Recycling of detoxification wastewater from Semen Armeniacae Amarum

. Detoxification of Semen Armeniacae Amarum requires large amounts of water, and the detoxifying wastewater contains highly toxic hydrocyanic acid. This paper aims to explore the recovery of water after the removal of cyanide from it by chemical precipitation. The results show that when the dosage of ferrous sulfate is 1.76 times of the theoretical value, the cyanide removal in water can be reduced to about 2.70×10-5mol/L, the hydrocyanic acid in Semen Armeniacae Amarum can be entirely removed when the treated water is recycled.


Introduction
Almonds and almond products are appearing more regularly in people's lives as their standard of living increases. Almonds are used in a growing number of table dishes, casual meals and plant-based beverages. Products made from apricots, such as apricot juice, apricot cake, apricot candy, are gaining popularity with consumers. However, amygdalin contained in Semen Armeniacae Amarum can be hydrolyzed to benzaldehyde, glucose, and hydrocyanic acid in the presence of amygdalinase or under acidic conditions 1. Hydrocyanic acid is highly toxic and can cause poisoning and even death in humans and animals. Thus, the detoxification of Semen Armeniacae Amarum is a necessary process. Currently, the processing and detoxification of almonds is mostly done by boiling water cooking, soaking, biological fermentation, etc. These processes are performed by hydrolyzing amygdalin under certain conditions to release cyanide. As a result, large amounts of cyanide will be produced in the detoxified wastewater. There is a high probability of contamination incidents if detoxifying wastewater is directly discharged or improperly treated2. Currently, treatment methods for cyanide-containing wastewater include acidification, recovery, chlorine oxidation, chemical precipitation, electrolysis and microbial treatment [3,4], however, there has been little domestic and international research on the recovery of cyanide-containing wastewater from Semen Armeniacae Amarum detoxification. In the chemical precipitation method, ferrous sulfate is used as a precipitator, and the excess ferrous sulfate forms a bright blue Prussian blue precipitate in water with cyanide. In this process, cyanide will initially react with ferrous ions to form ferricyanide ions, and excess ferrous sulfate will eventually reduce the generated ferricyanide ions to bright blue ferricyanide precipitate [5,6], This method is simple in operation, low in treatment cost, non-toxic, and widely used in the treatment of cyanide containing wastewater [7,8,9]. Since the wastewater from the detoxification of Semen Armeniacae Amarum does not contain additional heavy metal ions, it is possible to recover the wastewater from the detoxification of Semen Armeniacae Amarum. Based on the above background, the aim of this study is to determine the feasibility and conditions for the recovery of detoxified wastewater from Semen Armeniacae Amarum after removal of cyanide by ferrous sulfate.

Experiment
In this study, the cyanide content of a solution of Semen Armeniacae Amarum immersed in water was determined from the mass ratio of Semen Armeniacae Amarum to the deionized water. The amount of cyanide in the soaking solution was determined using the pyramidal method with isotretinoic acid, and the soaking solution was recovered by treating it with ferrous sulfate.

Drawing of the standard curve of cyanide.
Prepare 1mg/L cyanide standard intermediate solution for cyanide composition analysis reference material (500mg/L) in water, measure 0.00ml, 0.01ml, 0.05ml, 0.10ml, 0.50ml, 1.00ml, 2ml, 3ml, 4ml and 5ml cyanide standard intermediate solution respectively with pipette, and put them into 25ml volumetric flask, use isonicotinic acid precision method to determine cyanide content in sample tube, and draw standard curve diagram.

Poison removal by soaking bitter almonds.
Weigh an appropriate amount of bitter almond, add deionized water, prepare a bitter almond soaking solution with a mass ratio of 1:15, change the water every 12 hours at a certain temperature, store the soaking solutions, respectively, and determine the cyanide ion content in the soaking solution by using the isonicotinic acid precision method.

The removal of cyanide in a soaking solution.
120 ml of the above soaking solution of bitter almonds was boiled for 12h, various doses of ferrous sulfate were added according to stoichiometric calculations, stirred under different conditions, filtered, and the cyanide ion content of the soaking solution was determined by the method of pyrazolone of isotretinoic acid.

Recycling soaking water.
Recycling of the soaking solution is explored by taking the treated soaking solution and reusing it under certain conditions.

Standard curve
The standard curves for cyanide are shown in figure. 1 below. Taking the cyanide concentration (mg/L) in the standard sample as the abscissa and the absorbance (Abs) as the ordinate, the standard curve y=0.1439x+0.0134 is obtained, where the correlation coefficient R 2 =0.9992, and the absolute value of the two independent determination results obtained under repeated conditions does not exceed 10% of the arithmetic mean. It shows that the linear equation obtained has excellent linear correlation, and the test method is reliable and effective.

Hydrocyanic acid concentration in different soaking solutions.
At room temperature, the temperature of ionized water is 20 ℃, and the content of hydrocyanic acid in the replaced immersion solution is determined. The results are shown in Table 2: With the replacement of immersion solution, the content of cyanide in the water is different, and with the increase of replacement times, the concentration of cyanide in the immersion solution changes from 1.09×10 -3 mol/L gradually approaches zero. The results showed that Semen Armeniacae Amarum was effective at releasing hydrocyanic acid through soaking, but produced large amounts of cyanide-containing wastewater while using large amounts of water.

