【作者】 闫峰；

【导师】 张兴昌；

【作者基本信息】 西北农林科技大学 ， 农业环境保护与食品安全， 2010， 博士

【摘要】 Cr（Ⅵ）对土壤的污染取决于Cr（Ⅵ）在土壤中的环境化学行为,如还原-氧化、吸附等等;而这些化学反应的发生从根本上又取决于具体土壤的理化性质。所以,土壤的理化性质不仅决定了Cr（Ⅵ）在土壤中的化学反应,又决定了Cr（Ⅵ）在土壤中的迁移与归宿。通过研究Cr（Ⅵ）在16种中国土壤上的吸持动力学和等温线,并结合相关分析和通径分析的统计学方法,得到如下结论,Cr（Ⅵ）在酸性土壤中吸持量较大,且吸持动力学可以用一级动力学方程和抛物线方程描述,土壤pH、活性Fe（Ⅱ）含量和物理粘粒含量与一级动力学模型中平衡时土壤对Cr（Ⅵ）的表观吸持量qe参数和抛物线模型中表观吸附扩散速率常数k呈正相关关系;而Cr（Ⅵ）在碱性土壤中吸持量较小,其动力学呈现“波动”趋势,不能用动力学方程描述。从土壤对Cr（Ⅵ）的等温线和吸持量上来划分,自然土壤大致可以分为4类:1)Cr（Ⅵ）还原型土壤;2)Cr（Ⅵ）吸附-还原型土壤;3)少量Cr（Ⅵ）吸附型土壤;4)痕量反应土壤。土壤pH是决定Cr（Ⅵ）在土壤中吸持的最重要的因素,活性Fe（Ⅱ）含量为第二重要因素,络合态铁和物理粘粒含量对Cr（Ⅵ）的吸持只产生有限的影响,CaCO₃含量和CEC影响较小,而有机质含量、无定形铁含量和易还原锰含量对Cr（Ⅵ）的吸持量基本没有影响,所以判定未知土壤中Cr（Ⅵ）的吸持能力时,土壤pH和活性Fe（Ⅱ）含量起到了绝对重要的作用。由于土壤pH是影响Cr（Ⅵ）在土壤中吸持的最重要的因素,所以本文主要研究了pH影响Cr（Ⅵ）在黄棕壤上的吸持机理,发现pH与Cr（Ⅵ）的总吸持、吸附和还原均为线性的负相关关系,且还原量随pH的变化是吸附量变化的3倍左右,即还原随土壤pH的变化较吸附敏感的多,所以,pH主要影响Cr（Ⅵ）在土壤中的还原量。并发现土壤pH通过影响土壤还原剂的溶解是pH影响土壤Cr（Ⅵ）吸持的一个重要机理。所以,土壤pH对Cr（Ⅵ）吸持的影响机理总结为:1)酸性条件更有利于Cr（Ⅵ）的吸附,2)土壤中Cr（Ⅵ）的主要还原剂有机质和含铁矿物只有在酸性条件下会还原Cr（Ⅵ）,3)较低的pH有利于土壤中的还原物质溶解到土-水溶液中,4)土壤pH显著影响了Cr（Ⅵ）的氧化还原电位。同时,本文还发现土壤溶液中的NO₃^-、Cl^-和Br^-,它们都是影响Cr （Ⅵ）在黄棕壤上吸持最弱的阴离子;阳离子对吸持量的影响顺序为K⁺= Ca²⁺ > Mg²⁺ > Na⁺;这些阳离子影响Cr（Ⅵ）在黄棕壤上的吸持,主要因为K⁺、Ca²⁺和Mg²⁺可以改变土-水体系的pH而间接影响Cr（Ⅵ）在黄棕壤上的吸持（吸附和还原）,而且,K⁺、Ca²⁺和Mg²⁺离子本身会提高了Cr（Ⅵ）在黄棕壤上的吸附。另一方面,土壤中的Cr（ⅡI）,无论是土壤本底的Cr（ⅡI）,还是Cr（Ⅵ）还原生成的Cr（ⅡI）,都可能被土壤中的MnO₂氧化为毒性较强的Cr（Ⅵ）;而土壤中的有机酸是土壤中重要的组分,这些有机酸易于与三价金属Cr（ⅡI）发生络合反应,这就会使土壤中Cr（Ⅲ）的氧化反应发生变化。本文通过研究有机酸对Cr（Ⅲ）氧化的影响,以及有机酸-Cr（Ⅲ）络合体的氧化特性发现,在pH 5-8范围内,柠檬酸对Cr（Ⅲ）在MnOx上的氧化表现为明显的抑制作用,而草酸、酒石酸和葡糖酸只在pH = 5时有一定的抑制作用。有机酸在酸性条件下的影响机理为,1)还原络合δ-MnO_x;2)与Cr（Ⅲ）络合生成难氧化的有机-Cr（Ⅲ）络合物;3)还原体系中生成的Cr（VI）;而在碱性条件下的抑制主要途径为有机酸与Cr （Ⅲ）生成少量的有机-Cr（Ⅲ）络合物所致。但是,无论是上述的何种机理,有机酸中的–COOH都是影响Cr（Ⅲ）氧化过程中尤为重要基团,随着-COOH浓度的增加,有机酸的络合Cr（Ⅲ）和其还原Cr（VI） （或MnO₂）能力都增强,从而达到最终抑制Cr（Ⅲ）氧化的效果。同时研究表明,有机酸-Cr（Ⅲ）络合体的氧化能力为:柠檬酸- Cr（Ⅲ） <酒石酸- Cr（Ⅲ） <草酸- Cr（Ⅲ） <葡糖酸- Cr（Ⅲ） < Cr（Ⅲ）。所以土壤中的有机酸与Cr（Ⅲ）离子发生络合反应,则可降低Cr（Ⅲ）离子在环境中的氧化,另一方面,即使有机酸不发生络合反应,自然界的游离有机酸也会对Cr（Ⅲ）离子的氧化产生抑制作用,所以有机酸的存在都会大大降低了土壤中Cr（Ⅲ）离子的氧化性能;但另一方面,低分子有机酸可能增加了自然界中沉淀态Cr（Ⅲ）的移动,可使其到达土壤中锰氧化物的表面,从而增加了Cr（Ⅲ）的氧化。所以自然界的有机酸对Cr（Ⅲ）氧化的影响是一把双刃剑,这主要取决于Cr（Ⅲ）在土壤中的存在形态。更多还原

