Research On Adsorption Of Cr(Ⅵ) From Low Temperature Micro-Polluted Surface Water By Modified Powdered Activated Carbon

The environmental safety of drinking water source area and drinking water supply has been threatened by heavy metals as its potential human toxicity and high pollution risk. Due to low selectivity and low effectiveness for certain metal species of existing PAC in adsorption of low concentration of heavy metal (<0.5 mg/L) from water, it causes increasing dose of PAC and wasting of resource, especially for low temperature and low turbid water in north area of China.The object of this theme is the surface modification technology of PAC for improvement selectivity and effectiveness for heavy metal species such as hexavalent chromium according to the character of low temperature and low turbid water. The modification methods used in this work are acid/base oxidation modification and immobilization of metal capture agent such as humic substances.In this work, in order to imitate the conditions of low temperature water in the wintertime of north-east China, all experiments in this study were conducted at a constant temperature of 280±1K to be representative of environmentally relevant conditions. The original PAC is woodiness carbon which is marked as HNC. The PAC modified by nitric acid, sodium hydroxide and food-grade tannic acid is marked as N-HNC, OH-HNC and TA-HNC respectively. Test results show that TA-HNC has the best specific surface area, total pore volume and surface polarity. Among the three modified carbons, TA-HNC also has the best adsorption properties for low concentration of hexavalent and total chromium from low temperature water. The variance analysis of orthogonal test indicates that the optimal preparation conditions of TA-HNC were determined as food-grade tannic concentration of 15 mg/L, pH value of 4.0, temperature of 40℃and the modified time of 9 h.Adsorption experiments performed that the pH has a significant influence on the removal efficiency of Cr by TA-HNC. The adsorption efficiency of Cr(VI) by TA-PAC was significant higher in the acidic medium than that of counterpart, and the optimal pH value is 4.0, with the optimal dose of 50 mg/L. Langmuir(I), Langmuir(II) and Temkin models didn't fit the data well with the occurrence of the piecewise node at 0.149 mg/L, 0.151 mg/L and 0.163 mg/L respectively. While Freundlich model gave the best fitting of the sorption data (R2=0.9941, Ce= 0.010 mg/L to 0.404 mg/L). Additionally, the adsorption of chromium on TA-HNC can be better described by all the nonlinear forms of three-parameter models with R2 above 0.99. Dynamic test results indicate that adsorption process of total chromium by TA-HNC obeys second-order reaction kinetics model, so the key controlling step is absorption reaction. Additionally, effect of humic acid on adsorption capacity of TA-HNC for hexavalent chromium was also studied. The pH also has significant influences on the removal efficiency of hexavalent chromium by complex adsorbent (IHA-TAHNC ), while the optimal pH value scope is more widened than TA-HNC. In the acidic condition, oxidation and reduction reactions with the adsorption of trivalent chromium is the primary adsorption mechanism.The results obtained by 3-D fluorescence spectra and FTIR show that soluble humic acid (SHA) will help to improve the removal efficiency of hexavalent and total chromium, and the SHA is primary to participate the reduction of hexavalent chromium.