The present study is aimed to reduce the environmental footprints resulted from chromite ore processing residues (COPR); a hazardous waste having Cr(VI). The solidification/stabilization of COPR through geopolymer coupled with green tea synthesized nano zerovalent iron (GT-NZVI) is an effective approach to deal with this challenge. Therefore, the blast furnace slag and metakaolin were used to prepare the composite based geopolymer and GT-NZVI particles were synthesized by oolong tea in current experiment. The GT-NZVI treated and untreated COPR was solidified in composite geopolymer. The efficiency of solidified products was evaluated through compressive strength and leaching analysis. The results depicted that varying sizes of GT-NZVI particles were successively synthesized which could be utilized for reduction of Cr(VI) existed in COPR. The solidified products having GT-NZVI treated COPR (GCM (GTNZVI) and untreated COPR (GCM) had compressive strength of 33Mpa and 47 MPa, respectively up to 50% addition of COPR waste. The immobilization effect of GCM (GT-NZVI) samples were better than GCM samples. The leaching toxicity of Cr(VI) in both GCM (GT-NZVI) and GCM samples was far below than safe limits(<5 mg/L). In addition, the fourier transform infrared spectrometry, X-Ray diffraction, scanning electron microscope equipped with energy dispersive spectrometer analysis had verified that COPR was effectively solidified in composite based geopolymer by physical and chemical means.

Introduction

In start of 19th century, the chromium salt production has gained popularity in industrial sectors due to its use in leather tanning, stainless steel production, metallurgy, dyestuff, pigment, metal polishing, wood preservative etc. (Lehoux et al., 2017). Basically, the chromium salt is produced by two ways which includes high lime and non-lime process. High lime process is considered as outdated because each ton of Cr produces 2e3 t of hazardous solid waste called chromium ore processing residues (COPR). The COPR is a hazardous waste due to presence of hexavalent chromium/Cr(VI) which is highly mobile and carcinogenic in nature. Therefore, COPR production through high lime process is prohibited in UK and USA. While, Pakistan, China, India and Russia are still using high lime process and facing the problems associated with COPR (Yu et al., 2012). The COPR associated problems are; (a) high utilization of lands during its storage as heaps and (b) increase in leaching concentration results in contamination of ecosystem (Dermatas et al., 2006). Hence, resourceful utilization/remediation of COPR is necessary which can prevent the leaching and contamination of Cr(VI). Generally, Cr(VI) remediation technologies are classified into three broad categories i.e. toxicity reduction, removal and contaminants. More specifically, the researchers had remediated Cr(VI) in COPR waste through wet detoxification, high temperature reduction, electro-kinetic remediation and solidification/stabilization (Asavapisit et al., 2005; Lehoux et al., 2017; Xu et al., 2011; Yoon et al., 2010). Among these technologies, solidification/stabilization is regarded as one of best available remediation technology that had been widely used for treatment of 57 types of hazardous wastes since 1970s (Batchelor, 2006).