Study On The Adsorption And Release Characteristics Of Chromium By Pipe Scale In Water Supply Network

Corrosion scales in water supply network are the main places for accumulation of heavy metal elements.When water quality or hydraulic conditions change suddenly,it may cause the release of heavy metal and increase drinking water risk.In this paper,the characteristics of four corrosion scales in water supply network were analyzed firstly,which laid a foundation for subsequent experiments.Secondly,the gray cast iron pipe scale was selected as the sample,which was divided into inner and outer layers.Hexavalent chromium and trivalent chromium were selected as the research objects.The characteristics of chromium adsorption and releasing from pipe scale were studied from two aspects of influencing factors and kinetics.The effects of influencing factors on the adsorption and release of chromium by corrosion scales of gray cast iron were explored,also the adsorption and release process of different valence chromium by corrosion scales were compared and analyzed.Finally,the adsorption and release models were established.The main conclusions are as follows:（1）The total three-state content（weak acid extraction state,reducible state and oxidizable state）of each element in the pipe scale of gray cast iron shows that the three-state content of porous layer is the least,which indicates that in the three-layer pipe scale,the migration ability of heavy metals in the surface layer and hard shell layer is stronger,and the activity of elements in the porous layer is weaker.（2）On one hand,the physical structure and crystal composition of the inner scale decide that it has more adsorption sites;on the other hand,chromium element in the inner scale mainly exists in the oxidation state with lower activity and the residue state without bioavailability.Under different environmental conditions,compared with the outer layer with higher activity,the chromium in the aqueous solution is more easily adsorbed and stably fixed in the inner layer.Therefore,in the single factor experiment,the adsorption rate of inner scale to hexavalent chromium/trivalent chromium is slightly higher than that of outer scale,but the trend of adsorption curves of inner and outer scale is similar.（3）Initial concentration of chromium,pH,temperature and reaction time all have certain effects on the adsorption of hexavalent chromium/trivalent chromium by inner and outer scale.Generally speaking,the higher initial concentration of chromium and the alkaline environment at 15-25℃lead to the relatively higher accumulation of chromium in the scale.（4）The kinetics of adsorption of hexavalent chromium by inner and outer scales coincides with Lagergen’s quasi-second-order,Elovich’s and Webber-Morris’s intraparticle diffusion models at the same time.（5）Compared with the inner scale,the existing form and the sum percentage of three states of chromium in the outer scale determine its higher activity and migration ability.Therefore,in the single factor experiment,the release of hexavalent chromium/trivalent chromium from the outer scale is slightly higher than that from the inner scale,but release curves of the inner and outer scale have similar trend.（6）Temperature,concentration of SO₄^2-,concentration of Cl^-and pH all have certain effects on the release of hexavalent chromium/trivalent chromium from inner and outer scale.In order to inhibit the release of heavy metals,the water quality should be kept in a neutral or alkaline condition,while the concentration of SO₄^2-and Cl^-should be controlled at a low level.Also much attention should be paid to the impact of water temperature changes on the release of heavy metals.The results of orthogonal experiments show that the factors affecting the release process have significant differences in chromium release under different levels.According to practical experience,from the initial stage to the stable stage of pipeline network operation,the optimal level of factors to reduce chromium release is:temperature is5℃,pH=7,the concentration of SO₄^2-and Cl^-is 50 mg/L.（7）The kinetics of hexavalent chromium release from inner and outer pipe scales coincides with Elovich model and quasi-second-order model at the same time.（8）The adsorption and release process of chromium in the pipe scale of gray cast iron is affected by its valence state.Compared with trivalent chromium,hexavalent chromium has stronger migration ability between the aqueous phase and the scale phase.（9）The adsorption mechanism of the scale on chromium is:at the beginning of the reaction,the scale depends on the specific surface area and the adsorption vacancy to physically adsorb the chromium with higher concentration in the solution,and the adsorption rate is faster.With the saturation of adsorption sites on the surface of scale,chromium begins to be adsorbed by surface exchange reaction.At this time,the adsorption process is caused by electrostatic force on the outer surface of scale,forming outer ring compounds.With the reaction time continues,chromium complexes or chelates with some functional groups on the surface of pipe scale,forming inner ring compounds.This stage is chemical adsorption,the adsorption rate is slower,and finally reaches equilibrium.（10）The release mechanism of chromium from pipe scale is:the release process may be controlled by a variety of complex reaction mechanisms.At the beginning of the reaction,the release rate of chromium is relatively fast.At this time,the part that is desorbed is the non specific adsorption part.This part is mainly the metal ions that are easy to desorb by combining with the scale with Coulomb force in the double electric layer.As the reaction time continues,the specific adsorption part begins to desorb.Some of them are mainly the inner ring compounds formed by metal ions and scale.The desorption rate slows down gradually,and finally reaches equilibrium.