Sustainable CO₂ capture using acid activated alkaline sludge from photovoltaic industry: longevity and recyclability study

Recycling industrial wastage offers an effective strategy to mitigate waste discharge, promoting development of low-cost CO₂ adsorbents aligning with the principles of the circular economy. This study utilized waste alkaline sludge (AS) originated from the photovoltaic industry, activated with 1–4 M of hydrochloric acid (HCl), to study its effect on CO₂ capture performance. Activated AS were characterized with N₂ adsorption-desorption isotherm, XRD, FESEM-EDX, CO₂ adsorption-desorption isotherm and TPD-CO₂. AS activated by 2 M HCl exhibited the highest surface area of 123.73 m²/g and dominated by mesopores which played a significant role in CO₂ adsorption. CO₂ capture by physisorption at 25 °C exhibited an adsorption capacity of 0.44 mg/g, which was a 20 times increment than inactivated AS. Meanwhile, CO₂ capture performance by chemisorption was 284 mg/g, with temperature ranging from 207 to 644 °C and around 9 times higher than inactivated AS. Longevity study revealed that weight loss after prolonged CO₂ exposure for 24 h remain around 1.4%, indicating adsorption stop after 24 h. Longer exposure time induce morphological transformation from irregular ellipsoid into packed and aggregated nano coral, thus lessening its adsorption capacity. Based on adsorption isotherm breakthrough, it was determined that 25 °C was the optimal temperature for both adsorption and regeneration process. The adsorbent also demonstrated stable recyclability for 5 cycles, showing only 14% capacity reduction at the 2nd cycle and achieving a regeneration efficiency of 88.6%.