土壤和灰尘中铜锌铅生物可接受性的体外消化方法研究
摘要
为了快速而可靠地对土壤/灰尘中重金属的健康风险进行评价,模拟人体消化过程的体外消化实验在过去的二十多年里得到了快速的发展,并被广泛地应用于土壤/灰尘中重金属生物可接受性的测定。土壤/灰尘中重金属的生物可接受性指的是土壤/灰尘中重金属在人体模拟消化液中的溶解度。至今为止,虽然提出和发展了许多体外消化方法,但仍没有建立一种通用的体外消化方法。不同设计的体外消化方法在消化物质组成和操作上都存在较大的差异,这不仅使得不同体外消化方法的结果缺乏可比性,也造成了对体外消化方法选择和使用的随意性。体外消化方法研究的最终目的是完成体外消化方法的统一化、标准化,并使之能在实际应用中被广泛地接受和推广。为了能最终实现这一目标,在目前的研究阶段需要完成的工作主要有：1)明确体外消化方法组成(如肠相消化、消化酶)的作用、意义及其对结果的影响,为体外消化方法的选择和使用提供理论参考和指导；2)研究重金属体外消化方法和其它污染评价方法的差异及联系,丰富重金属生物可接受性的内涵和意义；3)发展和推广体外消化方法,扩大体外消化方法在实际应用中的影响和范围。
本研究选择了三种较为常用但设计不同的体外消化方法：SBET法(Simple Bioaccessibility Extraction Test)、PBET法(Physiologically Based Extraction Test)和SGET法(Simple Gastrointestinal Extraction Test)。SBET法是一个仅由胃相消化组成,且不添加胃蛋白酶的体外消化方法。PBET法是一个由胃肠液消化组成,且添加了所有消化酶(胃蛋白酶、胆汁盐和胰液素)的体外消化方法。SGET法则是一个由胃肠相消化组成,但仅添加了胃蛋白酶,没有添加肠酶(胆汁盐和胰液素)的体外消化方法。在研究中选取了3种性质差异较大的土壤,比较了三种方法对土壤Cu、Zn和Pb提取能力的差异,探讨体外消化方法的不同设计,即有无肠相消化、是否添加消化酶(胃酶：胃蛋白酶；肠酶：胆汁盐和胰液素)等对结果的影响,对肠相消化和消化酶在体外消化方法中的意义和作用进行了讨论,明确了土壤中Cu、Zn和Pb适用的体外消化方法类型。在此基础上,选择PBET法作为同时测定土壤中Cu、Zn和Pb生物可接受性的方法,比较了PBET法与六种常用单一提取剂法(0.1M HNO3、0.4M HOAc、0.1M NaNO3、0.01M CaCl2.0.05M EDTA和0.5M DTPA)对土壤中Cu、Zn和Pb的释放效果,探讨了土壤重金属生物可接受性和植物有效性整合的可能性。考虑到PBET法胃液组成成分在提取土壤重金属功能上的相似,对PBET法胃液中成分(柠檬酸钠、苹果酸钠、醋酸和乳酸)在提取土壤Cu、Zn和Pb中的作用进行了分析讨论,并对简化PBET法消化液组成的可能性进行了验证。最后,应用PBET法对杭州城市土壤和街道灰尘中Cu、Zn和Pb的生物可接受性及对人体的健康风险进行了评价。本研究主要得到以下结果：
(1)三种体外消化方法(SBET、PBET和SGET法)对土壤重金属的提取能力有显著差异,仅由胃相消化组成的SBET法对土壤Zn和Pb的提取效率最高,且在PBET法和SGET法中,土壤Zn和Pb在胃肠相中提取量远低于其在相应胃相中的提取量。研究表明,消化液pH值是影响土壤中Zn和Pb生物可接受性的主要因素,当土壤经过酸性胃相消化进入中性肠相后,在胃液中已经溶解的Zn和Pb会因pH的升高而发生沉淀,降低溶解度,所以土壤中Zn和Pb的胃相消化结果代表了其在消化道中的最大溶解度。但由于消化酶的作用,在PBET法和SGET法中,从胃液进入肠液后,土壤Cu的溶解度不但不一定降低,甚至有可能升高,所以土壤Cu的胃相消化结果不能代表其在消化道中的最大溶解度。由此可知,出于保护和预防的目的,在土壤重金属生物可接受性评价中,可以使用仅由胃相消化组成的体外消化方法(如SBET法)对土壤中Zn和Pb的生物可接受性进行测定,但对土壤Cu,则应选择由胃肠相消化组成,且添加了消化酶的体外消化方法(如PBET法)进行测定。
(2)为了揭示消化酶(胃蛋白酶、胆汁盐和胰液素)在体外消化方法释放土壤重金属中的作用,分别比较了三种体外消化方法(SBET、PBET和SGET法)中添加消化酶和不添加消化酶时,土壤重金属提取量的差异。结果表明,对于重金属的提取量而言,在三种方法中,与没有添加消化酶的结果相比,消化液中添加消化酶后,土壤重金属的提取量增加、降低或不变。这主要与消化酶与金属离子的反应有关：消化酶与金属离子的结合可以促进和保持土壤重金属的溶解,但另一方面,消化酶与金属离子的结合可能会促使消化酶发生沉淀,从而使与之结合的金属离子也变得不溶,所以消化酶对土壤重金属在消化液中溶解性的影响主要取决于消化酶提取的土壤重金属与随消化酶沉淀的重金属之间的差值。除此之外,消化酶在消化液中的作用因方法的不同而有所差异：PBET法中添加消化酶(胃蛋白酶、胆汁盐和胰液素)能保持或提高土壤Cu在胃肠液中的溶解度；SGET法中同时添加了胃酶和肠酶时,其结果更能体现土壤和元素类型、性质的差异。由于消化酶对土壤重金属提取量的影响主要表现在肠液中,所以消化酶添加在由胃肠液组成的体外消化法(如PBET和SGET法)中是必不可少的,但对于仅有胃相消化组成的体外消化方法(SBET法),胃蛋白酶添加并不是必须的。
(3)通过与六种常用单一提取剂法(0.1M HNO3、0.4M HOAc、0.1M NaNO3、0.01M CaCl2、0.05M EDTA和0.5M DTPA)结果的比较发现,PBET法对土壤Cu、Zn和Pb的提取能力较强,其对土壤重金属的提取效率明显高于中性盐提取,且与稀酸和络合剂提取相近。由相关分析可知,PBET法胃相和胃肠相对土壤Cu和Pb的提取量与土壤粘粒呈显著负相关,与土壤全P、全K、全Mg、全Ca及相应的重金属全量呈显著正相关；PBET法胃相和胃肠相对土壤Zn的提取量则与土壤金Fe、全Zn呈显著正相关。多元逐步线性回归分析表明,土壤中Cu、Zn和Pb的全量很好地解释了PBET法胃液和胃肠液中土壤Cu、Zn和Pb的溶解性,且土壤重金属在PBET法胃肠液中的溶解度在很大程度上取决于土壤重金属在胃液中的提取量；但和胃液消化结果相比,PBET法胃肠液的消化结果更能体现土壤和元素类型、性质的差异：生物可接受态土壤Cu、Zn和Pb在PBET法胃相消化中的变异系数分别是70.4%、80.1%和119.9%,但肠相消化结束后它们的变异系数分别增加到了71.7%、91.8%和128.7%。在六种常用的单一提取剂中,EDTA的提取能力和PBET法相似,而且其提取结果和PBET法一样,都能被土壤中重金属全量很好地解释,因此,在某种程度上来说,PBET法和EDTA对土壤Cu、Zn和Pb的提取结果同时代表了土壤Cu, Zn和Pb的植物有效性和生物可接受性。
