Enhanced pre-treatment of sepiolite on coal gasification wastewater: Performance and adsorption mechanism

Abstract

As a simple and feasible treatment method, adsorption can be applied in the pretreatment of coal gasification wastewater (CGW) to remove high-concentration contaminants. This study initially selected granular activated carbon (PAC) and different clay materials (sepiolite (SPI), bentonite (BTE), kaolin (KL), diatomite (DTI) and attapulgite (APG)) as adsorbents, and results showed that chemical oxygen demand (COD) removal efficiencies of by PAC, SPI, DTI, KL, BTE and APG were 54.28 %, 44.19 %, 27.15 %, 17.68 %, 16.08 % and 5.58 %, respectively. In contrast, SPI and PAC had high adsorption capacity for high concentrations of organics. Ammonia nitrogen (NH3-N) removal efficiency by SPI exceeded 27 %, while the effective capacity of PAC was only 9.3 %. Moreover, pseudo-second-order kinetics better fitted kinetic data for PAC and SPI, revealing that the adsorption of organic pollutants on SPI is multi-molecular layer adsorption. The Sips model well described the adsorption process of contaminants by SPI, and more remarkably, the maximum adsorption capacity of SPI for COD was 438.4 mg/g. Accordingly, SPI and PAC both had significant removal effects on phenol, 2-methylphenol, 4-methylphenol, catechol and other refractory organic pollutants through gas chromatograph-mass spectrometer (GC-MS). Simultaneously, acute biotoxicity of CGW was greatly reduced after PAC and SPI adsorption treatment. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of strong polar hydration bonds enabled SPI to adsorb organic pollutants through intermolecular forces. SPI still exhibited more than 30 % of COD removal for CGW after 5 cycles. And COD and total phenol (Tph) removal efficiencies in the stable period reached 37.34 % and 31.04 % during the anaerobic process, respectively. Specially, the cost of removing COD per kilogram of SPI is only 2.73 ¥ while the cost of PAC is as high as 26.61 ¥. On the whole, this study provided an important insight for the feasibility application of SPI in the CGW pretreatment process.