| Coking wastewater is a typical kind of industrial organic wastewater containing high concentration pollutants and toxic substances that are hard biodegradable. After treatment by a biological process, the coking-plant effluent still retains ammonium nitrogen (NH3-N) and non-degraded organic compounds such as long-chain hydrocarbons, benzene series and heterocyclic compounds which can do harm to human health and ecological balance seriously. In the paper, zeolite with high adsorption property was synthesized by hydrothermal conversion from coal fly ash that originally is a kind of industrial waste. The synthesized zeolite was used as adsorbent to depurate coking wastewater. Meanwhile, adsorption effect, adsorption behavior and kinetic characteristics of chemical oxygen demand (COD) and NH3-N on zeolite were investigated successively.Results from hydrothermal conversion experiments of fly ash into zeolite indicated:(1) Cremation temperature affected removal efficiency of the unburned carbon and activation grade of amorphous aluminosilicate in fly ash. Both the conversion rate and product types of zeolite mainly depended on liquid-solid ratio, while both the crystallization degree and types of zeolite were subject to the sodium hydroxide (NaOH) concentration and crystallization time. (2)The cation exchange capacity (CEC) value of synthesized zeolite increased with the enhancement of cremation temperature, NaOH concentration, liquid-solid ratio and crystallization time until a constant, and then the CEC value reduced. Optimum synthesis conditions obtained by experiments were as follows:cremation temperature 700℃, NaOH concentration 1mol·L-1, liquid-solid ratio 5mL·g-1 and crystallization time 36 hours. (3)Compared with raw fly ash, the CEC, removal rate of COD and NH3-N of synthesized zeolite increased 11.8 times,25.7%,32.3 times respectively. Furthermore, both the CEC value and NH3-N removal rate of synthesized zeolite were higher than natural zeolite. Also, synthesized zeolite had even higher CEC value than all the acid or alkali wet modificated or fire modified fly ash, therefore the former had better treatment effect on NH3-N of coking wastewater.The synthesized zeolite was applied to decontaminate the bio-chemical effluent of coking plant. The results show that:(1) The adsorption of COD and NH3-N on synthesized zeolite was a rapid process. More than 95% adsorption capacity completed less than 60min. (2)The removal rate increased as a function of zeolite dosage, reached a maximum value at a dosage of 2g·100mL-1, then the increasing trend weakened. (3)The pH of wastewater rose after zeolite added, and it had no effect on removal of COD. However, the removal of NH3-N was affected by values of pH significantly. When pH was between 6.0 and 9.0, the removal rate of NH3-N by zeolite could reach the maximum of 46%. (4)The adsorption capacity of synthesized zeolite improved when the initial concentration of pollutants increased, though its removal rate decreased simultaneously. (5) The most suitable operation conditions for treatment of coking wastewate which initial concentration was COD 240.0mg-L"1, NH3-N 117.4mg·L-1 were obtined:no pH value regulation to wastewater, reaction time 60min, added dose of zeolite 2g·100mL-1. The removal rate of COD, NH3-N was 17.1%,46.7%, and effluent concentration of coking wastewater were COD 189.9mg·L-1, NH3-N 62.6mg-L"1.Results from adsorption isotherms experiments indicated:(1)Adsorption isotherms of synthesized zeolite and raw fly ash were L type. NH3-N adsorption capacity on synthesized zeolite was higher than that on raw fly ash obviously, and the difference of adsorption capacity increased greatly as a function of NH3-N initial concentration. (2)It was found that adsorption capacity on synthesized zeolite decreased as the temperature of coking wastewater ascended which showed that the adsorption would belong to one kind of exothermic reactions. (3)Both Langmuir and Freundlich models fit the adsorption isotherms of synthesized zeolite. The fitting correlation coefficient of Freundlich model was closer to 1 than that of Langmuir model. Additionally, error between fitted data and measured results of the former was less than that of the latter. Therefore, the Freundlich model could describe the adsorption behavior better than Langmuir model.The adsorption kinetic characteristics of NH3-N on synthesized zeolite were studied. The results showed that the adsorption process confirmed to pseudo-second-order kinetics equation very well. However, the adsorption process was controlled by liquid-film diffusion and intra-particle diffusion jointly. Both the pseudo-second-order rate constant K2 and intra-particle diffusion coefficient D decreased as NH3-N initial concentration enhanced, though intra-particle diffusion rate constant kp increased relatively. Research results put forward a new direction for the reuse of coal fly ash. It could solve problems of land occupancy and environmental pollution caused by coal fly ash, also applied to purification of coking wastewater. This technology of wastewater treatment by synthesized zeolite from fly ash showed the method of waste control by waste with high economical and environmental benefits.
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