(4)对PBET法胃液中成分(1.25g胃蛋白酶、0.50g柠檬酸钠、0.50g苹果酸钠、500μl醋酸和420μl乳酸)在提取土壤重金属中的作用进行了研究,以PBET去原始胃液为对照,比较了在胃液pH值一定的条件下,消化液中仅分别含有1.25g胃蛋白酶、0.50g柠檬酸钠、0.50g苹果酸钠、500μl醋酸和420μl乳酸时,土壤重金属提取量的变化情况。结果表明,PBET法原始消化液的提取能力最强,其次是仅含有柠檬酸钠的消化液。对PBET法原始消化液的提取过程进行分析：溶液中金属离子优先与分子量较小的有机酸,如乳酸或醋酸,结合形成复合物,当消化液中游离的小分子有机配合体大量减少时,柠檬酸才能与金属离子结合形成复合物。因此,柠檬酸钠有可能通过逐步多级电离的方式模拟PBET法原始消化液中有机酸(苹果酸、醋酸、乳酸)对土壤中重金属的提取作用。对PBET法消化液进行简化,即胃液中仅含1.25g胃蛋白酶和1.0g柠檬酸钠,发现PBET法简化消化液的结果并不能完全代替PBET法原始消化液的提取结果,因为：对某一具体的土壤而言,简化消化液的提取量与原始消化液的提取量之间存在一定的差异。如果使用PBET法简化消化液对某一具体的土壤中重金属的健康风险进行评估,其结果可能存在较大偏差；而且,土壤Cu、Zn和Pb在PBET法原始消化液中的结果更能体现土壤间性质的差异。但回归分析表明,土壤中Cu、Zn和Pb在PBET法简化消化液中提取量与他们在PBET法原始消化液中提取量的相关系数分别为0.968、0.977和0.977,呈显著线性正相关。所以,PBET法简化消化液对土壤Cu、Zn和Pb的提取结果可用于推测土壤Cu、Zn和Pb在PBET法原始消化液中的提取结果。
(5)根据中国土壤环境质量二级标准,杭州城市土壤未被Cu污染,但Zn和Pb有不同程度的污染。杭州市街道灰尘中Cu、Zn和Pb的含量平均为221.67、478.70和105.31mg/kg,与文献中报道的我国其他城市的街道灰尘中Cu、Zn和Pb的含量相比,杭州市街道灰尘中除Cu含量略有偏高以外,Zn和Pb的含量属于一般水平。应用PBET法对杭州城市土壤和街道灰尘中Cu、Zn和Pb的生物可接受性进行了测定,并分别根据重金属全量和PBET法对重金属的提取量计算杭州城市土壤和街道灰尘的人体每日可摄入量限值,以儿童每人每日平均摄入0.208g土壤／灰尘为参照,对杭州城市土壤和街道灰尘中重金属的人体健康风险进行了评估。结果表明：1)杭州城市土壤和街道灰尘中重金属对人体健康风险的大小顺序为Pb>Cu>Zn;2)根据PBET法对土壤中Cu、Zn和Pb的提取量,杭州城市土壤最低的人体每日摄入量限值分别为23.1g(延安路)、84.5g(杭钢)和3.01g(武林广场)；3)根据PBET法对灰尘中Cu、Zn和Pb的提取量,人体每日灰尘摄入量限值最小的分别为25.64g(凤起路)、24.29g(火车站)和3.39g(武林广场)。因此可知,以PBET法对土壤和灰尘中Cu、Zn和Pb的提取量为依据,杭州城市土壤和街道灰尘中Cu、Zn和Pb对人体健康无显著影响。
To assess the health risk of heavy metals in soils/dusts quickly and reliably, in-vitro digestion test, which has progressed rapidly in the past two decades, is widely used to determinate the bioaccessibility of heavy metals in soils/dusts. The bioaccessibility of heavy metals in soils/dusts refers to the solubility of heavy metals in simulated human digestive juice. Although various in-vitro digestion tests have been developed, no universally accepted method is established. Differently designed in-vitro digestion tests have apparently differences in compositions and procedures, which has caused the lack of comparability between results of different in-vitro digestion tests and the subjectivity in selection and application of in-vitro digestion tests. The aim of in-vitro digestion test research is to establish a unified, standardized, well accepted and widely used in-vitro digestion test. Before achieve this goal, several issues must be confirmed. They are:1) the effects of intestinal digestion and digestive enzymes addition on the results of in-vitro digestion test;2) the differences and connections of in-vitro digestion test and other methods for assessing soil heavy metal pollution;3) the application meaning of in-vitro digestion test in evaluating the health risk of soil heavy metals.
