Application of Fe(II)/peroxymonosulfate for efficient alkali lignin wastewater treatment: Insight into the synergistic interactions between redox reactions and coagulation

In this work, Fe(II)/Peroxymonosulfate (PMS) treatment with dual functionality of pre-oxidation and coagulation was applied to enhance the removal of alkali lignin in water. The removal efficiency of alkali lignin (2 g/L) could reach ∼ 99% at an optimal dosage of [Fe(II)] = 2.4 mM and [PMS] = 7.8 mM, which was efficiently superior than that using ferric and polymeric coagulant individually. Pre-oxidation, involving hydroxyl radical (•OH) and sulfate radical (SO4•−), has been demonstrated to play a crucial role in both oxidative degradation (∼30%) and reducing the molecular weight and hydrophilicity of lignin. Meanwhile, the formation of polynuclear Fe-hydroxide complexes was also fostered, which in turn greatly contribute to the removal of lignin through an enhanced coagulation mechanism. The reductive units present in alkali lignin was demonstrated to initiate a dynamic transition between Fe(III) and Fe(II) by donating electrons. This redox activity is anticipated to continuously promote the formation of electro-neutral and loose flocs, thereby resulting in an enhanced coagulation performance. Furthermore, ultrafiltration (UF) was also demonstrated as a viable liquid/solid separation method following Fe(II)/PMS treatment, effectively promoting membrane flux recovery rate to ∼90%. This highlights the remarkable potential of this method when combined with other treatment processes. This study aims to offer a complementary strategy for remediating alkali lignin wastewater while advancing the fundamental understanding of redox-reaction enhanced in-situ coagulation in water treatment applications.