Precipitation of metal sulfides through nuclei-induced mechanism to prevent metal coagulant substitution.

The treatment of heavy metal-laden wastewater commonly involves precipitating metal ions using sulfides and inorganic coagulants (Fe3⁺/Al3⁺). However, the coagulants substitute metal ions from the generated sludge, leading to the re-dissolution of the captured metals. This study introduces hollow glass microspheres (HGM), a silica-based nucleus, to inhibit substitution by inducing the precipitation of metal sulfides. The formation of crystalline metal sulfide through heterogeneous nucleation, combined with the "push-pull" effect of sulfide (S2-) and Fe3+/Al3+ by HGM nuclei, prevents coagulants from attacking metal sulfides. For Ni treatment, HGM inhibited 95 % of Ni substitution from NiS and removed 99 % of Ni2⁺ from the effluent. HGM also enhanced the solid-liquid separation properties of NiS particles, increasing their self-floating velocity by 22 times compared to conventional sedimentation. The substitution inhibition effect and the improved separation performance by HGM are also applicable to other metals like Cu2+, Zn2+, Co2+, and Mn2+. Utilizing the rapid response of the self-floating system, we developed an integrated precipitation-coagulation reactor without stirring, which removed 97 % of Ni and retains 88 % of sludge simultaneously. This research offers a new strategy for optimizing the use of Fe³ ⁺/Al³ ⁺ coagulants in heavy metal treatment, supported by the role of silica-based nuclei in inhibiting ion substitution.