Three differently designed in-vitro digestion tests:SBET (Simple Bioaccessibility Extraction Test), PBET (Physiologically Based Extraction Test) and SGET (Simple Gastrointestinal Extraction Test), were chosen in this study. SBET is a method only with gastric digestion but without pepsin addition. PBET is a method including gastric and intestinal digestion with corresponding digestive enzymes. SGET is composed of gastric digestion with pepsin and intestinal digestion without bile salts and pancreatin. In this study, the differences of extraction ability between these three methods were compared, the suitable types of in-vitro digestion tests for the assessment of soil Cu, Zn and Pb was revealed, the role of digestive enzymes (pepsin, bile salts and pancreatin) in simulated digestive juices of SBET, PBET and SGET was discussed, the extraction efficiency of in-vitro digestion test (PBET) and six commonly used single-extraction methods (0.1M HNO3,0.4M HO Ac,0.1M NaNO3,0.01M CaCl2,0.05M EDTA and0.5M DTPA) was contrasted, a possibility for the integration of soil heavy metal bioaccessibility and phytoavailability was analyzed, the function of constituents in PBET (citrate, malate, acetic acid and lactic acid) was discussed, the compositions of PBET was modified and simplified, and at last the bioaccessibility of Cu, Zn and Pb in urban soils and street dusts from Hangzhou city was evaluated. Then, several conclusions were obtained:
(1) The extraction ability of different in-vitro digestion tests varied significantly. SBET, which only included gastric digestion, had the highest efficiency to extract Zn and Pb from soils. In PBET and SGET, compared to the results of gastric phase, the solubility of soil Zn and Pb in the intestinal phase was obviously low. The observations indicated that the bioaccessibility of soil Zn and Pb is mainly pH-dependent. When soil entered the neutral intestinal phase, the dissolved soil Zn and Pb in acid gastric phase would be deposited as a result of pH rise. Therefore, the bioaccessibility of soil Zn and Pb in gastric digestion represented the maximum solubility of soil Zn and Pb in digestive tract. However, in PBET and SGET, when soil entered the intestinal phase, the solubility of soil Cu did not necessarily decrease or sometimes even increased due to the digestive enzymes. It was suggested that the bioaccessibility of soil Cu in gastric digestion did not represented the maximum solubility of soil Cu in digestive tract. Therefore, it is concluded that the in-vitro digestion tests only including gastric digestion, such as SBET, can be used to assess the bioaccessibility of soil Zn and Pb for the purpose of protection and precaution, but the in-vitro digestion tests comprised of gastric and intestinal digestion with digestive enzymes, such as PBET, should be selected to assess the bioaccessibility of soil Cu.
