A review of the environmental impact of nanomaterial synthesis using continuous flow hydrothermal synthesis

Continuous flow hydrothermal synthesis (CFHS) is an emerging process technology for the manufacture of nanomaterials. Recent work has been undertaken to establish the green 'credentials' of CFHS amongst alternative techniques for nanosynthesis. In this paper we review how the production of silver sulfide (Ag2S) nanoparticles using CFHS is possible at 400 °C and how the environmental impact compares to other production methods. α-Ag2S nanoparticles were produced with a particle size <50 nm and an ex-ante environmental assessment was carried out (>1000 tonnes per year). The total greenhouse gas emissions (GHG) for the industrial production of 1 kg of Ag2S nanoparticles are 68.60 kg CO2-eq per kg of Ag2S nanoparticles, with the silver nitrate precursor being the main contributor CO2 emissions (∼81%). This review shows how the environmental impact of AgS production at full scale is predominantly through the production of the precursors themselves (i.e. environmental legacy) and not the CHFS itself.