Remediation of cyanide in biologically treated coke plant wastewater by chemical treatment method

ABSTRACT Biological treatment with a stable activated sludge process, followed by chemical treatment is one of the potential and accepted cyanide remediation processes for coke plant wastewater treatment. Biologically treated coke plant wastewater contains free cyanide above permissible limit. Presently, chemical treatment with NaOCl is being used to attenuate free cyanide below permissible limit in biologically treated water. This process increases the TDS and colour content in discharge water. Ca(OCl)2 can be used as an alternative to NaOCl for cyanide remediation in biologically treated coke plant wastewater without increasing TDS. In the present work, cyanide removal efficiency of NaOCl and Ca(OCl)2 for real coke plant wastewater after biological treatment has been studied. Optimisation of chemical dosage, treatment time and pH study has been done for Ca(OCl)2 and NaOCl treatment. It was found that up to 90% of free cyanide removal could be achieved through Ca(OCl)2 treatment without increasing the TDS value. In addition, more than 50% colour of the wastewater was removed. pH elevation step required in NaOCl treatment can be eliminated in Ca(OCl)2 treatment, thereby reducing caustic consumption. The study indicated that the use of Ca(OCl)2 is economically more viable than that of NaOCl in cyanide treatment.


Introduction
Integrated steel plant is a water intensive industry, where almost every process consumes a huge amount of water.Wastewater from steel industry operation transport various organic, inorganic pollutants and toxic substances that have adverse effects on the environment [1].Due to the toxic effects, discharge of this polluted water without detoxification causes severe damage to the environment.Coke plant is one of the major contributors for generation of Wastewater in an integrated steel plant.Nearly 4 m 3 of water is used to produce 1 ton of coke, and the process generates a large volume of heavily polluted wastewater [2].Coke plant wastewater is generated during quenching, cleaning and recovery of valuable by-product of coke oven gas produced during carbonisation of coal in the coke oven batteries.The wastewater generated during these processes contains various organic and inorganic toxic compounds like ammonia, thiocyanate, cyanides, sulphides, pyridine, phenols and other poly aromatic hydrocarbons (PAH) [3,4].Cyanide is the most toxic chemical present in the coke plant discharge water among these pollutants [5,6].The wastewater from the coke plant and blast furnace blowdown have been identified as the major contributors of aqueous cyanide emissions in iron and steel industries [7,8].Thus, steel industry is under increasing scrutiny of environmental regulators to meet more stringent water discharge limits.
Metal-complexed cyanides are classified according to the strength of the metal cyanide bond.Cyanide in wastewater are classified into three types: a. free cyanide (CN F ), which include CN-and HCN; b. weak acid dissociable (CN WAD ), weak cyanide complexes with metals such as copper, nickel, cadmium and zinc; c. strong-acid dissociable (CN SAD ), strong cyanide-complexes with metals such as gold, cobalt, iron and silver.Toxicity of these cyanides are in the order of CN F > CN WAD > CN SAD [9].All cyanides are classified as hazardous with respect to the characteristics of acute and chronic toxicity [10].But the legislation of the Government for the cyanide discharge deals with only CN F , as it is the most toxic to living elements and sometimes deadly in nature.The discharge limit of CN F to the environment is up to 0.2 ppm [11] as guided by Central Pollution Control Board of India.
Because of the environmental concerns and potential hazards due to the toxic effects, control and remediation of cyanide containing industrial wastewater is essential.There are several physical, chemical and biological treatment methodologies available for cyanide removal in coke plant wastewater.But all the processes have some limitations for implementation in the plant scale.SM Table A1 presents some of the commonly adopted cyanide treatment technologies and their limitations.It also shows the fate of cyanide after the treatment and the reagents used in each process.
To overcome these limitations, different combined treatment methodologies are used for the remediation of cyanide in coke plant wastewater.Combined treatment of biological, followed by chemical treatment, is one of the potential and accepted cyanide treatment processes for coke plant wastewater [2].Present process uses the biological treatment through a stable activated sludge process followed by chemical treatment with sodium hypochlorite.The treatment process is schematically represented in SM Fig. A2.
After biological treatment, the discharge water contains high amount of CN F , which is taken care by sodium hypochlorite (NaOCl) treatment.In this chemical treatment, oxidation transforms CN F to cyanate (CNO − ), posing as an environmental hazard 1,000 times lower than cyanide [12].However, this popular treatment method has the major disadvantage of increasing TDS content in the discharge water [13,14] and is also expensive due to the extraneous dosing of sodium hydroxide [15].Moreover, biologically treated water gets more intense dark brown colour than the untreated water due to the presence of degraded phenolic compounds [16].This acute colour cannot be taken care by NaOCl treatment.Calcium hypochlorite [Ca(OCl) 2 ] can be used as an alternative chemical for reduction of cyanide in biologically treated coke plant wastewater (BTCPW), which decrease TDS and colour along with CN F content.Ca(OCl) 2 is more stable and potentially more efficient for cyanide removal as compared to the NaOCl by virtue of having higher available chlorine [17].Powdered Ca(OCl) 2 has the highest oxidation power among other chemicals used for chlorination [18].Moreover, removal of cyanide through Ca(OCl) 2 treatment is more effective considering the dosage and cost [19].Incidentally, very few studies have been reported on the application of this method to biologically treated coke plant effluent.
Present research aims to develop a continuous, effective and economical process by using Ca(OCl) 2 in cyanide treatment process of BTCPW.Comparative study of cyanide removal efficiency in BTCPW has been carried out with Ca(OCl) 2 and NaOCl.TDS and colour of the treated solution were also compared in both the treatments.In addition to this, optimisation of Ca(OCl) 2 treatment process by changing chemical dosage, treatment time and pH of the treatment solution has been carried out.Lab scale trial of Ca(OCl) 2 treatment in BTCPW was also performed in optimum conditions and the results are presented.

