Valorisation of Waste Guanidine Thiocyanate into a Guanidinium-Modified Silica Sol/Poly(Vinyl Alcohol) Adhesive: CuCl2/Cu0 Synproportionation Route, Colloidal Stability and Antibacterial Performance

Guanidine thiocyanate (GT), a solid by-product of guanidine-salt manufacture, combines a valuable antimicrobial guanidinium cation with a toxic, oxidation-sensitive thiocyanate anion. Here, both ions are converted into useful products. In stage I, aqueous GT is treated with CuCl2 and metallic copper at 20–30 °C; Cu2+/Cu0 synproportionation (ΔE° = +1.14 V) precipitates β-CuSCN in 92 % yield without thiocyanate oxidation or acid formation, leaving guanidinium chloride (GuCl) in solution. In stage II, GuCl reacts with acrylic acid; speciation analysis (K ≈ 7•10−11) and FTIR show that the product is a mixture of guanidinium salts and partially polymerised acrylic acid rather than guanidinium acrylate. In a formalin-modified poly(vinyl alcohol) (PVA)/silica sol adhesive, the modifier shifts the ζ-potential of silica from −28.6 to +2.5 mV; at the resulting ionic strength (≈0.26 mol kg−1) DLVO theory predicts no electrostatic barrier, so the 120-day stability is attributed to steric stabilisation by adsorbed PVA. Compared with an unmodified PVAc reference, the adhesive shows 44–62 % higher lap-shear strength (5.2–6.8 MPa), lower 24 h water uptake (1.8 vs. 7.6 %) and antibacterial activity that is 91.8 % retained after washing, at a net raw-material cost of ≈2.1 kUSD t−1.