(2) To uncover the function of digestive enzymes (pepsin, bile salts and pancreatin) on the release of soil heavy metals in in-vitro digestion test, three in-vitro digestion tests (SBET, PBET and SGET) were selected. The bioaccessibility of soil Cu, Zn and Pb in each method were respectively evaluated with and without digestive enzymes and the differences compared. The results showed that the bioaccessibility of soil Cu, Zn and Pb increased, decreased or hardly changed after the addition of digestive enzymes. This phenomenon can be explained by the reaction between digestive enzymes and metal ions. The combination of digestive enzymes and metal ions can help soil heavy metals stay soluble, but on the other hand, the combination may cause the precipitation of digestive enzymes and the heavy metals combined with digestive enzymes become insoluble. It can be known that the effects of digestive enzymes addition are mainly dependent on the difference between the amounts of heavy metals extracted by digestive enzymes and deposited with digestive enzymes. Besides, the role of digestive enzymes played in simulated digestive juice is different with methods. Compared to the results of gastric digestion of PBET, the bioaccessibility of soil Cu after entered the intestinal phase stayed constant or even increased because of the addition of digestive enzymes. The results of SGET when all digestive enzymes (pepsin, bile salts and pancreatin) were added reflected more differences resulting from soil and element types. And considering the effects of digestive enzymes on the bioaccessibility of soil heavy metals were mostly seen in the intestinal phase, it is concluded the addition of digestive enzymes is indispensable to the gastrointestinal digestion methods, such as PBET and SGET, but for gastric digestion methods, such as SBET, the addition of digestive enzymes is not important.
(3) The extraction results for soil heavy metals by PBET and six commonly used single-extraction methods (0.1M HN03、0.4M HOAc、0.1M NaNO3、0.01M CaCl2、0.05M EDTA and0.5M DTPA) were compared. It was showed that the ability of PBET to extract Cu, Zn and Pb from soils is strong, obvious higher than neutral salt solution, and close to diluted acid solution and complex agents. The effects of soil properties on the extracted amounts of soil heavy metals by PBET were analyzed. The extractable Cu and Pb by the gastric phase and gastrointestinal phase of PBET were impacted by the similar soil properties. They decreased as the content of soil clay increased while as the total content of soil P, K, Mg, Ca and corresponding heavy metals reduced. The extracted amounts of soil Zn in the gastric and gastrointestinal phase of PBET were both significantly positively linearly correlated with the total content of soil Fe and Zn. According to the stepwise multiple linear regression analysis, the solubility of soil Cu, Zn and Pb in the gastric phase and gastrointestinal phase of PBET was both well explained by the total content of Cu, Zn and Pb in soils, respectively. The solubility of soil Cu, Zn and Pb in the gastrointestinal phase of PBET was determined to a great extent by their solubility in the gastric phase of PBET, but compared to the gastric digestion results, the gastrointestinal digestion results reflected more differences resulting from soil and element types. The coefficients of variation of bioaccessible soil Cu, Zn and Pb in the gastric phase of PBET were70.4,80.1and119.9%, respectively, but after the gastrointestinal digestion, the coefficients of variation of bioaccessible soil Cu, Zn and Pb increased to71.7,91.8and128.7%. Among six commonly used single-extraction methods, the extraction efficiency of EDTA was similar to that of PBET, and its results were well explained by the total contents of soil heavy metals, the same as PBET. Therefore, in some extent, the extraction results of PBET and EDTA represent the bioaccessibility and phytoavailability of soil Cu, Zn and Pb at the same time.
(4) The effects of the compositions of simulated gastric juice of PBET (1.25g pepsin,0.50g citrate,0.50g malate,500μl acetic acid and420μl lactic acid) on the solubility of soil heavy metals were studied. Compared with the results of original PBET, the changes of soil heavy metal bioaccessibility were analyzed when the simulated gastric juices only contained1.25g pepsin,0.50g citrate,0.50g malate,500μl acetic acid or420μl lactic acid at a constant pH, respectively. The results suggested that the extraction ability of original PBET was highest and that of the simulated digestive juices including only0.50g citrate was next. The extraction procedure and mechanism of PBET was uncovered. Firstly, metal ions in solution combined with organic acids whose molecular weight was lower, such as acetic acid and lactic acid. When the organic ligands of lower molecular weight organic acids were almost combined with metal ions, the citrate acid can start to form complex with metal ions. Therefore, the citrate acid solution can mimic the extraction of soil heavy metals by organic acid (malate, acetic acid and lactic acid) in PBET. The gastric juice of PBET was simplified:the gastric juice contained1.25g pepsin and1.0g citrate. The extraction efficiency of PBET and simplified PBET was studied and found the simplified PBET can not replace the original PBET, because the extracted amounts of heavy metals in a certain soil by simplified PBET could be higher or lower than that by PBET. If the health risk of heavy metals in a certain soil was assessed based on the results of simplified PBET, then the risk could be overestimated or underestimated. Moreover, the results of original PBET reflected more differences resulting from soil types. However, the regression analysis showed that the extraction results for soil Cu, Zn and Pb by the simplified PBET had a significantly positive linear correlation with those by the original PBET. Therefore, the simplified PBET can be used to predict the bioaccessibility of soil Cu, Zn and Pb in PBET.
(5) According to the Second Degree of Soil Environmental Quality implemented in China, the urban soils from Hangzhou city were not polluted by Cu, but they were contaminated by Zn and Pb in different degrees. The average concentrations of Cu, Zn and Pb in street dusts from Hangzhou city were221.67,478.70and105.31mg/kg, respectively. Compared to the contents of Cu, Zn and Pb in streets from other cities of China, the concentration of Cu in street dusts from Hangzhou city was a little high; the concentrations of Zn and Pb in street dusts were normal. The bioaccessibility of Cu, Zn and Pb in soils and dusts from Hangzhou city were evaluated using PBET and the limits of daily intake soils and dusts were calculated based on the bioaccessible heavy metals and the total content of heavy metals, respectively. The results showed:1) the sequence of heavy metals in soils and dusts from Hangzhou city was Pb>Cu>Zn;2) according to the bioaccessibility of Cu, Zn and Pb in soils, the lowest limits of daily intake soils were23.1g,84.5g and3.01g, respectively;3) according to the bioaccessibility of Cu, Zn and Pb in dusts, he lowest limits of daily intake dusts were25.54g,24.29g and3.39g. A child averagely intake0.208g soils/dusts per day. Therefore, it was known based on the extraction results of PBET, the Cu, Zn and Pb in soils and street dusts from Hangzhou city have no obvious negative effects on human health.
