Optimization of Efficiency Mercury (Hg) Removal with Electrocoagulation Using Zinc (Zn) Electrode by RSM Methods

. Good and optimal management of the Final Processing Site (TPA) can provide benefits to the community and the surrounding environment. One of the impacts of poor management of the landfill is that the decomposition of waste that occurs at the landfill will produce leachate which will potentially contaminate groundwater. Leachate from landfill can contaminate groundwater if it seeps into the ground and eventually into the groundwater. In one of the landfill’s in Medan City, namely Terjun landfill, it is known that the level of mercury (Hg) in leachate is 0.04012 which is categorized as exceeding the quality standard. Electrocoagulation is a promising treatment technology because it has the potential to remove organic matter and persistent pollutants in landfill leachate. The electrocoagulation process was carried out with variations of the electrocoagulation time used, namely 10, 20, and 30 minutes using zinc electrodes. As well as other variations used, namely the electrode spacing of 1 cm, 2 cm and 3 cm and the voltage of 8 volts, 10 volts and 12 volts. The results showed that the optimal variation was obtained at a distance of 2 cm, 30 minutes, and 12 volts with a mercury reduction efficiency of 98.108%.


Introduction
With the expansion of human activity and industry, such as the plating and electroplating sector, batteries, pesticides, mining, rayon, tanning, fluidized bed bioreactors, textile, metal smelting, petrochemical, paper, and electrolysis applications, the amount of heavy metals in wastewater has been rising.Wastewater tainted with heavy metals seeps into the environment, endangering both the ecology and human health.As well as, the increasing volume of waste due to increasing population and community activities will have an impact on the management of landfill locations.Good and optimal landfill management can provide benefits to the community and the surrounding environment.Environmental pollution including water, soil and air pollution can be caused by improper landfill management [1].Decomposing waste in landfills can pollute the air, emit unpleasant odors and also produce leachate.Leachate is a hazardous and toxic liquid that emerges from the compression of waste and the chemical reactions that take place within landfills.Leachate contains high levels of organic and inorganic compounds, is brown in color and smells [2,3].Leachate has the potential to contain dissolved solids, organic materials, suspended solids, salts both organic and inorganic, ammonia-nitrogen, and heavy metals, one of them is mercury (Hg) [4].Leachate resulting from waste degradation has the potential to pollute groundwater.Therefore, it is necessary to carry out good and optimal management, especially in leachate management.Wastewater treatment employs multiple conventional systems, including physical, chemical, biological methods, or various combinations of these approaches [5].Among various technologies, electrocoagulation is a promising treatment technology because it has the potential to remove organic and persistent pollutants in leachate from landfills.The potential of electrocoagulation as a treatment technology is promising since it can effectively eliminate organic substances and persistent contaminants present in landfill leachate.The advantages of electrocoagulation include easy operation, faster retention time, and lower operating costs.Electrocoagulation has been used successfully to remove heavy metals [6] [7].

Research Time and Location
Experiments in this study were carried out at the Water Quality Laboratory of Environmental Engineering, Universitas Sumatera Utara.This research was conducted from October 2022 to December 2022.The sample used in this study was leachate from the Terjun Landfill in Medan.

Research Methods
Sampling was done by grab sampling.The samples were then processed using an electrocoagulation process using a batch system to determine the final concentration of mercury metal (Hg) in the leachate after processing.The variables in this study were voltage (8.10, 12 Volt), time (10, 20, 30 minutes), and distance (1, 2, 3 cm).The sample was then put into a glass reactor with a volume of 1 L. The electrode used was a zinc (Zn) plate measuring 4 cm × 10 cm × 1 mm.Furthermore, the electrodes are supplied with direct current (DC) electricity with a predetermined voltage, time, and distance.The electrocoagulation results were then tested for mercury (Hg) levels by ICP (Inductively Conducted Plasma).Data from the test results will then be analyzed using Design Expert software with the Box Bhenken method to determine the effect of the research variables on the efficiency of mercury (Hg) removal.

Groundwater Vulnerability Level Zoning
This experiment was carried out in groups based on the Box Behnken experimental design.Experimental data can be seen in Table 1.Based on Table 1, it can be seen that the highest mercury removal efficiency was 98.108% from the 15th running with a variation of 12 volts, 30 minutes of time, and 2 cm of electrodes space.Meanwhile, the lowest mercury removal efficiency was 90.877% from the 8th run with a variation of 8 volts, 10 minutes of time, and 2 cm of electrodes space.The final level of mercury obtained in all experiments was below the quality standard.

