Hybrid adsorbent prepared from renewable lignin and waste egg shell for SO₂ removal: Characterization and process optimization

The lignin (L) precipitated from black liquor was mixed with eggshell (ES) and followed by carbonization to prepare a hybrid porous adsorbent. The performance of removing of trace sulphur dioxide (SO₂) from the air at room temperature was investigated. The optimization and statistical work was conducted by using the central composite design (CCD) together with the response surface methodology (RSM). Among the investigated operational parameters, the effect of carbonization temperature was found to be the most significant to the performance of SO₂ removal by the prepared hybrid adsorbent. With an increase of carbonization temperature, the specific surface area increased appreciably reaching 130 m².g⁻¹ at 800 °C (L-ES-50-800-50). XRD analysis indicates the crystallite phase transition from calcite (CaCO₃) to limestone (CaO) with an increase of carbonization temperature. The FT-IR study indicates the disappearance of out-plane vibration peaks of C-O from CaCO₃ and gradual appearance of peaks from Ca-O of limestone on the surface of prepared hybrid material as an increase of carbonization temperature. The increased specific surface area and amount of limestone in the prepared hybrid adsorbent are found to contribute to the increased SO₂ removal by the prepared hybrid adsorbent. The final optimal preparation condition was determined using RSM by setting maximum SO₂ removal performance as optimization goal. The optimal preparation conditions achieved were: 50% = L/(L + ES), carbonized at 800 °C for the duration of 50 min. The prepared hybrid adsorbent at the optimal preparation condition showed excellent SO₂ removal performances in both wet and dry conditions.