The roles of suspended solids in persulfate/Fe²⁺ treatment of hydraulic fracturing wastewater: Synergistic interplay of inherent wastewater components

The high content and low removal efficiency of suspended solids (SS) in wastewater derived from shale gas development via hydraulic fracturing has raised advanced treatment problems (e.g., organics removal) and environmental concerns. In this study, two kinds of suspended solids were separated from synthetic flowback wastewater (FWW) after 1 or 14 days of sedimentation. The physicochemical properties, porous structures, surface chemistry, and particle stability of the obtained 1-/14-day solids were characterized. To illustrate the specific roles of SS during radical-based FWW treatment, the removal of di-(2-ethylhexyl) phthalate (DEHP, a typical recalcitrant organic that has been identified in FWW) by a ferrous iron/peroxysulphate (Fe²⁺/PS) system was examined as a representative case. Results showed that DEHP was simultaneously adsorbed onto the solids and degraded by Fe²⁺/PS. The removal processes were governed by the phase distribution of DEHP and Fe²⁺, and the distribution was influenced by interactions between the 1-/14-day solids, Fe²⁺/PS, and various FWW components. Polyacrylamide (PAM), an anionic surfactant commonly detected in FWW, can attach to the solids and aid in maintaining the structural and compositional integrity of the solids. Due to stronger adsorption with PAM, the 14-day solids retained less DEHP and Fe²⁺, contributing to enhanced DEHP degradation in the solution phase as compared to the 1-day solids. Thus, sufficient sedimentation is advantageous for the degradation of recalcitrant organic contaminants (e.g., DEHP) in FWW treatment. Moreover, certain FWW components (i.e., PAM and ethylene glycol) can interact with FWW solids, creating a positive synergistic effect on organic removal efficiency.