Reducing Zinc and Chromium in Wastewater Treatment Biosolid Through Sulfuric Acid Extraction

The separation of heavy metals, zinc (Zn) and chromium (Cr), in biosolid waste can be carried out by an extraction process using H 2 SO 4 as a solvent. The extraction process was carried out with a variation of time agitation and sulfuric acid concentration. The extracted cake was then analyzed using the atomic absorption spectrophotometry (AAS) method. The most effective reduction in Zn and Cr levels was obtained at 120 minutes and a solvent concentration of 1.5M with an efficiency of 98.80% and 97.89%, respectively.


Introduction
Biosolid waste is organic solid waste rich in nutrients derived from the by-products of aerobic and anaerobic biological treatment of industrial, household wastewater, and laundry.Biosolid solid waste from the wastewater treatment industry contains complete nutrients, macro, and micronutrients.In addition, biosolid waste also contains heavy metals depending on the wastewater being treated (Setiyono, 2017).Each sector can produce biosolid waste in large quantities, up to 30 tons per month.This biosolid waste is a serious problem for the industry because of the large amount and heavy metals in the biosolid waste.The content of heavy metals includes arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), and zinc (Zn).The processing and utilization of biosolid waste have not been maximized.Biosolid management is currently only accommodated on vacant land, unhandled and unmanaged properly, so it can pollute the environment and harm human life (Said, 2010).
Based on the quality of the existing biosolid waste, one alternative to reduce environmental pollution/damage is to reduce the heavy metal ions present in the solid waste.Heavy metal ions in biosolid waste can be reduced by further processing and utilized in goods with a higher use value.The decrease of heavy metal ions in biosolid waste is carried out by the extraction process (leaching).
In the research of Hutagaol et al. (2015) with the title of ZnO solid-liquid extraction with acid from filter dust in the steelmaking process, 50 grams of raw materials were added into 500ml H2SO4 and HCl with different concentrations.In that study, the best conditions for separating ZnO using H2SO4 solvent at a concentration of 0.75M resulted in the extracted ZnO content of 97.73%.
In the research of Yulianti et al. (2014) with the title the effect of extraction time and concentration of ethanol solvent on the physicochemical properties of stevia leaf extract using the MAE method.The method used is descriptive experimental using five microwave extraction times, namely 30, 60, 90, 120, and 150 seconds, as well as two variations of ethanol solvent concentration, namely 80 and 90%.The results obtained from the extraction time of 60 minutes to 90 minutes experienced an increase in the extracted substance.At a concentration of 90%, the ability of the solvent to remove substances is better than the concentration of 80%.
According to Pomantouw (2011), reported research with the title separation of heavy metals (Pb and Cd) in biosolids by an acid-base leaching process.In that study, heavy metals contained in the biosolid waste were extracted by pulverizing 30 grams of biosolid waste dissolved into 500 ml of H3PO4 and KOH solution.The best condition for separating heavy metals Pb and Cd in biosolid waste was the condition using H3PO4 solvent at pH 6, stirring speed of 100 rpm, and extraction time of 90 minutes.The percentage decrease in Pb was 97.43%.Initial lead 362.33 mg/L; after processing 9.57 mg/L.While the percentage decrease in Cd is 97.20%.Initial Cd 92.63 mg/L; after process 2,671 mg/L.

Materials
The materials used in this research were biosolid solid waste taken from PT. Sier -Surabaya, sulfuric acid and aquadest.

The procedure of biosolid extraction
First, the biosolid waste was dried, crushed, and sieved with a size of 100 mesh.After that, 50 grams of that waste were weighed.Then H2SO4 solution was prepared with a concentration variation of 0.5M, 0.75M, 1 M, 1.25M, and 1.5M.The biosolid waste was mixed with the sulfuric acid and stirred at 100 rpm for variations of 40, 60, 80, 100, and 120 minutes.The mixture was then filtered to obtain the residue.It was washed using aquadest.Zn and Cr content were then analyzed with the AAS (Atomic Absorption Spectrophotometry) test.

