| The extensive application of enhancing oil recovery (EOR) technique to oil exploration in China results in significant quantities of wastewater containing polyacrylamide (PAM). The main methods to treat such wastewater are biodegradation, advanced oxidation and adsorption. Fenton catalytic oxidation, one of advanced oxidation processes, is considered as an effective approach to remove PAM. Liquid homogeneous catalysts are typically used in Fenton oxidation, which generally suffers from several drawbacks, such as the loss of catalytic active species, low utilization of H2O2, strict limitation of pH and production of iron-containing sludge. In the case of adsorption method, the bottle neck lies in the relatively low adsorption capacity of adsorbent. Herein, two methods were used to remove PAM from wastewater, one is Fenton oxidation over heterogeneous solid catalyst, the other is adsorption based on molecular zeolites.For heterogeneous Fenton catalytic oxidation, a series of solid catalysts using LDH、γ-Al2O3 and NaY as supports were prepared through impregnation (IM), mixing (MX) and microwave irradiation (MW) methods, respectively. The effects of supports and catalysts preparation methods on the catalytic performance were investigated. Moreover, the reaction conditions including time, temperature, pH, catalyst and H2O2 dosages were also investigated.For adsorption method, various molecular sieves were used to adsorb PAM in wastewater. The effects of molecular sieves type on the adsorption performance were investigated. The results show that Beta exhibits superior adsorption performance. Therefore, a series of Beta zeolites with different physiochemical properties were prepared at conditions to investigate their adsorption performance. Furthermore, the adsorption behavior and mechanism of PAM over Beta zeolite was studied based on the adsorption kinetics and adsorption isotherms. The results are given below.1. The heterogeneous catalyst loading 5% Fe2O3 prepared via MW exhibits high catalytic activity, when Fe(NO3)3 is used as precursor and NaY as support. Under the optimal reaction conditions of reaction time 1 h, temperature 30℃, pH 2-4, catalyst dosage 5 g/L, H2O2 dosage 7 mL/L, the removal ratio can reach 86.6% for the water with PAM concentration of 200 mg/L2. Among Y, ZSM-5 and Beta zeolites, Beta exhibits high adsorption performance due to its high surface area, large pore volume, notable mesopore proportion and opening channel. The adsorption performance of Na-Beta increases with the crystallinity (Rc). During the hydrothermal crystallization process, Re increases with crystallization time and temperature. H-Beta is more active than Na-Beta. PAM can be effectively adsorbed on H-Beta through electrostatic attraction between cations in molecular sieve and anions of PAM, as well as H-bonding between Si-O and Al-OH groups of molecular sieve and amide groups of PAM. The adsorption performance of PAM on Beta decreases with the SiO2/Al2O3 ratio. At the initial PAM concentration in 100-500 mg/L, the adsorption of PAM on H-Beta agrees with pseudo-second-order adsorption kinetic model. The values of active energy suggest that, chemical adsorption predominates at low PAM concentration, while physical adsorption predominates at high PAM concentration. The adsorption isotherm of PAM over H-Beta shows that, in low PAM balance concentration, the isotherm is consistent with Langmuir model and the monolayer saturation adsorption capacity reaches 70.2 mg/g, whereas in high concentration regime, multiple layers adsorption occurs due to the enhancement of hydrophobic associations of PAM. The optimal adsorption conditions are:pH 2-4, adsorbent dosage 5 g/L, adsorption time 4 h. At this conditions, H-Beta can remove 95.2% PAM in the waster water with PAM concentration of 200 mg/L, much higher than some conventional adsorbents including active carbon, active clay, montmorillonite and bentonite. |
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