本研究选择了三种较为常用但设计不同的体外消化方法：SBET法(Simple Bioaccessibility Extraction Test)、PBET法(Physiologically Based Extraction Test)和SGET法(Simple Gastrointestinal Extraction Test)。SBET法是一个仅由胃相消化组成,且不添加胃蛋白酶的体外消化方法。PBET法是一个由胃肠液消化组成,且添加了所有消化酶(胃蛋白酶、胆汁盐和胰液素)的体外消化方法。SGET法则是一个由胃肠相消化组成,但仅添加了胃蛋白酶,没有添加肠酶(胆汁盐和胰液素)的体外消化方法。在研究中选取了3种性质差异较大的土壤,比较了三种方法对土壤Cu、Zn和Pb提取能力的差异,探讨体外消化方法的不同设计,即有无肠相消化、是否添加消化酶(胃酶：胃蛋白酶；肠酶：胆汁盐和胰液素)等对结果的影响,对肠相消化和消化酶在体外消化方法中的意义和作用进行了讨论,明确了土壤中Cu、Zn和Pb适用的体外消化方法类型。在此基础上,选择PBET法作为同时测定土壤中Cu、Zn和Pb生物可接受性的方法,比较了PBET法与六种常用单一提取剂法(0.1M HNO3、0.4M HOAc、0.1M NaNO3、0.01M CaCl2.0.05M EDTA和0.5M DTPA)对土壤中Cu、Zn和Pb的释放效果,探讨了土壤重金属生物可接受性和植物有效性整合的可能性。考虑到PBET法胃液组成成分在提取土壤重金属功能上的相似,对PBET法胃液中成分(柠檬酸钠、苹果酸钠、醋酸和乳酸)在提取土壤Cu、Zn和Pb中的作用进行了分析讨论,并对简化PBET法消化液组成的可能性进行了验证。最后,应用PBET法对杭州城市土壤和街道灰尘中Cu、Zn和Pb的生物可接受性及对人体的健康风险进行了评价。本研究主要得到以下结果：
(1)三种体外消化方法(SBET、PBET和SGET法)对土壤重金属的提取能力有显著差异,仅由胃相消化组成的SBET法对土壤Zn和Pb的提取效率最高,且在PBET法和SGET法中,土壤Zn和Pb在胃肠相中提取量远低于其在相应胃相中的提取量。研究表明,消化液pH值是影响土壤中Zn和Pb生物可接受性的主要因素,当土壤经过酸性胃相消化进入中性肠相后,在胃液中已经溶解的Zn和Pb会因pH的升高而发生沉淀,降低溶解度,所以土壤中Zn和Pb的胃相消化结果代表了其在消化道中的最大溶解度。但由于消化酶的作用,在PBET法和SGET法中,从胃液进入肠液后,土壤Cu的溶解度不但不一定降低,甚至有可能升高,所以土壤Cu的胃相消化结果不能代表其在消化道中的最大溶解度。由此可知,出于保护和预防的目的,在土壤重金属生物可接受性评价中,可以使用仅由胃相消化组成的体外消化方法(如SBET法)对土壤中Zn和Pb的生物可接受性进行测定,但对土壤Cu,则应选择由胃肠相消化组成,且添加了消化酶的体外消化方法(如PBET法)进行测定。
(2)为了揭示消化酶(胃蛋白酶、胆汁盐和胰液素)在体外消化方法释放土壤重金属中的作用,分别比较了三种体外消化方法(SBET、PBET和SGET法)中添加消化酶和不添加消化酶时,土壤重金属提取量的差异。结果表明,对于重金属的提取量而言,在三种方法中,与没有添加消化酶的结果相比,消化液中添加消化酶后,土壤重金属的提取量增加、降低或不变。这主要与消化酶与金属离子的反应有关：消化酶与金属离子的结合可以促进和保持土壤重金属的溶解,但另一方面,消化酶与金属离子的结合可能会促使消化酶发生沉淀,从而使与之结合的金属离子也变得不溶,所以消化酶对土壤重金属在消化液中溶解性的影响主要取决于消化酶提取的土壤重金属与随消化酶沉淀的重金属之间的差值。除此之外,消化酶在消化液中的作用因方法的不同而有所差异：PBET法中添加消化酶(胃蛋白酶、胆汁盐和胰液素)能保持或提高土壤Cu在胃肠液中的溶解度；SGET法中同时添加了胃酶和肠酶时,其结果更能体现土壤和元素类型、性质的差异。由于消化酶对土壤重金属提取量的影响主要表现在肠液中,所以消化酶添加在由胃肠液组成的体外消化法(如PBET和SGET法)中是必不可少的,但对于仅有胃相消化组成的体外消化方法(SBET法),胃蛋白酶添加并不是必须的。
(3)通过与六种常用单一提取剂法(0.1M HNO3、0.4M HOAc、0.1M NaNO3、0.01M CaCl2、0.05M EDTA和0.5M DTPA)结果的比较发现,PBET法对土壤Cu、Zn和Pb的提取能力较强,其对土壤重金属的提取效率明显高于中性盐提取,且与稀酸和络合剂提取相近。由相关分析可知,PBET法胃相和胃肠相对土壤Cu和Pb的提取量与土壤粘粒呈显著负相关,与土壤全P、全K、全Mg、全Ca及相应的重金属全量呈显著正相关；PBET法胃相和胃肠相对土壤Zn的提取量则与土壤金Fe、全Zn呈显著正相关。多元逐步线性回归分析表明,土壤中Cu、Zn和Pb的全量很好地解释了PBET法胃液和胃肠液中土壤Cu、Zn和Pb的溶解性,且土壤重金属在PBET法胃肠液中的溶解度在很大程度上取决于土壤重金属在胃液中的提取量；但和胃液消化结果相比,PBET法胃肠液的消化结果更能体现土壤和元素类型、性质的差异：生物可接受态土壤Cu、Zn和Pb在PBET法胃相消化中的变异系数分别是70.4%、80.1%和119.9%,但肠相消化结束后它们的变异系数分别增加到了71.7%、91.8%和128.7%。在六种常用的单一提取剂中,EDTA的提取能力和PBET法相似,而且其提取结果和PBET法一样,都能被土壤中重金属全量很好地解释,因此,在某种程度上来说,PBET法和EDTA对土壤Cu、Zn和Pb的提取结果同时代表了土壤Cu, Zn和Pb的植物有效性和生物可接受性。
