| Generally speaking,there are few heavy matels cadmium for irrigation water into the soil except sewage irrigation.China’s irrigation water quality standards(GB5084-2005)provides that the total amount of irrigation water Cd should not be higher than 0.01 mg/L.The irrigation quota of rice field in southern China is 15000 m3/hm2,the average annual water is nearly 9000 m3/hm2,even if irrigation water is consistent with the standard,the total amount of cadmium that bring into the rice field by irrigation water is not to be underestimate.The contribution of Cd which concentration is low and total is big in irrigation water to Cd in soil is still not paid enough attention,there is little research on the purification model of cadmium pollution in irrigation water.In this study,cadmium in irrigation water,farmland soil and brown rice were investigated in a typical irrigation area in central Hunan.Candmiun formations,the content of organic matter in irrigation river sediment and the correlation between varous factors were analyzed.In this paper,an artificial wetland system is constructed and combined with the screening of adsorbent materials to try to establish an economical and efficient technology model for the purification of cadmium in irrigation water.The main research results are as follows:(1)The average concentration of Cd in sampling was 6.354 ug/L for the whole growth period of rice in irrigation river,and the average concentration of cadmium in Xiangjiang River was 4.006 ug/L.The average concentration of Cd in irrigation water for early rice was 0.447 ug/L,and the average concentration of Cd in irrigation water for late rice was 1.759 ug/L,Sampling results,the most concentration of Cd in irrigation water do not exceed standard.The average content of Cd in early rice and late rice paddy soil was 0.556 mg/kg and 0.694 mg/kg,respectively,exceeding 1.85 times and 2.31 times.Similarly,the average content of Cd in brown rice exceeded 2.82 times.The results of single factor pollution assessment showed that most of the irrigation water was not polluted by Cd,only 2.2%of the samples were mildly polluted.Most of the soil samples were polluted by Cd,mild,moderate and severe pollution accounted for 63.2%,18.4%,15.8%;brown rice was the most serious pollution,slight,moderate and severe pollution accounted for 16.7%,44.4%,36.2%.(2)13 samples of sediments were collected along the irrigated river,the average content of Cd was 1.351 mg/kg,and exchangeable,organic-bound and Fe-Mn oxides bound were the main form of Cd existence,accounting for 51.04%,28.18%and 18.95%which indicated that the activity of Cd in the sediment is high.The average carbon contents of humic acid,fulvic acid and humicin were 2.32,7.48 and 9.12 g/kg,respectively.The average contents of DOC,ammonia nitrogen and DON were 0.34,0.10 and 0.07 g/kg.Correlation analysis indicated that there was a significant positive correlation between the content of humic acid and the concentration of Cd in the water of early rice(p<0.05).There was a significant positive correlation between the water-soluble Cd content in the sediment and the Cd content in the late rice(P<0.05).There was a highly significant positive correlation between the exchangeable Cd and Cd in the soil(p<0.01).The organic-bound Cd was highly significantly positively correlated with the fulvic acid,DOC,DON(P<0.01),The Fe-Mn oxides bound was alse highly significantly positively correlated with fulvic acid(p<0.01).The migration and transformation of Cd in irrigation water were closely related to the composition of organic matter in sediments.(3)The modified corn straw biochar and peanut shell biochar have good adsorption properties to Cd in solution.The results indicated that under the conditions of pH 6.0,the inital concentrations of Cd2+ 100 mg/L,wastewater temperature 20℃,contact time 12 h,biochar dosage 1.0 g/L,the removal rates of cadmium by modified corn straw biochar and peanut shell biochar were 67.03%,46.10%,the removal rates were increased to 3.8,6.2 times as compared with unmodified biochars.The adsorption process of modified corn straw biochar and peanut shell biochar can be fitted by Langmuir and Freundlich models,with modeling of Langmuir equation,the theoretical maximun adsorbance of modified corn straw biochar is 68.97mg/g and the adsorption capacity of peanut shell biochar is 55.55mg/g.The adsorption process of two types of biochars could be well described using pseudo-second-order reaction kinetics model,which indicated that the adsorption was mainly chemical.The Cd2+ adsorbed on the two types of biochars can be easily desorbed by using NaOH solution,the two types of biochars still had 31.40 and 24.10 mg/g adsor:ption capacity for Cd2+ after recycling 4 times.(4)In the late rice growing season,the artificial wetland system was used to treat the irrigation water,and the average concentration of Cd in irrigation water was 4.157 ug/L.After three times of artificial wetland treatment,the concentration of Cd decreased by 79.01%on the three sampling periods in July,August and October.The average Cd concentration of effluent after treated with artificial wetland was 0.764 ug/L,which was much lower than that of the national irrigation water quality standard(GB5084-2005)Cd<10 ug/L,indicating that the constructed wetland system could intercept Cd in irrigation Water,effectively reducing the concentration of Cd in irrigation water.(5)The treatment of irrigation water with artificial wetland system for field plot test,the Cd content of brown rice in field which irrigation water was not treated was up to 0.621 mg/kg,the average Cd content of rice brown in field with purification of irrigation water was 0.503 mg/kg.The contents of Cd in brown rice were decreased by 23.76%,respectively.The Cd content of brown rice in field which irrigation water was treated by wetland system and modified corn straw biochar was down to 0.437 mg/kg which decreased by 29.66%.Although the content of Cd in the brown rice of the tested area was decreased,the Cd content of brown rice was still higher than that of the national food safety standard(GB2762-2012)Cd≤0.2 mg/kg,which may be related to the soil Cd background value,or other sources of Cd contamination. |
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