|Kevin Cleary Wanta||Department of Chemical Engineering, Faculty of Industrial Technology, Parahyangan Catholic University, Ciumbuleuit 94, Bandung, 40141, Indonesia|
|Catherine||Department of Chemical Engineering, Faculty of Industrial Technology, Parahyangan Catholic University, Ciumbuleuit 94, Bandung, 40141, Indonesia|
|Arry Miryanti||Department of Chemical Engineering, Faculty of Industrial Technology, Parahyangan Catholic University, Ciumbuleuit 94, Bandung, 40141, Indonesia|
|Anastasia Prima Kristijarti|
industry wastewater containing metals must be treated so as not to threaten the
environment or human life. One of the wastewater treatments is the biosorption
process using living microalgae. Although living microalgae can provide better
results as a biosorbent, the mechanism of this biosorption process is complex
because it involves two steps of the process, active and passive uptake, which
run simultaneously. In addition, several process parameters need to be adjusted
for the biosorption process to operate optimally. This study aims to
investigate the effect of several parameters such as microalgae concentration,
salinity, and light color. Synthetic CuSO4 solution at a
concentration of 40 mg/L and pH 5 is used as artificial waste, while microalgae
Chlorella sp. is used as biosorbent. The biosorption process was
operated in a batch system at room temperature for 6 days. The experimental
results show that 96.83% of the Cu(II) ions could be removed when the
microalgae concentration, salinity, and light color were conditioned at 1.5 x
106 cells/mL, 3,000 mg/L, and red light, respectively.
Biosorption, Chlorella sp., Copper removal, Living biosorbent, Wastewater treatment
various chemical industries are emerging and developing rapidly in order to
fulfill human needs. It has had a positive impact on various sectors, including
the economy. However, on the other hand, it also creates new challenges and
problems, particularly in terms of industrial waste treatment. Improper
industrial waste processing will negatively impact the environment, especially
hazardous and toxic waste. Metal wastewater is one of the hazardous wastes
requiring proper treatment. This waste can originate from a variety of chemical
industries, such as the mining, electroplating, and metallurgical industry (Kim et al., 2018; Sun
et al., 2013). Metal wastewater is classified as hazardous waste
because some metal elements have toxic and even carcinogenic properties (Gunatilake, 2015; Yang
et al., 2019). It has a significant potential to endanger human
life and public health. Therefore, reducing and eliminating this hazardous
potential must be carried out effectively and efficiently.
One method that can be used to treat metal wastewater is the biosorption process. The definition of biosorption process is a process that utilizes the ability of biological materials (living and/or non–living) to reduce metal content in the wastewater (Kusrini et al., 2021; Volesky, 2007). This process will adsorb and/or consume the metal ions physicochemical and metabolically (Volesky, 2007). Biosorption is an option that should be considered because it has many advantages, which include low capital and operating costs, using fewer chemicals, producing less sludge, being effective at low metal concentration, and high efficiency (Anuar et al., 2019; Abdi and Kazemi, 2015; Abbas et al., 2014; Fomina and Gadd, 2014). Bacteria, fungi, microalgae, and yeast are the biological materials used as biosorbents (Wang and Chen, 2009). As a biosorbent, microalgae has some advantages, including the ability to remove metals from wastewater with high efficiency, the ability to be regenerated, does not produce toxic sludge, a high growth rate, can be applied in batch and continuous systems, being safe, inexpensive, and can live in open water (fresh or marine water) (Daneshvar et al., 2018; Brinza, Dring, and Gavrilescu, 2007; Borowitzka, 1999).
Most studies related to the biosorption process of metal wastewater use non–living microalgae (biomass) as biosorbents (Chu and Phang, 2019; Kücüker, Nadal, and Kuchta, 2016; Utomo et al., 2016). However, another interesting thing to be applied is the use of living microalgae as biosorbent and this is still not much to be explored further. Theoretically, living microalgae provide better removal results because of their mechanism which involves two steps. The first step is called passive uptake, and this step will bind (or adsorb) the metal ions to the cell surface. The second step is active uptake which the metal ions will be accumulated in the cell across the cell membrane (Das, Vimala, and Karthika 2008). Generally, the passive uptake has a faster rate process than the active uptake (Hawari and Mulligan, 2006). Living microalgae as a biosorbent has an advantage where passive and active uptake will run simultaneously during the biosorption process. On the other hand, when the biosorption process uses non–living microalgae, active uptake will not occur. Thus, living microalgae have a big potential to develop further to remove the metal ions from wastewater.