The effect of temperature.
Amygdalin releases hydrocyanic acid under conditions of enzyme catalysis or weak acidity. The activity of the enzyme and the stability of the hydrocyanic acid are easily influenced by temperature. Therefore, it is necessary to soak for 12h at 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃ respectively. The results are shown in Table 1. As the temperature increases, the concentration of cyanide in the immersed solution varies. When the temperature exceeds 35 ℃, the concentration of hydrocyanic acid in the soaking solution starts to decrease. It shows that the stability of hydrocyanic acid is extremely sensitive to temperature under acidic conditions. When the temperature exceeds 35 wen ℃, the hydrocyanic acid begins to volatilize. However, a proper increase in temperature can increase enzyme activity and improve the soaking effect. Therefore, we will soak Semen Armeniacae Amarum at 30 ℃ for detoxification.

Recycling of soaking solution
It is known that ferrous ions can react with cyanide ions, and the generated complexes can be removed in the form of precipitation, which provides a theoretical basis for the removal of cyanide in water and the recycling of soaking solution.

Dosage of ferrous sulfate.
As mentioned above, for the cyanide ion of about 1.15×10 -3 mol/L, theoretically about 158mg/L of ferrous sulfate is needed, but theoretically, ferrous ion is not enough to complexe with all cyanide ions [9], Therefore, at 20 ℃, we use 158mg/L of ferrous sulfate as the starting dose of precipitator, and the interval between other doses is 30mg/L, add excessive ferrous sulfate, and use sodium hydroxide and dilute sulfuric acid to adjust the acidity and alkalinity of the solution before and after reaction, React under magnetic stirring for 30min, and filter after standing for 30min. The results are shown in Table 2: the theoretical addition of ferrous sulfate is not sufficient to entirely remove the cyanide. Within a certain range, the removal efficiency of cyanide increases continuously with the amount of ferrous sulfate. Treatment is best when ferrous sulfate is used in amounts up to 278 mg/L. Beckenn 10 suggested that in order to effectively remove cyanide, the molar ratio of Fe: CN should be 16, which is unnecessary, because this will lead to elevated consumption of the precipitant used. It is essential to note that an excess of ferrous sulfate also precipitates ferrous hydroxide, which plays a role in the precipitation process as an adsorbent.

Reaction time.
The reaction of ferrous sulfate with cyanide to produce precipitate is a chemical reaction process. Therefore, at 20 ℃, the immersion solution with 278mg/L ferrous sulfate was stirred for 15min, 30min, 45min, 60min, 75min, and left for 30min. The results are shown in Table  4. For a reaction time of 30 min, the concentration of cyanide in the water reaches a minimum. When the stirring time exceeds 60 min, the cyanide concentration increases and it is possible that the mixing time is too long to destroy the resulting Prussian blue. Therefore, we set the reaction time as 30min.

The effect of temperature.
To explore the effect of temperature on the removal of cyanide by ferrous sulfate, an immersed solution of 278mg/L ferrous sulfate was stirred for 30min at various temperatures. The results are shown in Table 5: within a certain temperature range, the resulting Prussian blue precipitate is extremely stable and can effectively reduce the cyanide concentration in the immersed solution. Therefore, we react at 20 ℃. To sum up, at 20 ℃, adding 278mg/L of ferrous sulfate, stirring reaction for 30min, the content of cyanide ion in the soaking solution is reduced to about 2.54×10 -5 mol/L, the cyanide ion in the soaking solution is removed effectively, and the generated precipitation is stable, which makes it possible to recycle the soaking solution.

Reuse the soaking water.
Next, explore the recycling of the soaking solution: take 15g of unsinked Semen Armeniacae Amarum, add deionized water to form a mass ratio of 1:30, soak at 30 ℃, switch the water every 12 hours, treat the first soaking solution under the best conditions, and then recycle it. The acidity and alkalinity of the solution were adjusted before and after reaction with sodium hydroxide and diluted sulfuric acid. It is essential to note that the amount of immersion solution lost due to each cyanide content determination, precipitation and filtration operation is supplemented by the same volume of deionized water.
The results are given in Table 6 below: after applying ferrous sulfate to the immersed solution, the multiplexing is excellent and the concentration of cyanide in the immersed solution is continuously reduced. To investigate whether the hydrocyanic acid in the Semen Armeniacae Amarum had been completely removed, the Semen Armeniacae Amarum that had been soaked seven times in a circular manner was again immersed in deionized water. After the Semen Armeniacae Amarums have been soaked seven times in a cycle, they are re-soaked and cleaned with deionized water. The results are presented in Table 7 below. Semen Armeniacae Amarum is nearly free from hydrocyanic acid after one cycle of soaking. The traces of cyanide in the first soaking solution should be the same as those in the soaking solution of Semen Armeniacae Amarum adhered to, and may be effectually removed by soaking and cleaning. It can be seen that the soaking solution can be recovered after treatment with ferrous sulfate.

Conclusion
After the cyanide in the detoxification wastewater of Semen Armeniacae Amarums is adsorbed by ferrous sulfate heptahydrate, the cyanide concentration in the soaking solution can be reduced to about 2.70×10-5mol/L, which is excellent for the recycling use of the treated soaking solution in the detoxification of Semen Armeniacae Amarums soaking. After seven cycles of cyclic soaking, most of the hydrocyanic acid in Semen Armeniacae Amarums has been removed, the ferrous sulfate heptahydrate is inexpensive, the treatment is simple and effective, and it can be used as a reference for the detoxification process of Semen Armeniacae Amarums.