Collection of effluent samples
For the present study, effluent samples were collected from Biological Oxygen Treatment (BOT) plant of coke plant in an integrated steel plant situated in the eastern part of India.This plant produces approximately 9 MT of coke per day and about 200 m 3 of wastewater is generated daily during the process of coke making.

Characterisation of coke plant wastewater
The water samples collected from coke plant after biological treatment have the characteristics, as shown in Table 1.

Experimental setup and procedure
The experiment for optimisation study was carried out in 1 L biologically treated water from coke plant in a glass beaker.Water samples were taken in a beaker, and the pH was adjusted with sodium hydroxide and dilute hydrochloric acid.For lab scale trial, 5 L of BTCPW was taken and treated with Ca(OCl) 2 at its optimised condition.After maintaining the pH, 4% Ca(OCl) 2 was added drop wise from the burette.The samples were kept in continuous stirring condition up to the end of the treatment.Cyanide content was tested at every 10-min time interval after filtration through Whatman No. 1 paper.Each experiment was performed in duplicate, and the average of the results has been reported.All parameters including CN F were tested in triplicate throughout the study, and average value has been reported.

Analysis of pH
pH of the water samples was measured using the pH metre (Systronics, India, digital pH metre, model no: 335).

Analysis of TDS
TDS is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionised or micro-granular suspended form [20].
During the study, TDS of the water samples has been measured by using TDS metre (Systronics, India, model no: 308).

Determination of free cyanide concentration by ion-selective method
Free cyanide was determined by filtering water sample through Whatman No. 1 filter paper and taking 10 mL of the filtered water for further analysis.It was analysed potentiometrically using Ion-Selective cyanide electrode (Thermo Scientific) according to the procedure given by manufacturer.Simultaneous tests can be done through this method, due to the low analysis time (about 5 min).The cyanide electrode was calibrated using the standard cyanide solutions at pH 10.0 (as per the manufacturer guideline).Calibrated cyanide probe was used for direct determination of cyanide.

Determination of ammonia concentration using ion-selective electrode
Ammonia was measured potentiometrically with ammonia Ion-Selective electrode (Thermo Scientific) according to the procedure given by manufacturer.

Determination of thiocyanate concentration spectrophotometrically
Thiocyanate concentration in ppm was measured using spectrophotometer (Thermo Scientific, Genesys 10S UV-VIS spectrophotometer).The wavelength used was 460 nm.This was performed after development of a blood red colour using ferric nitrate [Fe(NO 3 ) 3 .9H 2 O] solution according to the standard procedure [21].

Measurement of colour
Coke plant water gets more intense brown colour after biological treatment due to the presence of aromatic-coloured compounds such as ortho-and para-benzoquinone, which form through the degradation of phenol [16].The colour of the wastewater was measured by the colour measuring instrument (Make: Lovibond) and expressed in PtCo unit.

Chemicals and reagents
Sodium hypochlorite (NaOCl) and calcium hypochlorite (Ca(OCl) 2 ) of analytical reagent (AR) grade as obtained from Merck.Ultra-pure distilled water was used for making the solution and other use throughout the study.All other chemicals used in this study were that of AR grade.

