Adsorption of Heavy Metals Fe and Mn in Acid Mine Drainage from Coal Mining Waste Using Calcium Oxide and Fly Ash

Acid mine drainage for Fe and Mn metals have a negative impact on the environment. This study studies the use of calcium oxide and fly ash as adsorbents, used to absorb Fe and Mn metals contained in acid mine drainage. This study aims to analyze the effectiveness of Calcium Oxide and fly ash as adsorbents, that an adsorbent of Fe and Mn metals in acid mine drainage. The method used is experimental with a laboratory scale, the test is carried out by adsorption using a batch system. Magnetic stirrer is used as a stirring medium in the adsorption process. Calcium oxide with various doses (0.1; 0.2; 0.3 grams) and fly ash with various doses (10; 14; 17 grams) were mixed with 250 ml acid mine drainage, then stirred at a speed of 150 rpm and (30; 60; 90 minutes) stirring time. The characterization of the adsorbent was carried out by SEM testing. The results show that fly ash adsorbent has a better adsorption effectiveness than calcium oxide. The effectiveness of Fe is 85.35% and Mn 78.14%. While calcium oxide has the effectiveness of Fe 72.91% and Mn 61.81%. SEM testing of holes that increase and enlarge after adsorption, as well as the filling of the cavity by the material indicates the success of the adsorption process.


Introduction
Human necessity derived from natural resources (minerals, oil, and coal) are increasing along with the development of technology for production, communication, and transportation. The mining industry plays an important role in regional and state economic growth. Opening high exports and promising prices also encourage business actors to invest in the coal mining industry.
In addition to the benefits and positive impacts of the mining industry, there are negative impacts arising from mining waste, namely acid mine drainage formed due to the oxidation of sulfide minerals. The presence of water and oxygen in the air as a source of oxidation. Sulphide minerals are often found in the coal mining industry and ore minerals in which the digging and backfilling activities caused by the exposure of sulfide minerals that were previously below the soil surface [1]. This causes an increasing production in the water body which is characterized by low pH, and dissolved metal concentrations in the receiving water body [2].
Treatment of acid mine drainage generally uses calcium oxide (CaO), which is the result of burning calcium carbonate (CaCO3). Based on limestone which is formed as a result of a calcination reaction using heating at a temperature of 900 o C, it causes carbon dioxide (CO2) to come out and leaves calcium oxide (CaO) [3]. The calcium oxide has a high stability which when reacted with water can directly neutralize the acid solution [4], but it is not effective in reducing heavy metals in acid mine drainage [5].

I N V O T E K
Jurnal Inovasi Vokasional dan Teknologi P-ISSN: 1411-3414 E-ISSN:  Steam power plants (PLTU) used coal as a fuel source produce residues in the from of bottom ash and fly ash, know as FABA. Physically, fly ash is fine with a size of <20 μm, with a surface area of 300-500 m 2 /kg. Some fly ash is acidic with a pH of 3-4, but it is generally alkaline with a pH of 10-12 [6]. In addition, fly ash contains large amounts of unburned carbon, and has a high adsorption capacity [7]. Fly ash is also a great alternative as a conventional adsorbent [8].
With large power plant production, fly ash waste is generally only disposed of around industrial areas. Limitations in the use of fly ash waste, have an impact on the accumulation of waste. This case has the potential to contaminate dissolved heavy metals in the surrounding waters. By blownd the wind which will adversely affect breathing [9]. Various efforts have been made to increase the utilization of fly ash waste, including the issuance of fly ash waste from the B3 waste category by the government [10].
Adsorption is a process that can be used for the purpose of reducing heavy metals in acid mine drainage. The adsorption process has several advantages, namely the processing is relatively simple, effective, high efficiency, and does not have a negative impact on the environment [11]. Adsorption is also the process of separating or removing low concentration pollutants from large volumes of wastewater and aqueous solutions [12]. The adsorption process has an influence on contact time, stirring speed, adsorbent mass, and particle area [13].
This research is a development of previous research, where the use of the adsorbent dose is varied and the stirring speed is increased, so that more representative results are obtained. The purpose of this study was to analyze the effectiveness of using calcium oxide and fly ash as adsorbents for the absorption of Fe and Mn metals in acid mine drainage, and comparing the surface structure before and after adsorption using a scanning electron microscope.