【Abstract】 The risk of Cr（Ⅵ） pollution in soils depend on its reactions such as sorption, reduction-oxidation, precipitation, which in turn is intimately related to the specific soil properties. In this study, 16 kinds of Chinese soils were selected to study the effect of soil properties on Cr（Ⅵ） attenuation, and the combination of correlation analysis and path analysis were used to analyze the soil properties factors. There was significant different Cr（Ⅵ） attenuation in these soil. Usually, higher Cr（Ⅵ） attenuation exists in low pH soils and its kinetics can be fitted by the first-order equation and the intra particle diffusion model, and soil pH, the content of active Fe（Ⅱ） and clay were correlated with qe （first-order equation） and k （intra particle diffusion model）. While alkaline soils always have little or no Cr（Ⅵ） attenuation, and its kinetics cannot be described by any kinetic equations. Based on different Cr（Ⅵ） attenuation, the soils could be divided into 4 types: 1) reduction dominated soil; 2) reduction-adsorption soil; 3) slight adsorptive soil; 4) no reaction soil. Cr（Ⅵ） uptake by soil was correlation with soil properties such as soil pH, the content of Organic matters, Fe（Ⅱ） , OM-Fe, CaCO₃ and Clay. Of which, the soil pH play the most important direct role in Cr（Ⅵ） chemical attenuation, active Fe（Ⅱ） content was the secondary factory, the content of complex iron and clay were the third factors, the content of CaCO₃ and CEC exhibited little effect Cr（Ⅵ） attenuation, while the content of organic maters, the armorphurs and easily reducible manganese have no influence on Cr（Ⅵ） attenuation. Thus, we could estimate the Cr（Ⅵ） attenuation in unknown soils based on soil pH and the concentration of active Fe（Ⅱ） in soil.Based on the fact of that soil pH play the most impotant role in Cr（Ⅵ） attenuation, the mechanism of pH effected Cr（Ⅵ） attenuation in yellow-brown soil was investigated.The soil pH correlated with Cr（Ⅵ） reduction, adsorption and total uptake. And the Cr（Ⅵ） reduction was more sensitive than Cr（Ⅵ） adsorption as pH changes, since pH changed 1 unit, the amount of Cr（Ⅵ） reduction changed was 3 times than the amount of Cr（Ⅵ） adsorption changed. Low pH could effect the soil reductants dissolution from soil matrix was an important mechanism of pH effect on Cr（Ⅵ） reduction.Thus, the mechanisms of pH effect on Cr（Ⅵ） attenuation were summarized: 1) low pH could increase Cr（Ⅵ） adsorption on soil, 2) the reductants such as the organic matters and iron minerals only can react directly with Cr（Ⅵ） at low pH, 3) low pH could increase dissolution reductants power from soil