Interference Mechanism And Elimination Method Of Sulfides For Soil Cr(Ⅵ) Determination And Application Research

The Cr（Ⅵ）contamination problem of soil heavy metal pollution in our country has become more and more serious.With the development and popularization of soil remediation work,accurately evaluating its remediation effectiveness plays a guiding role in the field of soil remediation.In engineering practice,chemical reduction technology is one of the most important remediation technologies for Cr（Ⅵ）-contaminated soil.Commonly used reductants include sodium sulfide（Na₂S）,calcium polysulfide（Ca S_x）and ferrous sulfate（Fe SO₄）.However,the large amount of reductants remaining in the remediated soil will cause interference to the Cr（Ⅵ）detection process,especially sulfides,causing serious deviations in the detection results and leading to misjudgment of remediation performance.It will make the remediation projects fail to be accepted on time or soil Cr（Ⅵ）concentration detection value increase after acceptance,which brings severe challenges to remediation work.Therefore,the interference of sulfide to soil Cr（Ⅵ）detection has become a key issue that needs to be solved urgently in the current field of Cr（Ⅵ）-contaminated soil remediation.At present,the interference mechanism of sulfides on soil Cr（Ⅵ）determination is still unclear and the current soil Cr（Ⅵ）detection standard does not provide methods to eliminate the interference of sulfides.In order to solve the above problems,it is of great significance to carry out an intensive and systematic research on this.This thesis started with the interference mechanism of sulfides to soil Cr（Ⅵ）detection process which can be divided into three phases according to the change of p H to study the key location and influencing factors of interference.On this basis,a pretreatment method was proposed for eliminating the interference of sulfides by precipitation of metal ions and the elimination effectiveness of Zn²⁺and Cu²⁺were compared.Based on the detection results after eliminating interference,the actual remediation effectiveness of several widely used reductants（Na₂S,Ca S₄,Fe SO₄）and their combination Fe SO₄-Na₂S on Cr（Ⅵ）contaminated soil were evaluated and verified.Also,the long-term effectiveness of residual reductants under leaching condition and the changes of long-term soil Cr（Ⅵ）detection results after remediation were investigated to explore the main reason and essence of increase of detection results.Through the study of the above aspects,the following conclusions are obtained:（1）The sulfides remaining in the remediated soil continuely reducing Cr（Ⅵ）at various phase of soil Cr（Ⅵ）detection process is the main reason and mechanism for detection interference.The detection process is divided into alkaline digestion phase,p H adjustment phase and diphenylcarbazide（DPC）-spectrophotometry phase,and the change of initial concentration of S^2-is directly related to the specific phase of interference.When the concentration of S^2-is too high,the reduction of Cr（Ⅵ）begins in the first phase（alkaline digestion phase）.The high temperature condition of alkaline digestion process and the effects of Fe^2+/3+will further promote the reaction of Cr（Ⅵ）with S^2-,aggravating the interference of S^2-.Elemental sulfur（S⁰）,as the remediation product,will generate S^2-(or S_x^2-)during alkaline digestion process,which also leads to the reduction of Cr（Ⅵ）.As the key location for the occurrence of interference,it is necessary to take pretreatment measures before alkaline digestion phase to avoid the interference of residual sulfides on soil Cr（Ⅵ）determination at the source.（2）Using Zn²⁺to pretreat the residual S^2-in the soil can significantly decrease the interference of S^2-on soil Cr（Ⅵ）detection.Zn²⁺can react with S^2-under alkaline condition to form relatively stable Zn S precipitation,reducing the amount of S^2-in solution.Other main ions(OH^-,CO₃^2-and PO₄^3-)in alkaline digestion solution will not react with Zn²⁺under strong alkaline condition to cause the loss of Zn²⁺.However,the effectiveness of Zn²⁺for eliminating the interference of S⁰and Fe S is relatively poor,which limits the application scenarios of this method.（3）The effectiveness of Cu²⁺for eliminating the interference of S^2-,S⁰and Fe S is better than that of Zn²⁺.The reaction rate of Cu²⁺and S^2-is extremely fast and the products are very stable.Adding Cu²⁺before alkaline digestion phase can significantly decrease the reduction of Cr（Ⅵ）during the alkaline digestion process and increase the spiked recoveries of Cr（Ⅵ）.The excess Cu²⁺in the solution can interfere with the color development process of Cr（Ⅵ）and DPC at the third phase of soil Cr（Ⅵ）detection process（DPC-spectrophotometry phase）,while OH^-in alkaline digestion solution and heating condition can convert most of the excess Cu²⁺into Cu O precipitate and separate them from the solution,and a small amount of residue Cu²⁺can be masked by the complexation effect of EDTA.（4）The detection results after eliminating the interference of reductants help to evaluate the soil remediation effectiveness more accurately.The soil Cr（Ⅵ）detection results without eliminating the interference show that the order of remediation effectiveness is Ca S₄>Na₂S>Fe SO₄.While the results after eliminating the interference show that the remediation effectiveness of Ca S₄and Na₂S is not better than that of Fe SO₄,and the remediation effectiveness of Ca S₄is not better than that of Na₂S.（5）The attenuation of reductant-induced interference on soil Cr（Ⅵ）detection is an important reason for the increase of soil Cr（Ⅵ）detection results.The residual amount and reducing ability of reductants in soil will show a downward trend over time and leaching process will further accelerate the loss and oxidation of reductants,so the reductant-induced interference to the Cr（Ⅵ）detection process gradually decreased and the negative bias of detection gradually narrowed,which leads to the increase of soil Cr（Ⅵ）detection results over time.