and regional distribution characteristics of rice wereresearched. The relationship between absorption accumulation and the changes of Cd formswas analysed.(1) The root of rice was the main part to accumulate soil Cd and the average contentwas0.50mg·kg-1. The average amount in brown rice was0.15mg·kg-1, which was lower thanthe maximum levels of contaminants in foods (GB2762-1012). In the Chengdu plaineconomic area, the Cd content of brown rice in Jingyang district (0.25mg·kg-1) and Shuangliucounty (0.19mg·kg-1) exceeded or close to the maximum levels of contaminants in foods. Thelowest content of brown rice was Dujiangyan city (0.09mg·kg-1) and Qionglai city(0.08mg·kg-1). The range and variation coefficient of each part were all larger(38.77%~75.56%), and aboveground parts (straw, husk, brown rice) were the most obvious.The absorption and transfer efficiency of Cd in rice is relatively high, and the enrichmentorder to Cd of root, straw and brown rice was: Root>Straw>Brown rice.(2) Chengdu plain economic region showed high content of Cd in farmland, and also inbrown rice. But it also had contrary phenomenon, such as Guanghan. The content in soil washigh (0.50mg·kg-1) while rice in this area was low (0.11mg·kg-1). There was a significantpositive correlation between Cd in crown rice and the content of exchangeable and carbonatebound in soil, yet a remarkable negative correlation with organic combination bound. The enrichment coefficient that soil Cd transported to grain was strongly influenced by thespeciation changes of Cd in soil, but the absorption-accumulation coefficient of Cd in thestraw to soil Cd was no significant correlation with the speciation changes of Cd in soil. Thechange of soil Cd patterns was mainly affected by soil pH, organic matter and content ofmid-small powder particles.3.Farmland soil, typical rice Cd pollution region were chose to study the soilorganic-inorganic modifiers combined application technology and screen the exclusion ricevarieties to Cd, then constructed a pollution control technology for farmland soil-rice systemin Chengdu Plain.(1) Manure and straw returning combined with inorganic amendment (sepiolite, limeand calcium magnesium phosphate fertilizer) could significantly decrease exchangeable statecontent of soil Cd at the early growing period of rice (after transplantting30d, tillering stage)and late growth stage(after transplantting90d, mature stage), and increase the Fe-Mn oxidebound and organic combination bound content, especially the organic combination bound, anddecline the accumulation in brown rice at the early growing period of rice (after transplantting30d, tillering stage).Grain yield, biological effectiveness of soil Cd and absorption and accumulationefficiency all taken into consideration, straw returning with rice-rape rotation and manureapplication combined inorganic amendment such as sepiolite, lime with rice-wheat rotationare the agronomic regulation measures technologies that are worth to popularize in ChengduPlain.(2) The89soil-rice samples, which contained17rice varieties that was researchedunder the condition of mid-heavy pollution farmland, the results indicated that the content inbrown rice of K you817, Xie you527and Zhong you838exceed the standard seriously, allover0.3mg·kg-1. Ai you82, Jin you527, Gang you22, Fu you130, Ⅱyou906, Yixiangyou1577and Jin you725were the low absorption and accumulation efficiency(enrichmentcoefficient and transfer coefficient) breeds. Comprehensive analyzing the grain yield level,Ⅱyou906was the most spread rice variety with low enrichment and migration characteristics.The content in brown rice of Fu you130was low, but the enrichment factor (EFs=0.65) andtransfer factor (TF=0.46) were relatively high, therefore soil Cd pollution problem should beattached importance in the process of straw and root rootstalk returning. |