The effect of stirring time and sulfuric acid concentration on Zn content
Figure 1 shows a decrease in the metal content of Zn due to differences in stirring time and solvent concentration.The greatest decrease in Zn content occurred at a solvent concentration of 1.5M with a stirring time of 120 minutes.The initial condition of the metal content of Zn 10400 ppm after processing the Zn metal content decreased to 124.37 ppm with a percentage decrease of 98.80% efficiency.The results of reducing Zn metal levels according to solid waste quality standards were obtained at a stirring time of 60 minutes with a solvent concentration of 0.5M.During that time, it has been able to reduce Zn metal levels below the quality standard, which is 498.2 ppm of Zn metal content.Initial, with a percentage of 95.21%.
For 40 minutes of stirring time at 1.5M of sulfuric acid obtained a final Zn content of 261.73 ppm, while for stirring time of 60 minutes with the same solvent concentration, the final concentration of Zn metal was 124.37 ppm.The data showed that the longer the stirring time, the greater the decrease in Zn metal content.This phenomenon was due to the length of contact between the solvent and the material extracted could increase the diffusion.The mass transfer of Zn from the particle surface into the solution was also increasing.Besides, Fig. 1 also shows that the greater the concentration of the H2SO4 solvent, the greater the decrease in Zn levels.

The effect of stirring time and sulfuric acid concentration on Cr content in biosolid
Figure 2 shows that the concentration of Cr metal decreased due to increased stirring time and solvent concentration.The greatest decrease in Cr metal concentration occurred at a solvent concentration of 1.5M with a stirring time of 120 minutes.The initial condition of the Cr metal content of 990ppm after processing the Cr metal content decreased to 20.91 ppm with a decreasing efficiency of 97.89%.The results of reducing Cr content that met quality standards were obtained at 40 minutes of stirring time and with a solvent concentration of 0.75M.Under these conditions, the Cr metal content has reduced below the quality standard, 95.56 ppm from the initial concentration level, with a reduction efficiency of 90.35%.Besides, Fig. 2 also represents that the greater concentration of H2SO4, the greater the decrease in the concentration of Cr.This phenomenon occurred because the Cr could not dissolve completely at a low concentration of H2SO4.In contrast, with a high concentration of H2SO4, the Cr dissolved easily.

Conclusion
Zn and Cr levels in wastewater biosolid could be reduced using sulfuric acid as a solvent extraction method.The results showed that the longer the stirring time, the greater the decrease in the levels of Zn and Cr as well as the higher the concentration of sulfuric acid, the greater the metal concentration decrease.The greatest decrease in Zn metal content was obtained at a time of stirring for 120 minutes and a solvent concentration of 1.5 M, with the final result of Zn metal content of 124.37 ppm with a reduction efficiency of 98.80%.The largest decrease in Cr metal concentration was obtained at a time of stirring for 120 minutes and solvent concentration of 1.5 M, with the final result of Cr metal content of 20.91 ppm with a reduction efficiency of 97.89%.

Figure 2 .
Figure 2. Stirring time effect on the Cr concentration at various concentrations of sulfuric acid At a stirring time of 40 minutes and solvent concentration of 1.5M obtained a decrease in Cr metal content of 62.35 ppm, while at a stirring time of 120 minutes with the same solvent concentration, a reduction in Cr metal content of 20.91 ppm was obtained, this indicated that the stirring time affected the decrease of Cr content in biosolid.The data showed that the longer the stirring time, the greater the decrease in Cr concentration.Because the length of contact between the solvent and the extracted material could increase diffusion, the mass transfer of Cr from the particle surface into the solution also increased.Besides, Fig.2also represents that the greater concentration of H2SO4, the greater the decrease in the concentration of Cr.This phenomenon occurred because the Cr could not dissolve completely at a low concentration of H2SO4.In contrast, with a high concentration of H2SO4, the Cr dissolved easily.