Research Methodology
The tests were carried out using experimental methods on a laboratory scale. The adsorption process is carried out using a batch system, namely adsorption by immersion in a solution containing heavy metals, which will then be absorbed and observed quality changes at certain intervals. The material used is acid mine drainage from the coal mining sump location of PT X. Acid mine drainage uses a purposive sampling method and refers to the wastewater sampling method [14]. Furthermore, calcium oxide was taken at the location of the people's mining industry in the Padang Panjang area, while the fly ash sample came from the PLTU which was taken at the TPS location. Dry oven, analytical balance, watch glass, beaker, stirring bar, magnetic stirrer, elenmeyer, glass funnel, filter paper, mortal and pestle are tools used in adsorption testing. This research was conducted in an accredited and standardized laboratory.

Characterization of adsorbent
Characterization of calcium oxide and fly ash samples was carried out using a scanning electron microscope (SEM). The test was carried out to determine the surface structure of the adsorbent before adsorption, then carried out again after the adsorption experiment. The test was carried out aimed at knowing the differences and changes in the structure of the calcium oxide and fly ash adsorbents. The followingis the SEM testing process which can be seen in Figure 1. b. Figure 1a. The adsorbent sample is placed in the holder, b. The sample is loaded into the SEM Hitachi S-3400N tool

Fe and Mn metal content in acid mine drainage
Measurement of the metal content of Fe and Mn in acid mine drainage was carried out with atomic absorption spectrophotometry (AAS) before and after the adsorption experiment. Water quality testing refers to the Indonesian Nasional Standard on Water and Wastewater Quality, the AAS method of iron (Fe) testing [15] and the AAS method of manganese (Mn) testing [16]. Based on the quality standard of coal mining waste [17], the maximum concentration for Fe metal is 7 mg/l and Mn metal is 4 mg/l.

The process of adsorption uses calcium oxide and fly ash
The process of adsorption was carried out on a laboratory scale with a magnetic stirrer as a media stirrer. Before used, the adsorbent must be heated in an dry oven to remove the water content of the adsorbent. Furthermore, the adsorbent dose was weighed according to the predetermined dose, using an analytical balance which was measured on a watch glass. Then, the dose is mixed into a beaker glass, with acid mine drainage with a volume of 250 ml. Then a stirring bar is inserted as a magnet to stir in the magnetic stirrer. After that, stirring was carried out by adjusting the stirring speed at 150 rpm, and the stirring time was in accordance with the design of adsorption. Adsorption using adsorbents in the form of calcium oxide and fly ash can be seen more clearly in Table 1.
The next sample is poured into an erlenmeyer after the stirring process is complete, and filtered using filter paper placed on a glass funnel. The filtering results in the form of water are then put into a PET bottle, so AAS analysis is carried out to determine the remaining content of Fe and Mn metals. The results of the AAS analysis are calculated the effectiveness of the adsorption of Fe and Mn metals. The effectiveness of heavy metal reduction uses the following equation formula: Where Co is the initial metal concentration (mg/l), and Ct is the final concentration (mg/l). The results of the effectiveness is the form of a percentage level (%) of the decrease in Fe and Mn metals. Meanwhile, the sedimentation sample on filter paper was dried to remove the moisture content using a dry oven, then ground using a mortar and pestle. Furthermore, on the sedimentation sample that has been dry and smooth, SEM testing is carried out to see the surface morphology and pore structure after adsorption.