In simple terms, the biosorption process's main principle using living microalgae is (1) to ensure microalgae grow and reproduce as much as possible, and (2) an adsorption step through the cell surface of microalgae will occur. However, this process is categorized as complex. This process's complexity will be significantly influenced by several factors, such as solution pH, temperature, biosorbent dosage, ionic strength, initial solute concentrate, agitation rate, time, nutrients, light intensity, and salinity (Shihab, Dhahir, and Mohammed, 2020; Luangpipat and Chisti, 2017; Lee, Jalalizadeh, and Zhang, 2015; Das, 2010). These process parameters can either support or conflict with the performance of the biosorption process. For example, Wanta et al. (2020) studied the effect of the initial concentration of Cu(II) ions in the solution. The results of their study showed that there was a certain concentration that provides optimum biosorption results. A higher concentration of Cu(II) ions would kill the microalgae and decrease biosorption performance. It was characterized by a very low removal percentage when higher concentrations were used (Wanta et al., 2020). Thus, it is necessary to investigate the effect of other parameters on the biosorption process using living microalgae. When the process is implemented on an industrial scale, it can operate optimally.
The microalgae Chlorella sp. which was used as a biosorbent in this study was harvested at the Center for Study of Water Technology and Waste Management, Parahyangan Catholic University. Walne’s medium is a nutrient-rich growth medium for microalgae. The synthetic wastewater solutions were prepared using copper sulfate pentahydrate (CuSO4.5H2O) (Merck), while NaOH and HCl solutions were used as pH regulators. Sodium chloride (NaCl) (Merck) was also used to regulate the synthetic waste solution's salinity level. When the Cu2+ ion content in the liquid phase was analyzed, ammonia (NH3) solution (Merck) was also used as a complexing agent. The entire biosorption process was carried out using RO water as a solvent.
2.2. Cultivation of Microalgae Chlorella sp.
For the microalgae cultivation process, the first step was to prepare a nutrient solution. This solution was made by mixing 1 mL of Walne’s medium with 1 L of RO water. Then, this solution was then sterilized using an autoclave at 121oC for 15 minutes. After that, 500 mL of this solution was mixed with 1,500 mL of microalgae; then the solution is adjusted to a pH of 5. The equipment consists of a closed box, a 40-watt LED lamp, a glass bottle as a bioreactor, an air pump as an aerator, and also functions as a mixer. The cell density of microalgae Chlorella sp. was calculated daily using a hemocytometer for 6 days during the cultivation process.
2.3. Biosorption of Copper(II) Ions Using Microalgae Chlorella sp.
biosorption process was carried out using a CuSO4 solution where the
Cu2+ ions concentration was 40 mg/L, and the pH of
the solution was 5 (Wanta et al., 2020). 950 mL
of the solution was mixed with 50 mL of microalgae with
various concentrations of 1.5–4.5 x 106 cells/mL. In addition, the
solution's salinity level was also adjusted by adding 0, 3,000, and 6,000 mg/L
of NaCl solution into the bioreactor. The lights used in this study were also
varied in light color (white, red, and blue). After adjusting the settings for
the light intensity, the lights were programmed to be on for 12 hours and off
for 12 hours automatically.
The liquid phase was taken periodically for 6 days. That sample was centrifuged at 10,000 rpm for 5 minutes. The supernatant was analyzed for the remaining Cu2+ ion content using a UV–vis spectrophotometer (Mapada UV–6100 PC) at a wavelength of 610 nm. Before being analyzed, the supernatant was first mixed with 1 mL of NH3 (as a complexing agent). In addition to testing the remaining Cu2+ ions content, the supernatant was also analyzed for cell density using a hemocytometer.
