Development and evaluation of a sulfate precipitation method for separating key flocculated-species from titanium-based flocculant

Abstract

Key flocculated-species is the pivotal factor influencing the effectiveness of flocculants, which in turn directly determines the performance of mainstream wastewater treatment processes. Recovery of titanium-coagulated sludge and high-efficiency of titanium-coagulation have made titanium-coagulants an attractive hot point. However, the separation of key titanium-based flocculated-species remains a critical bottleneck limiting the advancement of titanium-based flocculant chemistry in water treatment. This study developed an efficient method for separating the key flocculated-species, named the sulfate precipitation method, which enables the effective purification of the key titanium-based flocculated-species. The electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) results demonstrated the successful separation of medium and large titanium-hydrolyzed-species. Ti-Ferron synchronous analysis was employed to indicate the separation of hydrolyzed products with different degrees of polymerization, including large, medium, and small species. The distribution of [OH-]/[Ti4+] (basicity) molar ratios was verified to confirm the reorganization of hydrolysis products after the reaction, resulting in varied basicities. Zeta potential results showed that species rich in medium and large hydrolyzed products had fewer positive charges, which was likely attributed to their superior flocculation efficiency, possibly due to sweep flocculation. The inductively coupled plasma mass spectrometry (ICP-MS) indicated that sulfur was present in the separated products. It was speculated that, on the one hand, the introduction of sulfate ions might have directly participated in the re-polymerization of titanium-hydrolyzed-species, potentially resulting in sulfate-doped titanium-based hydrolysis products. On the other hand, sulfate ions might have been involved in a substitution reaction with Cl- at active sites. Co-existence of these two pathways was deemed highly probable. The removal efficiency of organic matter was improved by approximately 20%, possibly owing to the rough surface (Ra = 16.2 nm). Additionally, larger flocs were generated, significantly shortening the sedimentation time in practical applications. This research presents a strategy for isolating key Ti-based flocculated species and establishes a base for their practical application.