High-purity nano particles ZnS production by a simple coupling reaction process of biological reduction and chemical precipitation mediated with EDTA

High-purity nanoparticles ZnS has been successfully synthesized using a simple coupling reaction process of biological reduction and chemical precipitation mediated with EDTA referred to as the CRBRCP-EDTA process. This research investigated the optimum conditions of the transformation of SO ₄^2- into S^2- by SRB, and the production of ZnS in the CRBRCP-EDTA process. The results showed that the molar ratio of Zn ²⁺ to EDTA = 1:1 was crucial for SRB growth and ZnS synthesis. At the ratio_{n(zn2+)/n(EDTA)} = 1:1, lower Zn²⁺ concentration enhanced both the growth of SRB and the reduction of SO₄ ^2-, leading to higher ZnS production. Although increase in Na ₂SO₄ concentration resulted in decrease in both SRB growth and SO₄^2- reduction, it improved the S^2- and ZnS production. Under the optimum conditions (0.05 mol L^-1 ZnCl ₂, 0.05 mol L^-1 EDTA, and 0.1 mol L^-1 Na ₂SO₄), the synthesized ZnS was characterized by X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The analysis showed that the obtained ZnS were high-purity and well-distributed solid spheres with diameters of about 15 nm for primary particles and around 400 nm for secondary particles. When polyacrylamide (PAM) was incorporated in the CRBRCP-EDTA process, the secondary particle's diameters were reduced to less than 100 nm. The photoluminescence (PL) spectra of produced ZnS centered at 396 nm, the spectrum with PAM added showed the gradual increase in absorption and stronger intensity in PL property. The present simple, low-cost, and safe method may be extended to prepare other metal-sulfide nanocomposites.