2.4. Data Analysis
2.4.1. Cell density analysis
cell density of microalgae indicates the number of cells in a solution. The
following equation was used to calculate cell density:
where x is the number of living microalgae cells counted with a hemocytometer.
2.4.2. Removal percentage analysis
The removal percentage indicates the number of metal ions successfully adsorbed by the microalgae. The following equation was used to calculate the removal percentage:
3.1. Effect of
Microalgae Concentration on the Copper(II) Ions Biosorption Process
As a biosorbent, the quantity of microalgae in the biosorption system greatly influences the ability of microalgae to adsorb metal ions both actively and passively uptake. In this study, the microalgae concentration was varied between 1.5 x 106, 3.0 x 106, and 4.5 x 106 cells/mL, while the salinity level of the solution was 0 mg/L.
Figure 4 shows
that the addition of 3,000 and 6,000 mg/L of NaCl salt has a positive effect on
the growth of Chlorella sp. microalgae, as evidenced by the increase in
microalgae cell density. However, this parameter's effect is not directly
proportional because there are optimal conditions for adding the salinity level
of the solution. When 6,000 mg/L of NaCl salt is added, it inhibits the growth
of microalgae cells (compared to the addition of 3,000 mg/L of NaCl salt). This
optimum condition occurs due to a decrease in microalgae's ability to absorb
water for growth and cell division needs. This phenomenon is known as the
salinity-water deficit effect.
The blue light spectrum plays no significant role in the growth process of Chlorella sp. microalgae, specifically in the photosynthetic process of the microalgae, as depicted in Figure 5. The density value of microalgae cells in white and red light is significantly higher than in blue light. On average, the cell density of microalgae under white and red light was 2.17 and 2.38 times higher than the density of microalgae cells in blue light, respectively. The poor performance of blue light in this cultivation process is due to blue light's characteristic wavelength. Blue light has a wavelength of 455–492 nm; therefore, this light cannot be absorbed properly in the photosystem step and affects the photosynthesis process.
On the other hand, white light is better than blue light. White light has a diverse spectrum of colors, including red light and blue light. This mixed color spectrum causes the photosystem to absorb red light from the white light spectrum even though that the quantity of red light absorbed is less than the full use of red light. According to Figure 5, the cell density in white light is less than in red light. This condition also indicates that the amount and intensity of light play an important role in light absorption in microalgae. The large amount and intensity of light have a positive tendency toward the photosynthesis process. However, this also depends on the cultivation volume and the microalgae density. The required light intensity increases proportionally to the microalgae density.However, it should be noted that the light factor is not the only factor that affects microalgae's growth process. Figures 1, 4, and 5 demonstrate and explain that microalgae growth is complex, and all factors will influence one another and can have other impacts, including nutritional competition and others.
biosorption process of metal wastewater using living microalgae Chlorella
sp. is complex because many process parameters influence it. The
experimental results of this study indicate that the effect of each parameter
is not always linear. For instance, the larger the number of microalgae in a
system, the less efficiently it can remove the Cu2+ ion
concentration from the solution, as the microalgae cell density increases,
thereby increasing competition for nutrients. This also applies to salinity
levels where optimum conditions must be adjusted for optimized biosorption
results. Moreover, the biosorption process is also related to the
photosynthesis process that occurs in these microalgae. It causes the influence
of the light color given to the system and will affect the microalgae's
photosynthesis rate. Based on the experimental results, the process conditions
that gave the maximum removal of Cu2+ ions were when the microalgae
concentration, salinity, and light color were conditioned at 1.5 x 106
cells/mL, 3,000 mg/L, and red light,
respectively. In this condition, 96.83% of Cu2+ ions were successfully
removed from the wastewater. These results revealed that this biosorption
process has promising potential and needs further development on a larger scale
(continuous system or industrial scale).
study was financially supported by the Parahyangan Catholic University Center
of Research and Community Service under contract number III/LPPM/2020–01/10–P.
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