(4)对PBET法胃液中成分(1.25g胃蛋白酶、0.50g柠檬酸钠、0.50g苹果酸钠、500μl醋酸和420μl乳酸)在提取土壤重金属中的作用进行了研究,以PBET去原始胃液为对照,比较了在胃液pH值一定的条件下,消化液中仅分别含有1.25g胃蛋白酶、0.50g柠檬酸钠、0.50g苹果酸钠、500μl醋酸和420μl乳酸时,土壤重金属提取量的变化情况。结果表明,PBET法原始消化液的提取能力最强,其次是仅含有柠檬酸钠的消化液。对PBET法原始消化液的提取过程进行分析：溶液中金属离子优先与分子量较小的有机酸,如乳酸或醋酸,结合形成复合物,当消化液中游离的小分子有机配合体大量减少时,柠檬酸才能与金属离子结合形成复合物。因此,柠檬酸钠有可能通过逐步多级电离的方式模拟PBET法原始消化液中有机酸(苹果酸、醋酸、乳酸)对土壤中重金属的提取作用。对PBET法消化液进行简化,即胃液中仅含1.25g胃蛋白酶和1.0g柠檬酸钠,发现PBET法简化消化液的结果并不能完全代替PBET法原始消化液的提取结果,因为：对某一具体的土壤而言,简化消化液的提取量与原始消化液的提取量之间存在一定的差异。如果使用PBET法简化消化液对某一具体的土壤中重金属的健康风险进行评估,其结果可能存在较大偏差；而且,土壤Cu、Zn和Pb在PBET法原始消化液中的结果更能体现土壤间性质的差异。但回归分析表明,土壤中Cu、Zn和Pb在PBET法简化消化液中提取量与他们在PBET法原始消化液中提取量的相关系数分别为0.968、0.977和0.977,呈显著线性正相关。所以,PBET法简化消化液对土壤Cu、Zn和Pb的提取结果可用于推测土壤Cu、Zn和Pb在PBET法原始消化液中的提取结果。
(5)根据中国土壤环境质量二级标准,杭州城市土壤未被Cu污染,但Zn和Pb有不同程度的污染。杭州市街道灰尘中Cu、Zn和Pb的含量平均为221.67、478.70和105.31mg/kg,与文献中报道的我国其他城市的街道灰尘中Cu、Zn和Pb的含量相比,杭州市街道灰尘中除Cu含量略有偏高以外,Zn和Pb的含量属于一般水平。应用PBET法对杭州城市土壤和街道灰尘中Cu、Zn和Pb的生物可接受性进行了测定,并分别根据重金属全量和PBET法对重金属的提取量计算杭州城市土壤和街道灰尘的人体每日可摄入量限值,以儿童每人每日平均摄入0.208g土壤／灰尘为参照,对杭州城市土壤和街道灰尘中重金属的人体健康风险进行了评估。结果表明：1)杭州城市土壤和街道灰尘中重金属对人体健康风险的大小顺序为Pb>Cu>Zn;2)根据PBET法对土壤中Cu、Zn和Pb的提取量,杭州城市土壤最低的人体每日摄入量限值分别为23.1g(延安路)、84.5g(杭钢)和3.01g(武林广场)；3)根据PBET法对灰尘中Cu、Zn和Pb的提取量,人体每日灰尘摄入量限值最小的分别为25.64g(凤起路)、24.29g(火车站)和3.39g(武林广场)。因此可知,以PBET法对土壤和灰尘中Cu、Zn和Pb的提取量为依据,杭州城市土壤和街道灰尘中Cu、Zn和Pb对人体健康无显著影响。
To assess the health risk of heavy metals in soils/dusts quickly and reliably, in-vitro digestion test, which has progressed rapidly in the past two decades, is widely used to determinate the bioaccessibility of heavy metals in soils/dusts. The bioaccessibility of heavy metals in soils/dusts refers to the solubility of heavy metals in simulated human digestive juice. Although various in-vitro digestion tests have been developed, no universally accepted method is established. Differently designed in-vitro digestion tests have apparently differences in compositions and procedures, which has caused the lack of comparability between results of different in-vitro digestion tests and the subjectivity in selection and application of in-vitro digestion tests. The aim of in-vitro digestion test research is to establish a unified, standardized, well accepted and widely used in-vitro digestion test. Before achieve this goal, several issues must be confirmed. They are:1) the effects of intestinal digestion and digestive enzymes addition on the results of in-vitro digestion test;2) the differences and connections of in-vitro digestion test and other methods for assessing soil heavy metal pollution;3) the application meaning of in-vitro digestion test in evaluating the health risk of soil heavy metals.
