Enhanced carbon adsorption treatment for removing cyanide from coking plant effluent
Introduction
In China today, most existing chemical industry wastewater treatment plants are hard pressed to meet increasingly more stringent effluent discharge limits. There are also urgent needs to recycle well-treated effluents for many beneficial reuse purposes. Relative to more highly developed countries, a much smaller fraction of industrial effluents is being recycled for reuse. Shanghai Coking Plant (SCP) is one of the largest chemical plants in the city with capacities to produce 3.2 million m3/a of manufactured gas, 1.9 million ton/a of coke, 350,000 ton/a of methanol and more than 100 additional products. About 7000 m3/day of wastewater from chemical production, cleaning, washing and other operations is treated in the anaerobic and aerobic biofilm reactors as depicted in Fig. 1. The SCP biotreated effluent does not meet the existing discharge limits for residual organic constituents and total cyanide (CODCr < 50 mg/L and TCN < 0.5 mg/L) [1]. Cost effective post treatment of the SCP effluent is desired to produce a final effluent that may be directly discharged and/or recycled for many reuse functions.
Alkaline chlorination, ozonization, and wet-air oxidation are chemical oxidation methods effective for treating cyanide containing wastewater [2], [3]. The high degree of chlorination required to meet the effluent objectives needs excessive doses of caustic and chlorine or sodium hypochlorite [4] which, in addition to the need for neutralization, would create a safety concern due to the large amounts of residual chemicals and by products [5]. Ozonization is expensive because it is not selective, while wet-air oxidation is only a viable alternative for small-scale applications because of the high temperature and pressure requirements. Other reported treatment methods, such as Caro's acid, copper-catalyzed hydrogen peroxide, electrolytic oxidation, ion exchange, acidification, AVR (acidification, volatilization, and re-neutralization) process, lime-sulfur, reverse osmosis, thermal hydrolysis, and INCO process (by SO2/air) [2], [3], [6], [7] are either too costly or unable to produce an effluent that would meet the discharge limits on both cyanide and organic.
Depending on the influent composition and season of the year, the SCP effluent contains variable concentrations CODCr (100–200 mg/L) and TCN (2–7 mg/L), which are all complex cyanides since any free cyanide would have been stripped by aeration (pKa of HCN = 9.3). Granular activated carbon (GAC) adsorption has been employed for removing both free and complex cyanides present in many industrial wastewaters and that its adsorptive capacities for Cu(CN)42− was much greater than for CN− [8], [9]. Activated carbon functioned both as an adsorbent and as a carrier of catalyst for cyanide oxidation [10], [11], [12], [13]. In the presence of dissolved oxygen (DO), the adsorbed cyanide may be oxidized to CNO− (Eq. (1)) which was hydrolyzed to NH4+ (Eq. (2)); further oxidation of CNO− to form N2 (Eq. (3)) is not expected since about the same total nitrogen concentrations were found in a SCP effluent before and after the copper/sulfite catalyzed oxidation treatment [14]:
Fenton oxidation (Fe2+ catalyzed oxidation by H2O2) is effective for breaking up large organic molecules and complex cyanides of the SCP effluent [15], [16], [17] making it possible for their long-term removal in GAC adsorbers which in effect function as biological activated carbon (BAC) systems, capable of removing both residual organic (COD and UV254) and TCN, in the SCP effluent [18]. Recently, Dash et al. have found the BAC process is more effective than adsorption and biodegradation alone for removing iron cyanide in batch reactors [19].
Loading a transitional metal, such as copper and silver, on the GAC has improved its adsorptive for cyanide due to chemisorption resulting from the interaction between complex ions and the surface groups of GAC and the catalytic oxidation of cyanide by adsorbed oxygen with metal compounds adsorbed on carbon surface as the catalyst [20], [21], [22]. To further improve the capacity of Fenton-BAC system in removing TCN, metal impregnation (loading) of GAC is investigated in this study.
The objectives of this study were to: (1) compare the cyanide removal effectiveness of a coal base activated carbon relative to several metal loaded carbons, (2) identify the best metal fixing method, (3) determine the effects of contact time and dissolved oxygen (DO) on the observed cyanide removal capacities of the batch treatment experiments, (4) validate the micro column rapid breakthrough (MCRB) method for simulating the breakthrough curve of a small carbon adsorber employed for removing TCN from K3Fe(CN)6 solution and effluent samples, (5) demonstrate that the Fenton oxidation/precipitation enhanced carbon adsorption treatment of SCP effluent will produce a high quality effluent that may be directly discharged and/or recycled for beneficial reuses, and (6) illustrate that using some metal impregnated GAC will ensure the adsorber effluent will ensure meeting the effluent discharge limits during the start-up phase of the BAC system for long-term treatment of the SCP effluent.
Section snippets
Materials, instruments and equipment
Since K3Fe(CN)6 is likely the major TCN constituent of a coking plant effluent [23], it was employed to prepare the test solutions and also to maintain a desired TCN concentration of the SCP effluent in the long-term treatment runs. Several batches of SCP effluent samples (pH 6–7, CODCr = 100–150 mg/L, TCN = 1.0–6.5 mg/L) were brought in from the plant during the study period and employed as the feed to the carbon adsorbers after pretreatment by coagulation/flocculation using polymeric ferric sulfate
Effects of metal loading and fixing method on TCN removal
Fig. 2 presents the 1-h TCN removal capacities of 5 carbons (Coal, Ag-GAC-1, Cu-GAC-1, Ni-GAC and Fe-GAC); the data clearly show that metal loading significantly enhanced the TCN removal capacity of the base carbon, consistent with literature reports [10], [20], [21], [34] and that, considering the % loading and cost of metal, copper was the best metal of the four studied. Fig. 3 presents the same comparative removal capacities of 6 Cu loaded carbons with and without chemical fixing; the data
Conclusions
- (a)
Adsorption of cyanide on pulverized activated carbon (45–75 μm) was rapid; the removal in 1 h was a good estimate of its adsorptive capacity.
- (b)
Metal impregnation (loading) of activated carbon enhanced its capacity for cyanide; chemical fixing of the loaded carbon further enhanced the capacity. Both the loading and fixing method affected the capacity enhancement. Copper loaded and KI fixed coal-based activated carbon (Cu/KI- GAC) was the most cost effective of all carbons studied.
- (c)
The observed
Acknowledgements
This research was supported by the National High Technology Research and Development Program of China (2007AA06Z331), National Natural Science Foundation of China (No. 40901148) and Wenruitang River Research Program of Wenzhou (No. Z090921421). We are grateful to Meining Li, Yuan Yu and Yinmei Cheng of Shanghai Coking Co. for providing information on its wastewater treatment plant and the effluent samples and also Youliang Liu of Shanghai Xingchang Activated Carbon Company for providing the
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