1.The mechanism of chromium adsorption on activated clay
Activated clay from bentonite by acid (usually sulfuric acid) activated activated acid clay or bleaching earth, its main component is montmorillonite. The main chemical composition of activated clay is SiO2 50% ~ 70%; Al2O3 10% ~ 16%; Fe 2O3 2% ~ 4%; MgO 1% ~ 6%; the rest is CaO + Na2O + K2O + FeO. Is generally believed that its adsorption capacity and chemical composition has nothing to do with the production of activated clay and inorganic acid used in the reaction conditions . In the activation process, the inorganic acid dissolved bentonite bentonite like impurities, montmorillonite which produce a lot of holes on the surface, the result of the crystal at both ends of the channel increases, increasing the diameter, so that the specific surface area increased, the adsorption capacity Increase, can produce effective physical adsorption; strong acid solution in the montmorillonite octahedral crystals Al3 +, Mg2 +, Fe3 +, Fe2 +, etc. were ion radius is very small H + part or all of the replacement, so that Mongolia The smectic cell is negatively charged and becomes a large negative ion, resulting in more surface acid hydroxyl exposure. These exposed surface acidic hydroxyl and water molecules can form stable complex ions with heavy metal ions. Its composition is hydroxyl or water molecules as ligands, metal ions as the central ion, the adsorption belongs to the surface of the complex adsorption; activated clay dispersed in water particles, is composed of several units in the C --- axis Stacked together, between the layers by van der Waals force link, layers can have ion exchange. Therefore, activated clay adsorption of heavy metal ions mainly due to the surface of the pore size of the physical adsorption, the surface layer of complex adsorption and interlayer ion exchange adsorption.
(1) Surface complexation adsorption According to the surface complexation model heavy metal ions adsorbed on the particle surface
Action is seen as a surface complex formation reaction, the reaction trend increases with the solution pH value. It is proved by spectroscopic analysis that a large amount of SiOH groups exist on the surface of activated clay, and the ionic reaction of hydroxyl groups is
SiOH → ※ SiO - + H + and SiOH + H + → ※ SiOH2 + When activated clay and heavy metal ions
Cr6 + occurs complex adsorption, the particle surface hydroxyl groups and its possible complex reaction under acidic conditions:
SiOH + Cr6 + → ※ SiOCr5 + + H +
SiO - + Cr6 + → ※ SiOCr5 +
Due to the H + participation, the surface complexation reaction is affected by the pH value. Obviously, when the pH value is small, the H + occupies more SiO - adsorption sites and is not conducive to Cr6 + adsorption complexation. This is in agreement with the experimental pH The value of about 6 activated clay adsorption capacity of chromium is the largest.
(2) layer ion exchange adsorption has a lot of literature shows that heavy metal ions in the montmorillonite
Grain and its interlayer between K +, Na +, Ca2 +, Mg2 + exchange effect. Ion exchange capacity in addition to the size of the ion itself and the nature of the surface of montmorillonite, but also depends on the size of doolitone, apparently greater dool, the exchange capacity greater. Bentonite modified dool value increases, which is conducive to Cr6 + deep layers of metal ions exchange. W Dubbin et al. Discussed from the theory of soft and hard acids (SHAB) that chromium readily forms spherical polymers with oxygen in siloxane groups between montmorillonites. SiO alkyl O belongs to soft alkali. The relative alkalinity of chromium determined by Misono's parameter method is 2.8, which is at the acid-base junction. Therefore, it is possible for chromium to form Cr-O bond with oxygen in the montmorillonite. Since Na, Al, etc. differ from the Cr ion type, the Cr outer layer has d3 electrons. Therefore, when Cr exchange occurs, Na, Al and the like form a Cr-O polymer stone, and the reaction can be overcome to overcome the energy barrier. Carr (1985), however, argues that the hydrolysis of OH-Cr can act as a catalyst between the layers and the formation of OH-Cr by chromium-hydrolyzed polymers acts as a catalyst for the substitution of Cr-O between layers.
Direct evidence of the formation of a polymer of chromium and oxygen between [SiO] 4 - layers is obtained by IR spectroscopy. In the montmorillonite without Cr substitution, the absorption peak in the vertical a-axis direction is -1100 cm -1 between the layers Chromium-substituted montmorillonite has an absorption peak at -1017 cm -1 (two samples). Carr thought that this decrease in absorption peak was due to the shift of the electron cloud from Cr-C to Cr-Cr between the original O-Si due to the formation of Cr-O-Si bonds. Due to the relative decrease of the electron cloud between the Si- Thus reducing its vibration frequency.
2.the conclusion and the existing problems
It is feasible to treat activated sludge with low concentration chromium-containing wastewater using activated clay as the adsorbent. Under the experimental conditions, it can meet the emission standards. However, there are still some problems: (1) Since the chromium-containing test solution used in the adsorption test is a simulated liquid and the single factor has no interference, the obtained conditions have a certain gap from the actual ones, and further research is needed. ② electroplating wastewater contains a variety of metal ions, in addition to chromium adsorption of metal ions other conditions discussed in another article. ③ active clay regeneration problems and to prevent secondary pollution issues still need further study.