Statistical analysis
The mean number of pH, TDS, and concentration of CN F , Thiocyanate, ammonia, colour of BOT water were calculated before and after the experiment and subjected to Student's t-test, and significant differences were calculated between treatments.Standard errors of the mean were calculated and presented for the above parameters before and after treatment.

Results and discussion
Ca(OCl) 2 and NaOCl react to form hypochlorite and hydroxide ion when added to water Hypochlorite ion (OCl − ) oxidises CN − in BTCPW to Cyanogen Chloride (CNCl).More the addition of Ca(OCl) 2 , the greater the cyanide reacts to form CNCl; therefore, the CN F content in the liquid waste is reduced.Further CNCl is oxidised into CNO and finally into CO 2 and N 2 [9].The oxidation reaction of cyanide and hypochlorite can be shown as follows

Treatment with sodium and calcium hypochlorite treatment (Ca(OCl) 2 and NaOCl)
In the present study, cyanide treatment by 4% NaOCl and 4% Ca(OCl) 2 solution was carried out in 1 litre BTCPW for 120 minutes.The residual CN F content was checked every 10 minutes by Ion-Selective electrode.No significant changes were observed in CN F concentration at the beginning of the treatment.But, significant decrease in CN F started after 20 minutes of NaOCl treatment and 30 minutes of Ca(OCl) 2 treatment, as shown in Figure 1.Up to 80% of CN F was removed in 50 minutes of NaOCl treatment, whereas Ca(OCl) 2 treatment took 60 minutes for the removal of 80% of CN F .This result is in line with the research work reported by Muntasir et al. (2016) [22] where optimisation of Ca(OCl) 2 treatment in gold mine wastewater has been reported as 60 minutes of contact time at a pH of 8.
In addition to CN F , significant changes were observed in TDS and colour (Figure 2) of the treated water.It was found that TDS content was increased in NaOCl treatment and decreased in the case of Ca(OCl) 2 treatment.Similar trend was observed in case of colour, where more than 55% colour was removed in Ca(OCl) 2 treatment against the increase in colour for NaOCl treatment.Similar work has been reported by Khandaker et al. (2020) and Massoudinejad et al. (2015) [23,24] in their research work, where significant removal of colour in textile industry wastewater was done by treatment with Ca(OCl) 2 .

Optimisation study
As per above experimental study, the colour removal and TDS decrease along with the CN F reduction therefore shows scope of enhancement in Ca(OCl) 2 treatment efficiency.Parameters like pH, treatment time and dosing rate can be optimised to get maximum efficiency of Ca(OCl) 2 treatment compared to NaOCl treatment in BTCPW.Maintaining proper pH allows calcium hypochlorite to react perfectly with CN F in wastewater [22,25].It has been found that the decrease in the CN F is maximum at optimum pH.This is in line with the research work reported by Lee and Tiwari (2009) [26].Optimum pH for NaOCl and Ca(OCl) 2 treatment in BTCPW has been found as 10.5 (Figure 3) and 8.5 (Figure 4), respectively, through the current experiment.As the pH of the BTCPW lies around 7.5-8.5,elevation of solution pH up to 10.5 is required in NaOCl treatment to get maximum efficiency.Whereas Ca(OCl) 2 treatment does not require such elevation of pH.It was also found during Ca(OCl) 2 treatment, that the colour and TDS content of the treated water were in lower range at pH 8.5, as shown in Figure 5 and Figure 6.For NaOCl treatment colour (Figure 5) and TDS (Figure 6) content of the treated water varied throughout the pH range.This has again confirmed the optimum pH of Ca(OCl) 2 treatment at 8.5.

