二乙三胺五乙酸-三乙醇胺-硝酸钙体系浸取土壤中8种重金属有效态

采用土壤中重金属有效态指标来评价耕地土壤污染状况对反映土壤真实污染状况有重要意义,而在对土壤中重金属有效态进行测定时需先对其进行浸取。在采用二乙三胺五乙酸(DTPA)-三乙醇胺(TEA)-CaCl₂复合浸取剂对土壤中重金属有效态元素浸取时发现有效态Zn的空白较高,对有效态Zn的测定有一定的影响。按DTPA-TEA-CaCl₂复合浸取剂中各组分浓度配制成相应的DTPA、TEA和CaCl₂单组分水溶液,并对其中有效态Zn空白进行测定,结果发现DTPA-TEA-CaCl₂复合浸取剂中有效态Zn空白的93%以上来源于CaCl₂。据此,实验提出采用Ca(NO₃)₂替代DTPA-TEA-CaCl₂体系中的CaCl₂来对土壤中重金属有效态元素进行浸取,即提出了一种浸取土壤中重金属有效态元素的新体系DTPA-TEA-Ca(NO₃)₂,并对其浸取的条件进行了优化。结果表明,采用DTPA-TEA-Ca(NO₃)₂体系浸取的最佳条件为:控制0.005mol/L DTPA-0.1mol/L TEA-0.01mol/L Ca(NO₃)₂溶液的pH值为7.2,土液比(土壤质量(g)与浸取剂体积(mL)之比)为1∶4,于20℃条件下在往复振荡器上以180r/min的速率振荡浸取2h时效果最佳。分别采用DTPA-TEA-Ca(NO₃)₂和DTPA-TEA-CaCl₂两种体系对土壤标准样品中Cu、Pb、Zn、Ni、Cd、Cr、As和Hg共8种重金属有效态进行浸取并对测定后的结果进行对比,结果表明,采用两种方法浸取后,各元素有效态的测定值保持一致,且与认定值相符,这表明采用这两种体系对这8种重金属有效态元素浸取的效果是一样的。试验进一步发现,采用DTPA-TEA-Ca(NO₃)₂体系浸取时,Zn有效态的测定值与其认定值更加接近,且其空白平均值也较DTPA-TEA-CaCl₂体系低约81.4%,这进一步验证了浸取时采用Ca(NO₃)₂替代DTPA-TEA-CaCl₂体系中的CaCl₂可有效降低Zn有效态空白对测试结果的干扰。

Extraction of eight available-state heavy metals in soil withdiethyltriamine pentaacetic acid-triethanolamine-calcium nitrate system

It is of great significance to evaluate the soil pollution status of cultivated land using the indicators of available-state heavy metals to reflect the real pollution status of the soil. The heavy metals in soil should be leached before the determination of available state.When diethyltriamine pentaacetic acid (DTPA)-triethanolamine (TEA)-calcium chloride (CaCl₂) composite leaching system was used to extract the available-state heavy metals in the soil, it was found that the blank value of available-state Zn was relatively high, which had a certain influence on the determination of Zn. According to the concentration of each component in DTPA-TEA-CaCl₂ composite leaching system, the corresponding solution of single component (i.e., DTPA, TEA and CaCl₂) was prepared for the determination of blank value of available-state Zn. It was found that more than 93% of blank value of available-state Zn was from CaCl₂ in DTPA-TEA-CaCl₂ composite leaching reagent. Therefore, it proposed to use Ca(NO₃)₂ to replace CaCl₂ in DTPA-TEA-CaCl₂ system for the extraction of available-state heavy metals. In other words, a new DTPA-TEA-Ca(NO₃)₂ system for the leaching of available-state heavy metals in soil was developed. Moreover, the leaching conditions were optimized. The results showed that the optimal conditions for DTPA-TEA-Ca(NO₃)₂ system were listed as follows: the pH of 0.005mol/L DTPA-0.1mol/L TEA-0.01mol/L Ca(NO₃)₂ solution was 7.2; the ratio of soil (mass of soil, g) to liquid (volume of leaching agent, mL) was 1∶4; the sample was leached on reciprocating oscillator with rate of 180 r/min at 20℃ for 2h. Eight available-state heavy metals in standard soil sample, including Cu, Pb, Zn, Ni, Cd, Cr, As and Hg, were leached with DTPA-TEA-Ca(NO₃)₂ system and DTPA-TEA-CaCl₂ system, respectively. The measured results were compared. It indicated that the measurement results of available states of elements were consistent with the certified values for both methods. In other words, the leaching effect of eight available-state heavy metals by two systems was same. It was further found that the determination result of available state of Zn was more close to the certified value in DTPA-TEA-Ca(NO₃)₂ system. Moreover, the average value of blank was about 81.4% lower than that in DTPA-TEA-CaCl₂ system, which further proved that the replacement of CaCl₂ in DTPA-TEA-CaCl₂ system by Ca(NO₃)₂ could effectively reduce the interference of blank value of Zn with the determination results.