matrix which was an indirectly way in increasing Cr（Ⅵ） reduction.,4) The other important reasons was the fact that low pH could raise redox potential of the Cr（Ⅵ）/Cr（Ⅲ） and change Cr（Ⅵ） to a very strong oxidant. In this experiment, we also observed that the anion NO₃^-, Cl^- and Br^- exhibited little effect on Cr（Ⅵ） uptake by soils, while cation such as K⁺, Ca²⁺, Mg²⁺, Na⁺ showed influence on Cr（Ⅵ） attenuation, which was because that these cations could lower the soil pH and can directly increase Cr（Ⅵ） adsorption on soil.On the other hand, Cr（Ⅲ） in soils,wether the nature Cr（Ⅲ） or from Cr（Ⅵ） reduction, may re-oxidize by MnO₂. And the organic acid was an important composontion of soil, since these organic acids can complex cations with valent of ⁺2 or ⁺3, thus the organic acids can complex Cr（Ⅲ） and inevitably affect on soil Cr（Ⅲ） oxidation. The organic acids influence on Cr（Ⅲ） oxidation were studied. At pH ranged from 5 to 8, citric acid exhibited strong inhibition of Cr（Ⅲ） oxidation by MnO₂, while oxalic acid, gluconic acid and tartaric acid only inhibited Cr（Ⅲ） oxidation at pH 5. The reaons of these organic acid effect at acid condation were 1) organic acid may reducte or complex withδ-MnO₂, 2) organic acid can complex with Cr（Ⅲ）, 3) organic acid may reduce the Cr（Ⅵ） which was produced from Cr（Ⅲ） oxidation; while at alkaline system, this mechanism was limited to that organic acid can complex with Cr（Ⅲ）. No matter which of the mechinsm mentioned above, -COOH play important role in affecting on Cr（Ⅲ） oxidation, with increase concentration of -COOH, the organic acid will more easily to complex with Cr（Ⅲ） and to reduce Cr（Ⅵ） orδ-MnO₂, thus inhibit the Cr（Ⅲ） oxidation. The organic acid-Cr（Ⅲ） complex have the lower oxidation potential than Cr（Ⅲ） ion with this order: citici acid-Cr（Ⅲ） < tartaric acid- Cr（Ⅲ） < oxalic acid-Cr（Ⅲ） < gluconic-Cr（Ⅲ） < Cr（Ⅲ） ion. Thus, whether organic acid complex with Cr（Ⅲ） or not, soil organic acid could inhibit the Cr（Ⅲ） ion oxidation. On the other hand, soil organic acid also could increase the Cr（Ⅲ） precipitation dissolution, leading Cr（Ⅲ） mobility to the surface of MnO₂, and increasing Cr（Ⅲ） oxidation. Thus soil organic acid was a double-edged sword, inhibition or increasing oxidation was decided by the species of Cr（Ⅲ） exist in soils.更多还原