The effects of granular activated carbon heating rate and moisture content on defluorination of per- and polyfluoroalkyl substances during microwave regeneration

Microwave (MW) regeneration of PFAS-laden granular activated carbon (GAC) offers rapid and more uniform control of the particle temperature, shorter regeneration time, compact process equipment, energy savings, and potential for on-site use at water treatment plants. This study investigated the effects of GAC heating rates (100, 200, 300, 400, 533, and 653 °C/min) and moisture contents (1-5 %, 35-40 %, and 65-70 %) on defluorination of per- and polyfluoroalkyl substances (PFAS) during microwave regeneration of PFAS-laden GACs. Heating of GAC at the rate of 653 °C/min to > 950 °C in <2 min and holding the temperature at this level for an additional 3 min, played the key role in the MW regeneration of PFAS-laden GAC. The higher moisture content of GAC (65-70 %) slightly increased the fluoride recoveries in the range of 5-12 %. Under the optimal conditions (653 °C/min, 65-70 % moisture), fluoride recovery ranged from 91-99 % for PFOA, PFOS, PFBA, and PFBS-laden GACs. Braunauer-Emmett-Teller (BET) surface area and pore volume distribution analyses confirmed that MW regeneration did not alter the physical characteristics of GAC regardless of heating rate. The results documented here highlight the unique nature of MW treatment in terms of fast heating of the GAC particles to very high temperatures resulting in over 90 % PFAS defluorination.