Research On Removal And Migration Regularity Of Hexavalent Chromium In Soil By Nanoscale Zero-valent Iron Coated With Pectin

Soil is the important component in ecological cycle indispensably as the foundation of natural ecosystem. However, along with the development of modern science and technology, soil pollution problems became increasingly serious, hexavalent chromium（Cr（Ⅵ）） pollution is one of the most major environmental problems. Generally, nanoscale zero-valent iron（nZVI） is used as a reducing agent to deal with heavy metal pollution with the characteristics of large surface area and high reaction activity. However, nZVI is easily suffered from oxidation and reunification, which limit severely its development potential in the field of the treatment of environmental pollution.As one kind of green and natural polysaccharides, pectin is cheap and obtained easily. In this study, pectin has been used for the preparation of nanoscale zero-valent iron coated with pectin（P-nZVI） for a set of tests in order to get the removal and migration regularity of Cr（Ⅵ）. The results of the experiment can be obtained as follows:（1）Through the study on the dispersive test as well as the characterization of X-ray diffraction and transmission electron microscopy between nZVI and P-nZVI, the result illustrates that phenomenon of demix start to appear after 1minutes and precipitation completely after 30 minutes standing, but P-nZVI can maintain a high degree of dispersion until 30 hours later. Thus, P-nZVI have a longer suspension in aqueous solution than nZVI.（2）Distilled water, pectin and P-nZVI have been added into soil column model for the leaching test, respectively. The study show that distilled water and pectin have little impact on removal of Cr（Ⅵ） separately, but there is a remarkable removal effect of Cr（Ⅵ） by adding P-nZVI, so the most essential substance in this study is P-nZVI.（3）The removal effect of Cr（Ⅵ） will be influenced by the concentration of pectin, P-nZVI dosage, pH, adding velocity of P-nZVI and bulk density of soil. The study show that increasing the dosage of pectin and adding velocity of P-nZVI appropriately are propitious to promoting the reduction of Cr（Ⅵ） under acidic conditions. The removal of Cr（Ⅵ） decreases with the increasing of bulk density of soil. Thus the optimum reaction conditions can be obtained as follows: initial concentration of Cr（Ⅵ） is 15.56 mg Cr⁶⁺/kg soil（concentration of Cr（Ⅵ） from leaching solution is 20 mg/L）,concentration of pectin is 0.6 g/L,P-nZVI dosage is 0.12 g/L, value of pH is 5.0, adding velocity of P-nZVI is 5mL/min and bulk density of soil is 1.27 g/cm³.（4）Three soil samples in different location are selected and numbered with sample A, sample B and sample C. Through the studying on reduction of Cr（Ⅵ） in three different samples, the results show that the removal of Cr（Ⅵ） in sample C is better than sample A and B, and sample A have the worst removal effect. This may be affected by the soil organic matter content. Higher the content of organic matter, higher the reduction of Cr（Ⅵ）.（5）The kinetic property can be described by pseudo-first-order reaction.The apparent rate constant( k_obs) is affected by the concentration of pectin,P-nZVI dosage, pH, adding velocity of P-nZVI and bulk density of soil.（a）With the increasing of concentration of pectin and pH, k_obs of them increases first and then decreases.（b）k_obs increases with the increasing of P-nZVI dosage and adding velocity of P-nZVI, the reduction reaction rate is positively correlated with both factors, k_obs increases at a exponential function with P-nZVI dosage and adding velocity of P-nZVI.（c）k_obs decreases with the increasing of soil bulk density, the reduction reaction rate is negative correlated with soil bulk density, and power function relationship can be found between both factors.