The test results of SEM
Based on the SEM test before the experiment on calcium oxide and fly ash, it showed that there were few pores and a hollow surface. After the experiment the changes in the pores were getting more and more visible and became denser. The surface morphology of calcium oxide and fly ash before and after adsorption can be seen in Figure 2 and Figure 3.   In Figure 2 and Figure 3, it can be seen that the pores increase and enlarge which indicates an increase in the surface area of the adsorbent. The presence of small pores that are getting bigger after the experiment, shows that acid mine drainage has acidic properties that can damage the surface of calcium oxide and fly ash. Silica and alumina are porous media which presence is indicated by white color, as well as active groups that have the potential as adsorbents for metal ions. As active groups, silica and alumina will come out to the surface if there is damage to the surface layer, so that they can adsorb heavy metals Fe and Mn.
In addition to the increasing number of pores, a significant thing shows that the adsorbent has adsorbed Fe and Mn metals on the pore surface. The pore surface in question is the surface of the adsorbent which was previously hollow which looks denser because the cavity is filled with material. This shows that the material enters and adheres to the surface of the adsorbent, then the voids are seen to be less than before the experiment. So it can be concluded that calcium oxide and fly ash adsorbents successfully absorb heavy metals from acid mine drainage.

Adsorption of Fe and Mn metals
Based on the results of the analysis before the adsorption experiment using atomic absorption spectrophotometry, it is known that the Fe and Mn metals contained in the acid mine drainage are Fe 8.8605 mg/l and Mn 7.0375 mg/l. After the adsorption experiment, there was a decrease in the concentration of Fe and Mn in acid mine drainage. The results of using calcium oxide to decrease Fe and Mn concentrations can be seen in Figure 4 and Figure 5.

The effectiveness of Fe and Mn metal adsorption
Based on decreasing in Fe and Mn metals, the effectiveness of the adsorption ability of calcium oxide and fly ash is calculated as shown in Table 2 and Table 3.  Table 2 shows the effectiveness of adsorption by using calcium oxide as an adsorbent. In Fe metal, the lowest adsorption effectiveness was 68.74%, namely the administration of a dose of 0.   Table 3    From the analysis between Fe and Mn metals, there is a difference that shows the adsorption of Fe metal better than the adsorption of Mn metal. This happens because the electronegativity of Fe is greater than Mn. The order of the ability to absorb ions in water due to the presence of ion selectivity towards the adsorbent, namely Fe3 + > Al 3+ > Pb 2+ > Ba 2+ >Sr 2+ > Zn 2+ > Cu 2+ > Fe 2+ > Mn 2+ > Ca 2+ > Mg 2+ > K + > NH 4+ > H + > Li + [18]. In addition, it is also influenced by the acidity of the acid mine drainage, where the adsorption of Mn in a solution goes well at high pH, whereas if the pH is below 8 the adsorption will be slow [19].

Conclusion
Calcium oxide and fly ash are good adsorbents for the adsorption of Fe and Mn metals in acid mine drainage, experiments with various variations succeeded in reducing Fe and Mn metals to below the specified quality standards. The SEM test shows the pores increasing and getting bigger, due to the acidic nature of the water which damages the surface of the adsorbent, which results in an increase in the surface area of the adsorbent. In addition, the previous hollow surface has been filled with material, which indicates the adsorbent has successfully adsorbed Fe and Mn metals. The effectiveness of calcium oxide on the adsorption of Fe metal is 72.91% and Mn is 61.81%, where the effectiveness occurs at the same dose and time, namely 0.3 grams and 90 minutes. Better effectiveness occurred in fly ash with Fe metal adsorption of 85.83% at a dose of 17 grams and 30 minutes, while Mn metal was 78.14% at a dose of 17 grams and 90 minutes. Fly ash is an alternative that can be used in acid mine drainage treatment, because of the abundant availability of fly ash and its not yet maximized utilization. The use of fly ash is also more economical than calcium oxide, because fly ash is a residue from the combustion of steam power plants, so there is no need to spend money to get it.