Optimum condition for dosage and time
Considering the oxidation reaction of CN − by chlorine compound, during which CN − was oxidised to CNO − , hypochlorite ion (OCl − ) is the active chlorine group in the oxidation process.This reaction can be slow, from 30 minutes to 2 hours.The Ca(OCl) 2 has two groups of OCl − , hence more effective in oxidation than NaOCl.The optimum condition is achieved by the equilibrium between the volume of Ca(OCl) 2 solution added and the cyanide content in the wastewater [27].To attain the best condition for maximum treatment efficiency with removal of CN F to its MPL (maximum permissible limit) and simultaneous removal of colour from BTCPW, different experiments were carried with NaOCl and Ca(OCl) 2 solutions.Condition was assumed to be optimum when the residual CN F concentration of the solution reached its MPL (0.2 ppm) with minimum time and minimum doses of the hypochlorite solution.
Treatment of NaOCl and Ca(OCl) 2 was done at optimum pH (10.5 and 8.5, respectively) for 60-minute reaction time to find out the dosage at which both methods are at their maximum efficiency level.The results are as shown in Figure 7.
From the above experiment, it was found that cyanide removal efficiency increases as the volume of NaOCl and Ca(OCl) 2 solution is increased.The removal rate was faster up to addition of 30 ml of hypochlorite solution and then gets slower.No significant changes were observed after the addition of 40 ml of the solution.This may be due to the faster reaction of CN F with OCl − to form CNCl and thereby reducing the CN − at higher concentration of OCl − in the initial stage of the treatment.The reaction gets slower as the concentration of hypochlorite ion is decreased for both (NaOCl and Ca(OCl) 2 ) treatment [27].
From the above treatment, it was clear that the maximum removal efficiency of CN F lies in between 30 and 40 ml of the NaOCl or Ca(OCl) 2 solution.To find out the exact dose and more precise condition, experiment was carried out at 2 minutes interval with five different volume of doses between 20 ml and 40 ml (20 ml, 25 ml, 30 ml, 35 ml and 40 ml).pH was kept constant at 10.5 and 8.5, respectively, for NaOCl and Ca(OCl) 2 .
Results show that, addition of 35 ml of Ca(OCl) 2 up to 62 minutes treatment reduces the residual CN F concentration to 0.2 ppm, as shown in Figure 8, which is the optimum condition for Ca(OCl) 2 treatment of BTCPW.Whereas optimum condition for NaOCl reached at 58 minutes of treatment time and 35 ml of NaOCl addition, as in Figure 9.

Lab scale trial
After the completion of the optimisation study, 5 litre BTCPW water was treated with 4% Ca(OCl) 2 solution at its optimum dose of (35 ml or 1.4 gm per litre) and treatment time (62 min).Physico-chemical parameters like pH, TDS, thiocyanate, ammonia and colour were analysed along with CN F (Table 2), to know the changes in water characteristics before and after the treatment.
In the table, data represents mean ±SE (Standard error) of n = 5.
The results presented in Table 2 show 92% CN F removal by calcium hypochlorite along with removal of more than 50% of colour without any increase in TDS content.In addition to this, no negative impact was observed in other important parameters like thiocyanate and ammonia content.

Economic aspect of Ca(OCl) 2 use over NaOCl
From the economic aspect of cyanide remediation from coke plant wastewater, the cost can be calculated for optimum dose of the two treatment processes.Using the price of 1 gm NaOCl (Rs 0.55) and 1 gm Ca(OCl) 2 (Rs 0.27), the cost of cyanide removal for one litre of wastewater can be computed as Rs 0.77 and Rs 0.38 for NaOCl and Ca(OCl) 2 , respectively.In addition to this, cost of pH elevation step is required in NaOCl treatment where as it is not required in Ca(OCl) 2 treatment.Therefore, use of Ca(OCl) 2 in cyanide removal from coke plant wastewater is economically more viable than that of NaOCl.

Conclusion
The removal of cyanide from steel industrial wastewater is possible using either NaOCl or Ca(OCl) 2 .The optimum condition for cyanide remediation in BTCPW with NaOCl or Ca(OCl) 2 treatment was established through experimental procedure.Ca(OCl) 2 could be more effective than NaOCl considering the cost, TDS and colour removal from the coke plant wastewater.There is no requirement of pH adjustment in case of Ca(OCl) 2 treatment as the optimum condition is close to the original pH of the feed water, thereby reducing expensive caustic consumption.The experiment showed that more than 90% removal of CN F could be achieved along with more than 50% reduction of colour by optimising calcium hypochlorite treatment.Therefore, Ca(OCl) 2 can be one of the promising chemical treatment method for reduction of cyanide treatment without increasing the TDS value of the Coke plant wastewater after biological treatment.

Figure 1 .Figure 2 .
Figure 1.Changes in residual CN F content with time during NaOCl and Ca(OCl) 2 treatment.

Figure 3 .Figure 4 .
Figure 3. Optimization of pH on cyanide removal at different concentration of NaOCl.

5 Figure 7 .
Figure 7. Changes in CN F content with dosage of NaOCl and Ca(OCl) 2 at their optimum pH.

Figure 9 .
Figure 9. Optimization of time and dosage for NaOCl treatment.

Table 1 .
General characteristics of Coke plant wastewater after biological treatment.

Table 2 .
Characteristics of BOT wastewater before and after Ca(OCl) 2 treatment.In the table SE stands for standard error.