Treatment of petrochemical wastewater by acid precipitation and carbon adsorption

Abstract

This paper deals with the physicochemical treatment of petrochemical wastewater emanating from a purified terephthalic acid (PTA) manufacturing unit. The PTA wastewater has a very high chemical-oxygen demand (COD=3,530 mg·dm-3) and contains various acids like terephthalic acid (TA), benzoic acid (BA), acetic acid, and p-toluic acid (p-TA), in addition to aldehydes. The treatment of this wastewater with 1 N sulfuric acid (pH 2.0) at 25°C resulted in the removal of 53.2% of the COD, >90% of TA, and >60% of BA in the form of solid precipitate. Subsequent treatment of the clear supernatant with granular activated carbon (GAC) resulted in an overall removal of >99% of TA and BA in addition to 82.7% of the COD. The GAC is basic in nature with its pH at the point of zero charge being 8.3. It was also predominantly microporous with a BET surface area of 134.69 m2·g-1. Fourier-transform infrared spectroscopy of the GAC, before and after the adsorption of PTA wastewater components, was carried out to understand the COD and acid-removal mechanism. The optimum conditions for the treatment were pH 4.0, GAC dosage of 20 g·dm-3, and ambient temperature. The COD removal mechanism was adsorption and precipitation of undissociated acid molecules within the pores. The adsorption was exothermic in nature and the Freundlich isotherm equation represented the experimental adsorption data well. Thermal regeneration of the spent GAC showed an improvement in the adsorption characteristics with the formation of mesopores, and resulted in the enhanced COD removal during five consecutive adsorption-desorption cycles. The spent carbon can be used as a cofuel for firing in the boiler furnaces for extracting its energy value. © 2014 American Society of Civil Engineers.