Redefining role of fluoride ions (F^–) in metal recovery: mechanistic insights into electrodeposition kinetics and crystallographic evolution

Abstract

Nickel (Ni) recovery from steel pickling wastewater using electrochemical technology is crucially influenced by interfering ionic species, particularly fluoride ions (F^–), which constitute a predominant fraction of impurities and exert a substantial influence on Ni electrodeposition dynamics. Although traditional approaches often prioritize F⁻ removal due to its corrosiveness and toxicity, F⁻ may conversely enhance Ni recovery during electrodeposition. Nevertheless, the mechanistic role of F⁻ in Ni electrodeposition remains systematically underexplored. In this study, 2000 mg/L F^– enhanced the current efficiency of Ni electrodeposition from 52.75% to 70.76%. Speciation analysis revealed F^–-mediated transformation of free Ni²⁺ into NiF⁺ complexes, with the Ni²⁺/NiF⁺ ratio shifting from 19.4:1 to 0.8:1. Electrochemical and crystallographic characterization identified dual enhancement pathways. 1) The EIS and in situ Raman spectro-electrochemical analyses demonstrated that NiF⁺ is the primary electroactive species, undergoing a two-step electron transfer through adsorbed NiF_ads intermediates. 2) Crystallographic analysis confirmed F^–-induced preferential growth of (220) crystal planes, thereby enhancing Ni(II) reduction. These results indicate that F^– enhances Ni electrodeposition through both electroactive complex formation and crystallographic regulation, redefining its role from pollutant to process enhancer. The practicability and feasibility of electrodeposition technology were validated by its long-term stability and high current efficiency (64.96%) in treating actual wastewater. This study provides a novel perspective that F^– synergistically enhances Ni electrodeposition kinetics, fundamentally challenging the traditional model that prioritizes the removal of F^– in wastewater management.

Graphical abstract

Keywords

Influence mechanism / Fluoride / Nickel recovery / Electrodeposition / Steel pickling wastewater / Crystallographic orientation

Highlight

	● NiF+ forms by complexation and preceding reactions in F–-containing electrolytes.
	● NiF+ acts as the direct charge-transfer species in F–-containing electrolytes.
	● NiF+ exhibits superior electroactivity compared to NiOH+.
	● F– promotes (220) plane exposure by specific adsorption, enhancing Ni(II) reduction.
	● F–-enhanced Ni electrodeposition has substantial application potential.

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