a) Response Model Selection Analysis
The results of the analysis of the selection of the mercury removal efficiency model based on the Sequential Model Sum of Squares in Table 2.The model that is accepted based on this test is a model that has a P value of less than 5% (significant) [8].
The selection of the model based on the lack of fit analysis can be seen in Table 3.The model selected based on the lack of fit tests is a model that has P value > 0.05 (not significant) which means that the model is in accordance with the response [8].
The selection of the model based on summary statistics analysis can be seen in Table 4.The third test of the mathematical model for mercury removal efficiency response is the Summary Statistics Model.The best models focus on the highest adjusted R 2 and predicted R 2 values [9].Analysis of variance (ANOVA) was used to determine the effect of each independent variable on the response.The results of the analysis of variance (ANOVA) can be seen in Table 5.
The value of the coefficient of determination (R 2 ) shows how much influence the independent variable has simultaneously on the dependent/response variable.In general, the R 2 value is in the range of 0 to 1.The closer the R 2 value is to 1, the effect will be stronger.If R 2 is negative, it is concluded that the independent variable has no effect on the dependent variable [8].

c) Effect of Voltage Variation on Mercury (Hg) Removal Efficiency
In this study, the voltage was varied to 8, 10 and 12 Volts.Based on Figure 2, it can be seen that the greater voltage used, the higher efficiency of mercury (Hg) removal.When the electric voltage is applied continuously, it will produce an increasing amount of Zn 2+ from the formed electrodes, so the number of Zn(OH)2 flocs also increases.This is in line with the research of [10], where the greater the voltage strength used, the more flocs are formed and it will stick to the electrodes during the electrocoagulation process.So, the greater voltage strength used, the higher resulting percent allowance.By applying a voltage of 8, 10 and 12 Volts, the flocs formed will float to the surface of the solution.This is due to the mixing between the floc and H2 gas which is formed from the reaction of the two electrodes used.Voltage directly determines the dose of coagulant and the rate of bubble formation, and greatly influences solution mixing and mass transfer at the electrodes.Thus, a series of experiments was carried out to determine the relationship between voltage and electrocoagulation performance.From the results of his research, it was found that the efficiency of pollutant removal increased with increasing the voltage used [11].In this study, variations of the electrocoagulation time used were 10, 20, and 30 minutes.Based on Figure 3, it can be seen that the longer electrocoagulation time used, efficiency of mercury (Hg) removal got higher.The longer the electrocoagulation time used, the more possibilities for oxidation and reduction processes to occur at the electrodes.The oxidation and reduction processes will produce OH -gas to bind Zn to Zn(OH)2 as flocs and O2 which will help the flocs flotation to the sample surface [12].Thus, this is in line with Putri and Purnama's research in 2022, that the more time used in the electrocoagulation process, the more interactions occur between the electrodes and the sample [13].So, the more flocs are formed, the higher removal of pollutant levels.In this study, the space between the electrodes used in the electrocoagulation process was varied to 1, 2 and 3 cm.Based on Figure 4, it can be seen that the closer space between the electrodes, the greater efficiency of mercury removal.The space between electrodes affects the rate of electron transfer between the anode, which accepts electrons, and the cathode, where the reduction process takes place.As the space between electrodes increases, processing power decreases, current resistance increases and conductivity decreases.This is in accordance with the research conducted, with increasing electrode spacing, processing efficiency decreases.Because the greater space between the electrodes, the less interaction of the ions in the solution with the coagulant.The greater space between the electrodes, the greater the voltage required [12].

Conclusion
The highest percentage of mercury removal was 98.108% with a voltage of 12 Volts, a time of 30 minutes, and a distance between the electrodes of 2 cm.Thus, the voltage, time, and distance of the electrodes have a significant effect on the efficiency of mercury (Hg) removal.The greater the voltage, the higher the mercury (Hg) removal efficiency.The longer the electrocoagulation contact time, the higher the mercury (Hg) removal efficiency.The farther the distance between the electrodes, the lower the mercury (Hg) removal efficiency.

Fig. 2 .
Fig. 2. Analysis of the Effect of Voltage on Mercury (Hg) Removal Efficiency Graphic d) Effect of Time Variation on Mercury (Hg) Removal Efficiency

Fig. 3 .
Fig. 3. Analysis of the Effect of Time on Mercury (Hg) Removal Efficiency Graphic e) Effect of Electrodes Space on Mercury (Hg) Removal Efficiency

Fig. 4 .
Fig. 4. Analysis of the Effect of Electrodes Space on Mercury (Hg) Removal Efficiency Graphic

Table 1 .
Efficiency of Mercury (Hg) Removal

Table 2 .
Sequential Model Sum of Squares Source Sum of Squares df Mean Square F-value p-value

Table 5 .
Analysis of Variance Result (ANOVA)

Table 6 .
Variance Parameter Coefficient