Three differently designed in-vitro digestion tests:SBET (Simple Bioaccessibility Extraction Test), PBET (Physiologically Based Extraction Test) and SGET (Simple Gastrointestinal Extraction Test), were chosen in this study. SBET is a method only with gastric digestion but without pepsin addition. PBET is a method including gastric and intestinal digestion with corresponding digestive enzymes. SGET is composed of gastric digestion with pepsin and intestinal digestion without bile salts and pancreatin. In this study, the differences of extraction ability between these three methods were compared, the suitable types of in-vitro digestion tests for the assessment of soil Cu, Zn and Pb was revealed, the role of digestive enzymes (pepsin, bile salts and pancreatin) in simulated digestive juices of SBET, PBET and SGET was discussed, the extraction efficiency of in-vitro digestion test (PBET) and six commonly used single-extraction methods (0.1M HNO3,0.4M HO Ac,0.1M NaNO3,0.01M CaCl2,0.05M EDTA and0.5M DTPA) was contrasted, a possibility for the integration of soil heavy metal bioaccessibility and phytoavailability was analyzed, the function of constituents in PBET (citrate, malate, acetic acid and lactic acid) was discussed, the compositions of PBET was modified and simplified, and at last the bioaccessibility of Cu, Zn and Pb in urban soils and street dusts from Hangzhou city was evaluated. Then, several conclusions were obtained:
(1) The extraction ability of different in-vitro digestion tests varied significantly. SBET, which only included gastric digestion, had the highest efficiency to extract Zn and Pb from soils. In PBET and SGET, compared to the results of gastric phase, the solubility of soil Zn and Pb in the intestinal phase was obviously low. The observations indicated that the bioaccessibility of soil Zn and Pb is mainly pH-dependent. When soil entered the neutral intestinal phase, the dissolved soil Zn and Pb in acid gastric phase would be deposited as a result of pH rise. Therefore, the bioaccessibility of soil Zn and Pb in gastric digestion represented the maximum solubility of soil Zn and Pb in digestive tract. However, in PBET and SGET, when soil entered the intestinal phase, the solubility of soil Cu did not necessarily decrease or sometimes even increased due to the digestive enzymes. It was suggested that the bioaccessibility of soil Cu in gastric digestion did not represented the maximum solubility of soil Cu in digestive tract. Therefore, it is concluded that the in-vitro digestion tests only including gastric digestion, such as SBET, can be used to assess the bioaccessibility of soil Zn and Pb for the purpose of protection and precaution, but the in-vitro digestion tests comprised of gastric and intestinal digestion with digestive enzymes, such as PBET, should be selected to assess the bioaccessibility of soil Cu.
(2) To uncover the function of digestive enzymes (pepsin, bile salts and pancreatin) on the release of soil heavy metals in in-vitro digestion test, three in-vitro digestion tests (SBET, PBET and SGET) were selected. The bioaccessibility of soil Cu, Zn and Pb in each method were respectively evaluated with and without digestive enzymes and the differences compared. The results showed that the bioaccessibility of soil Cu, Zn and Pb increased, decreased or hardly changed after the addition of digestive enzymes. This phenomenon can be explained by the reaction between digestive enzymes and metal ions. The combination of digestive enzymes and metal ions can help soil heavy metals stay soluble, but on the other hand, the combination may cause the precipitation of digestive enzymes and the heavy metals combined with digestive enzymes become insoluble. It can be known that the effects of digestive enzymes addition are mainly dependent on the difference between the amounts of heavy metals extracted by digestive enzymes and deposited with digestive enzymes. Besides, the role of digestive enzymes played in simulated digestive juice is different with methods. Compared to the results of gastric digestion of PBET, the bioaccessibility of soil Cu after entered the intestinal phase stayed constant or even increased because of the addition of digestive enzymes. The results of SGET when all digestive enzymes (pepsin, bile salts and pancreatin) were added reflected more differences resulting from soil and element types. And considering the effects of digestive enzymes on the bioaccessibility of soil heavy metals were mostly seen in the intestinal phase, it is concluded the addition of digestive enzymes is indispensable to the gastrointestinal digestion methods, such as PBET and SGET, but for gastric digestion methods, such as SBET, the addition of digestive enzymes is not important.
