Micro-interfacial Processes Of Steroid Hormones In Excess Sludge

Steroid hormones are widely used in pharmaceuticals for animal growth promotion,immune system regulation,and cancer treatment.As a group of potent environmental endocrine disruptors,steroids can cause abnormalities in endocrine functions,immune system,development and reproduction of organisms even at trace levels.Steroids enter the sewage treatment system via human excretion,and the effluent residues may contaminate surface water and potentially threaten the ecological system.In addition,a wide variety of contaminants including steroids are concentrated in excess sludge which could be a secondary source releasing steroids after ending up in landfill or agricultural field.In order to evaluate and control the ecological risks caused by steroids,it is of great importance to understand the sorption-desorption behavior and micro-interfacial mechanisms of steroids in excess sludge.To address these objectives,the present study investigated the occurrence and distribution characteristics of twenty-two steroids in three wastewater treatment plants of different treatment processes,and identified the main species remaining in the excess sludge;evaluated the sorption-desorption kinetics and isotherms of steroids in excess sludge;selected model organics based on analysis of sludge organic matter composition,and explored the molecular mechanisms of micro-interfacial interaction between steroids and organic surrogates.The main conclusions are as follows:(1)In the influent,fourteen steroids were detected,among which androstenedione(ADD)occurred at a high detection frequency up to 100%and epiandrosterone(EADR)and androsterone at high concentrations of 111.8-184.2 ng/L and 13.2-156.8 ng/L,respectively.In the effluent,thirteen steroids were detected,among which estrone,ADD,and ADR were the main species with concentrations of 0.1-34.4 ng/L,0.1-34.4 ng/L,and 33.4-210.3ng/L,respectively.Nineteen species were detected in the excess sludge,and ADD,EADR,and progesterone were found in all samples.In particular,EADR appeared higher detection frequency and higher concentration than the other species.Glucocorticoids were abundant in sludge samples,and cortisol(CRL)was detected in a range of 0.6-5.3μg/kg,higher than the reported values.Different treatment processes showed different removal capabilities of steroids.Biological treatment degraded most of the target compounds,and A~2O+MBR showed best performance with anaerobic tank and aerobic tank as the main contributors.(2)The sorption-desorption characteristics and micro-interfacial mechanisms of EADR and CRL in excess sludge were studied.Results showed that the pseudo second-order model was more suitable for the sorption kinetics of presentative steroids in excess sludge with a faster sorption rate for EADR than CRL.Freundlich model was more suitable for the sorption isotherm of EADR on the heterogeneous surface of excess sludge and aquatic sediment.Linear model was more suitable for CRL isotherm in excess sludge while both Langmuir and Freundlich models worked for the single-layered sorption of CRL in sediment.The desorption of EADR reached equilibrium at 72 h,however,CRL could not reach equilibrium within the experimental period.Considerable positive hysteresis was observed for both EADR and CRL.Compared with aquatic sediment,excess sludge containing higher content of organic matters showed stronger affinity and hysteresis to steroids.(3)Seven model organic matters,namely fulvic acid,humic acid,bovine serum protein,tyrosine,tryptophan,lignin,and cellulose,were selected to evaluate the interaction between steroids and sludge organic matter.Molecular docking results theoretically explained that hydrogen bond could be a main association mechanism,while van der Waals force,hydrophobic interaction,and electrostatic forces also participated in the binding.Fourier transform-infrared spectra(FT-IR)of organic matters with and without EADR/CRL showed that-OH,carbohydrate C-O or C-O-C,C=C could be the main interactive functional groups involved in the binding of steroids to organic matters.Four binding sites of EADR/CRL to organic matters were detected by hydrogen nuclear magnetic resonance(~1H NMR),which confirmed that hydrogen bond was a main association force.Ultraviolet-visible spectra and fluorescence emission-excitation matrixes showed that the presence of EADR/CRL could change the conformation of organic matters as well as the microenvironment of fluorophores,and the conjugation ofπ-πdouble bonds might also play an important role in the interaction between steroids and sludge organic matter.