Microwave Regeneration of Spent Activated Carbon: Restoring Adsorption Capacity
Activated carbon, with its large specific surface area, good adsorption performance and resistance to acids and alkalis, is one of the best adsorbent materials for industrial wastewater treatment. It is normally produced by physical or chemical activation. The activation process is complicated and the product expensive: activated carbon with an iodine value of 1000 costs as much as 10,000 to 12,000 RMB per tonne.
Activated carbon that has reached adsorption saturation is classified as solid waste, and because the adsorbed substances it carries are harmful to the environment, regenerating spent carbon has considerable economic value.
Two Routes to Regeneration
Regeneration falls essentially into two categories. The first uses added energy or chemicals to desorb the adsorbate from the carbon surface, but this only transfers the pollutant from a solid surface into a liquid or gas phase, which creates secondary pollution. The second uses oxidants or reductants to convert and degrade the adsorbate into harmless substances, completing the regeneration. The two routes complement each other, and used together they preserve activity and the integrity of the carbon while remaining environmentally sound.
Common Methods and Their Trade-offs
The regeneration methods in common use are thermal, ultrasonic, chemical, electrochemical and wet oxidation.
Thermal regeneration gives good regeneration and is widely applied, but consumes a great deal of energy, and the carbon loses its function after repeated cycles.
Ultrasonic regeneration is simple in process and low in energy consumption, but its regeneration efficiency is low.
Chemical regeneration is highly targeted and effective, but requires large quantities of chemicals and readily causes secondary pollution.
Biological regeneration was first applied to activated carbon in fixed beds: micro-organisms decompose the organic matter in the bed into carbon dioxide and water, and as large molecules are broken into smaller ones the pressure loss across the fixed bed falls, which is the regeneration effect. The intermediate products of that decomposition, however, tend to block the pores, so biological regeneration is generally of low efficiency.
How MACT Microwave Regeneration Works
The MACT microwave regeneration technology developed by SINOKLE grew out of thermal regeneration. Both methods use heating to change the adsorption behaviour of the carbon, so that the adsorbate desorbs and the carbon is re-activated — regeneration and reactivation in one step. Under microwave irradiation the water molecules and other organics inside the carbon are heated very rapidly. Part of the organic matter decomposes into CO2 gas and water vapour, and the resulting steam pressure is driven explosively outwards from inside the particle. Under that violent action most of the pore structure of the carbon is restored, producing a markedly porous material; part of the organic matter is carbonised and, at high temperature, reacts with the residual carbon in a water-gas reaction that restores further porosity.
Why Microwaves Differ from Conventional Heating
The essential difference from thermal regeneration is the way heat is generated between the microwaves and the dielectric material. Microwaves interact with a material in three ways — reflection, transmission and absorption — and on that basis materials divide into conductors, insulators and dielectrics. Activated carbon is a dielectric. At the microscopic level the heating effect of microwaves arises from ionic conduction and dipole rotation. Ionic conduction means that dissociable ions migrate directionally in the electromagnetic field, producing a current, and that current generates heat when it meets resistance in the medium. Dipole rotation means that molecules carrying positive and negative charges realign under the electromagnetic field and generate heat in doing so.
Because activated carbon is a high dielectric-loss material, microwave heating raises its temperature very quickly — to well over a thousand degrees Celsius within a few minutes — which is what makes rapid regeneration possible. The carbon can therefore be reused cyclically, cutting operating cost substantially. By changing the regeneration conditions, the same technology can be adapted to the targeted treatment of different kinds of wastewater, achieving deep purification.