(3) The extraction results for soil heavy metals by PBET and six commonly used single-extraction methods (0.1M HN03、0.4M HOAc、0.1M NaNO3、0.01M CaCl2、0.05M EDTA and0.5M DTPA) were compared. It was showed that the ability of PBET to extract Cu, Zn and Pb from soils is strong, obvious higher than neutral salt solution, and close to diluted acid solution and complex agents. The effects of soil properties on the extracted amounts of soil heavy metals by PBET were analyzed. The extractable Cu and Pb by the gastric phase and gastrointestinal phase of PBET were impacted by the similar soil properties. They decreased as the content of soil clay increased while as the total content of soil P, K, Mg, Ca and corresponding heavy metals reduced. The extracted amounts of soil Zn in the gastric and gastrointestinal phase of PBET were both significantly positively linearly correlated with the total content of soil Fe and Zn. According to the stepwise multiple linear regression analysis, the solubility of soil Cu, Zn and Pb in the gastric phase and gastrointestinal phase of PBET was both well explained by the total content of Cu, Zn and Pb in soils, respectively. The solubility of soil Cu, Zn and Pb in the gastrointestinal phase of PBET was determined to a great extent by their solubility in the gastric phase of PBET, but compared to the gastric digestion results, the gastrointestinal digestion results reflected more differences resulting from soil and element types. The coefficients of variation of bioaccessible soil Cu, Zn and Pb in the gastric phase of PBET were70.4,80.1and119.9%, respectively, but after the gastrointestinal digestion, the coefficients of variation of bioaccessible soil Cu, Zn and Pb increased to71.7,91.8and128.7%. Among six commonly used single-extraction methods, the extraction efficiency of EDTA was similar to that of PBET, and its results were well explained by the total contents of soil heavy metals, the same as PBET. Therefore, in some extent, the extraction results of PBET and EDTA represent the bioaccessibility and phytoavailability of soil Cu, Zn and Pb at the same time.
(4) The effects of the compositions of simulated gastric juice of PBET (1.25g pepsin,0.50g citrate,0.50g malate,500μl acetic acid and420μl lactic acid) on the solubility of soil heavy metals were studied. Compared with the results of original PBET, the changes of soil heavy metal bioaccessibility were analyzed when the simulated gastric juices only contained1.25g pepsin,0.50g citrate,0.50g malate,500μl acetic acid or420μl lactic acid at a constant pH, respectively. The results suggested that the extraction ability of original PBET was highest and that of the simulated digestive juices including only0.50g citrate was next. The extraction procedure and mechanism of PBET was uncovered. Firstly, metal ions in solution combined with organic acids whose molecular weight was lower, such as acetic acid and lactic acid. When the organic ligands of lower molecular weight organic acids were almost combined with metal ions, the citrate acid can start to form complex with metal ions. Therefore, the citrate acid solution can mimic the extraction of soil heavy metals by organic acid (malate, acetic acid and lactic acid) in PBET. The gastric juice of PBET was simplified:the gastric juice contained1.25g pepsin and1.0g citrate. The extraction efficiency of PBET and simplified PBET was studied and found the simplified PBET can not replace the original PBET, because the extracted amounts of heavy metals in a certain soil by simplified PBET could be higher or lower than that by PBET. If the health risk of heavy metals in a certain soil was assessed based on the results of simplified PBET, then the risk could be overestimated or underestimated. Moreover, the results of original PBET reflected more differences resulting from soil types. However, the regression analysis showed that the extraction results for soil Cu, Zn and Pb by the simplified PBET had a significantly positive linear correlation with those by the original PBET. Therefore, the simplified PBET can be used to predict the bioaccessibility of soil Cu, Zn and Pb in PBET.
(5) According to the Second Degree of Soil Environmental Quality implemented in China, the urban soils from Hangzhou city were not polluted by Cu, but they were contaminated by Zn and Pb in different degrees. The average concentrations of Cu, Zn and Pb in street dusts from Hangzhou city were221.67,478.70and105.31mg/kg, respectively. Compared to the contents of Cu, Zn and Pb in streets from other cities of China, the concentration of Cu in street dusts from Hangzhou city was a little high; the concentrations of Zn and Pb in street dusts were normal. The bioaccessibility of Cu, Zn and Pb in soils and dusts from Hangzhou city were evaluated using PBET and the limits of daily intake soils and dusts were calculated based on the bioaccessible heavy metals and the total content of heavy metals, respectively. The results showed:1) the sequence of heavy metals in soils and dusts from Hangzhou city was Pb>Cu>Zn;2) according to the bioaccessibility of Cu, Zn and Pb in soils, the lowest limits of daily intake soils were23.1g,84.5g and3.01g, respectively;3) according to the bioaccessibility of Cu, Zn and Pb in dusts, he lowest limits of daily intake dusts were25.54g,24.29g and3.39g. A child averagely intake0.208g soils/dusts per day. Therefore, it was known based on the extraction results of PBET, the Cu, Zn and Pb in soils and street dusts from Hangzhou city have no obvious negative effects on human health.
引文
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