An integrated eco-friendly and efficient approach towards heavy metal removal from industrial wastewater and co-generating bio-oil using wheat straw via hydrothermal liquefaction

Abstract

Industrialization and population growth have adversely affected our environment. Heavy metal-contaminated industrial wastewater and the extensive use of fossils fuel are the prime reason for the degradation of the environment. In this research work, removal of heavy metals (Ni, Fe, and Cr) was achieved from synthetically prepared wastewater in a two-step process, i.e., metal-impregnation and hydrothermal liquefaction (HTL). In the first stage, metal removal was investigated via batch impregnation process using wheat straw by varying parameters such as pH (2 − 12), biomass dosages (0.5 – 1.5 g), and contact time (up to 48 h). The maximum metal removal achieved at this stage was 55 – 65 % (Ni – 58.78 ± 1.5 %, Fe – 62.55 ± 1.8 %, and Cr – 63.51 ± 1.7 %). In the second stage, almost complete removal of heavy metals (99.9 %) was accomplished during the hydrothermal liquefaction of metal-loaded wheat straw and filtered wastewater (used as solvent) collected after the biosorption process. The HTL experiment of metal-impregnated wheat straw was performed by varying temperatures, biomass–to–wastewater ratio, and residence time. Nano-metals (oxides/hydroxides) were formed in the 10–50 nm range during the impregnation process, enhancing the HTL product's yield and selectivity. MP-AES analyzed the heavy metals concentration, and XRD confirmed the formation/reduction of nano-metals. TEM and FE-SEM analysis illustrated the particle size and morphology. CHNS, FTIR, and GC-MS analysis identified the heavy and light bio-oil compositions. In the present study, we have addressed the issues of heavy metal removal and fulfilling the energy demand gap via a greener and eco-friendly approach with the concept of waste–treat–waste and waste–to–energy. The present method provides an efficient outcome and can be a sustainable solution for the two prominent issues.