Kill two birds with one stone: Production of nanoscale CaCO3 and FeS2 from red gypsum via microbial degradation

Abstract

Red gypsum (RG) poses significant challenges to the sustainable development of enterprises due to its complex composition. This study aimed to evaluate the utilization of RG through microbial degradation, leading to the production of high-value products. Three anaerobic digestion (AD) systems were developed, with papermaking wastewater (PPW) serving as the carbon source and RG providing sulfur and calcium. The results demonstrated that adding 3 g/L of RG to the AD system (RG-3) enhanced biomethane production and improved the utilization of organic matter. As sulfate (SO₄^2-) in RG was reduced to H₂S, Ca²⁺ was released into the digestion liquid and reacted with CO₂ generated during wastewater degradation, forming calcium carbonate (CaCO₃) nanoparticles. The methanogenesis via CO₂ reduction metabolism was inhibited. In the RG-3 AD system, the abundance of genes (Mt) responsible for methyl transfer to form methyl-CoM increased fourfold, resulting in a 32% increase in biomethane yield. Furthermore, the abundance of genes (aprA、aprB) responsible for direct sulfite synthesis increased sixfold, achieving an almost 100% sulfate conversion rate. RG promoted mutual interactions between methylotrophic methanogens and sulfate-reducing bacteria in the RG-3 AD system. Pyrite (FeS₂) nanoparticles was successfully synthesized using the gas and liquid produced from microbial degradation of RG. This study, for the first time, proposes a treatment strategy that converts RG into nanoscale CaCO₃ and FeS₂ while facilitating initial degradation of wastewater via AD.