Detoxification of pulping black liquor with Pleurotus ostreatus or recombinant Pichia pastoris followed by CuO/TiO2/visible photocatalysis

Abstract

Abstract Cellulose-pulping requires chemicals such as Cl2, ClO2, H2O2, and O2. The black liquor (BL) generated exhibits a high chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD5), and chlorophenol content, along with an augmented colour and increased pH. BL is often discharged into water bodies, where it has a negative impact on the environment. Towards that end, laccases are of great interest for bioremediation, since they can degrade aromatic and non-aromatic compounds while reducing O2 to water instead of H2O2. As such, we evaluated Pleurotus ostreatus and Pichia pastoris (which produces rPOXA 1B laccase) in the treatment of synthetic BL (SBL) in an “in vitro” modified Kraft process followed by CuO/TiO2/visible light photocatalysis. Treating SBL with P. ostreatus viable biomass (VB) followed by CuO/TiO2/visible light photocatalysis resulted in 80.3% COD removal and 70.6% decolourisation. Toxic compounds such as 2-methylphenol, 4-methylphenol, and 2-methoxyphenol were eliminated. Post-treated SBL exhibited low phytotoxicity, as evidenced by a Lactuca sativa L seed germination index (GI) > 50%. Likewise, SBL treatment with P. pastoris followed by VB/CuO/TiO2 /visible light photocatalysis resulted in 63.7% COD removal and 46% decolourisation. Moreover, this treatment resulted in the elimination of most unwanted compounds, with the exception of 4-chlorophenol. The Lactuca sativa L seed GI of the post-treated SBL was 40%, indicating moderate phytotoxicity.