We have a system to treat water from an old industrial basin before discharging to the river. The system initially consisted of (in this order): a 3-stage bag filter, 3 granular activated carbon (GAC) adsorbers, 3 ion exchange vessels (WAC-Na) for heavy metals (mainly mercury, lead, and copper), then 2 activated alumina vessels for arsenic. We added acid injection to bring the pH down to 7.0 at the very end, and to our surprise, discovered the acid was *adding* Hg contamination. So we moved the 3rd GAC unit to after the acid injection as we procure "cleaner" acid. NOW, we find that As, Pb and Cu are drastically elevated at the end! Sampling confirmed that concentrations for those constituents are much lower before the treated water runs through that last GAC unit. Is it possible the GAC picked up metal contamination in its previous configuration, which is now being released to the treated water??
Due to unique physical properties of activated carbon for it's large surface area and porosity, so many solids, gaseous and dissolved substances are trapped through adsorption, occlusion and precipitation. Contaminated activated carbon implies the decreased level of all these properties and almost in saturated level or deactivated . As a result, this deactivated carbon can not remove any contaminants from water instead it may contaminate the treated water through release of so many contaminants may be through displacement reactions ,already present or by slipping due to overload. Hence, contaminated activated carbon must be reactivated by removing those contaminants and then could be used. In present case, addition of acid may leaching the adsorbed metals like lead, cadmium and arsenic from the contaminated activated carbon resulting in increase in levels of these toxic metals in the effluents, the treated water.
Hey Kimberly,
GAC will definitely adsorb metals and the extent of that adsorption will depend on water chemistry including pH. I suspect that the acid addition prior to the GAC vessel has changed the pH enough to shift the adsorption capacity of the GAC for those different metals. Thus, you are basically washing the previously adsorbed metals off of the carbon. I am also curious whether the acid you are using actually contains mercury or whether the acidification step is causing mercury that is present in the effluent water in a form that is not detected by your analysis (e.g., as a precipitate of some type) to become soluble and measurable? Mercury chemistry is pretty complex and this will depend on the geochemistry of the effluent and what anions are present, etc, but it is one other possibility to consider if the switch in acid does not solve the problem.
Hi Kimberly,
For every substance that is in GAC and in water, there is an equilibrium at steady state that exists between two phases (solid GAC and liquid water). That fact implies that if there is whatever contamination in GAC, it will partially dissolve in the water phase. How much that is, is a question of thermodynamics. How fast it occurs is a question of kinetics.
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