Ca(OCl)2 and NaOCl react to form hypochlorite and hydroxide ion when added to water
NaOCl + H2O → OCl- + OH- + Na+ (1)
Ca(ClO)2 + 2 H2O → 2 OCl- + 2 OH- + Ca+2 (2)
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 CNF content in the liquid waste is reduced. Further CNCl is oxidized into CNO and finally into CO2 and N2 (Johnson 2015).
The oxidation reaction of cyanide and hypochlorite can be shown as follows
CN- + HOCl → CNCl + OH- (3)
CNCl + OH → CNO- + Cl + H2O (4)
CNO- + OCl- + H2O → N2 + Cl- + HCO3 (5)
In the present study, cyanide treatment by 4% NaOCl and 4% Ca(OCl)2 solution was carried out in one litre BTCPW for 120 minutes. The residual CNF content was checked in every 10 minutes by ion selective electrode. No significant changes were observed in CNF concentration at the beginning of the treatment. But, significant decrease in CNF started after 20 minutes of NaOCl treatment and 30 minutes of Ca(OCl)2 treatment Fig. 2.
Up to 80 % of CNF was removed in 50 minutes of NaOCl treatment. Whereas Ca(OCl)2 treatment took 60 minutes for the removal of 80% of CNF. This result is in line with that reported by Muntasir et al. 2016. In addition to CNF, significant changes were observed in TDS and colour (Fig. 3) 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 has been 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. This is in accordance with the research work reported by Khandaker et al. 2020 and Massoudinejad et al. (2015).
The colour removal and TDS decrease along with the CNF reduction shows scope of enhancement in Ca(OCl)2 treatment efficiency by optimising treatment parameters like pH, treatment time and dosing rate, to get maximum efficiency compared to NaOCl treatment of BTCPW.
Optimisation study
Optimum condition of pH
For optimisation study, BTCPW containing CNF 4.08 ppm, TDS 2970 ppm and colour 2780 PtCo was considered. Treatment of Ca(OCl)2 and NaOCl were carried out in 1 L of BOT water at a constant treatment time of 60 minute. The experiment was done for three different dosing rates (20ml, 30ml and 50 ml) with 4% of NaOCl and 4% Ca(OCl)2 solutions. pH of the solution was varied from 7.5 to 12. Colour and TDS of the solution at different pH were also checked before and after the treatment along with CNF.
It has been found that cyanide removal efficiency of Ca(OCl)2 increases with pH and reaches the maximum at pH 8.5. Further increase in pH has not shown any increase in removal efficiency of CNF with 50ml, 30ml and 20 ml of Ca(OCl)2 solution as shown in Fig. 4a. However, for NaOCl, maximum CNF removal was achieved at pH 10.5 as shown in Fig. 4b.
Maintaining proper pH allows calcium hypochlorite to react perfectly with CNF in wastewater (Cidu et al. 2011 and Muntasir et al. 2016). At optimum pH decrease in the levels of CNF ismaximum. This is in line with the research work reported by Lee and Tiwary (2009). Optimum pH for NaOCl and Ca(OCl)2 treatment in BTCPW has been found as 10.5 and 8.5 respectively through this experiment. As the pH of the BTCPW lies around 7.5 to 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 that during Ca(OCl)2 treatment, colour and TDS content of the treated water were in lower range at pH 8.5 as shown in Fig. 5a and Fig. 5b. For NaOCl treatment colour (Fig. 5a) and TDS (Fig. 5b) content of the treated water varies throughout the pH range. This has again confirmed the optimum pH of Ca(OCl)2 treatment at 8.5.
Optimum condition of dosage and time
Considering the oxidation reaction of CN- by chlorine compound, during which CN- has been changed 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 2 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 waste water (Teixeira et al. 2013).
To attain the best condition for maximum treatment efficiency with removal of CNF to its MPL 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 CNF concentration of the solution reached its MPL (0.2ppm) with minimum time and minimum doses of the hypochlorite solution.
Treatment of NaOCl and Ca(OCl)2 was done at their 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 Fig. 6.
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 CNF 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 (Teixeira et al. 2013).
From the above treatment, it was clear that the maximum removal efficiency of CNF 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 has been carried out at 2 minutes interval with five different volume of doses between 20 ml and 40 ml (20ml, 25ml, 30ml, 35ml and 40 ml). pH was kept constant at 10.5 and 8.5 respectively for NaOCl and Ca(OCl)2.
Result shows that, addition of 35 ml of Ca(OCl)2 up to 62 minutes treatment reduces the residual CNF concentration to 0.2 ppm as shown in Fig. 7a. 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 35ml of NaOCl addition as in Fig. 7b.
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 have been analysed along with CNF (Table 3), to know the changes in water characteristics before and after the treatment.
Table 3: Characteristics of BOT wastewater before and after Ca(OCl)2 treatment
Parameters
|
Before Treatment Mean ± SE*
|
After Treatment Mean ± SE
|
pH
|
8.26 ± 0.03
|
8.25 ± 0.03
|
TDS, ppm
|
2812 ± 15.4
|
2345 ± 11.8
|
Free cyanide, ppm
|
2.75 ± 0.04
|
0.22 ± 0.01
|
Thiocyanate, ppm
|
12.6 ± 0.08
|
3.34 ± 0.02
|
Ammonia, ppm
|
66 ± 1.3
|
56 ± 1.3
|
Colour, (PtCo)
|
2610 ± 3.0
|
1240 ± 3.4
|
*SE: In the table SE stands for standard error.
In the table, data represents mean ±SE (Standard error) of n = 5.
The results presented in Table 3 show 92% CNF removal by calcium hypochlorite with removal of more than 50% of colour and no increase in TDS content. In addition to this, no negative impact was observed in other important parameter like thiocyanate and ammonia content.
Economic aspect of Ca(OCl)2 use over NaOCl
From the economic aspect of cyanide remediation from coke plant waste water, 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 waste water 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 of cyanide removal from coke plant waste water is economically more viable than that of NaOCl.