Cost-efficient and sustainable treatment of malachite green, a model micropollutant with a wide range of uses, from wastewater with Pyracantha coccinea M. J. Roemer plant, an effective and eco-friendly biosorbent

ABSTRACT

Malachite green is a pestilential micropollutant with a wide range of uses. In this work, cost-efficient and sustainable treatment of malachite green from aqueous medium was first studied using the biosorbent material obtained from Pyracantha coccinea M. J. Roemer plant. The biosorption kinetics followed the pseudo-second-order model (R²: 0.996). The standard Gibbs free energy change parameter ranging from −8.472 kJ mol⁻¹ to −7.357 kJ mol⁻¹ indicated a spontaneous and feasible treatment process. The biosorption isotherm was well described by Freundlich model with an R² of 0.991. SEM and FTIR studies showed an irregular and uneven biosorbent surface morphology rich in active site. The biosorbent exhibited a good biosorption performance compared to its competitors, with a capacity of 117.745 mg g⁻¹. As a result, this study indicated that P. coccinea M. J. Roemer-based biosorbent could be a promising candidate for the economical, effective, eco-friendly and sustainable treatment of malachite green micropollutant.

KEYWORDS:

• Sustainable treatment
• micropollutant
• malachite green
• Pyracantha coccinea M. J. Roemer
• eco-friendly biosorbent

1. Introduction

The economic development of a country is significantly dependent on its rapid rise in the industrial field. Unfortunately, excessive amounts of wastewater are mostly produced as a result of this rapid rise. Today, the concern of commercial gain, which does not take into account the environment, has reached a level that threatens human health and ecological security. Despite the fact that there are many water resources in the world, the fact that there is very little water of usable quality further aggravates the picture. Water pollution is mainly caused by a variety of micropollutants, including biological, chemical, physical and radioactive substances [1]. Malachite green is a chemical organic micropollutant with a wide range of uses. It is extensively applied in a wide variety of sectors, such as aquaculture, animal husbandry, leather, paper, textile, pharmaceutical and food for many purposes such as use as a colouration, antimicrobial, anthelminthic, antifungal, antiprotozoal and antiseptic agent [2–4]. These anthropogenic activities are the main sources of malachite green discharge into the natural aquatic environment. Unfortunately, malachite green is a pestilential xenobiotic compound that is resistant to degradation through biological, chemical, physical and photolytic processes. Today, it is widely known that this micropollutant has carcinogenic, mutagenic and toxic effects and causes many health problems not only on humans but also on fauna and flora [5–8]. Consequently, it is of vital importance to remove such micropollutants from the contaminated water environment, considering both the scarcity of usable water and their destructive effects on the ecological system.

Numerous biological, chemical and physicalapproaches are available to treat micropollutants from wastewater environment, such as ozonation, biological degradation, membrane separation, electrocoagulation, photocatalytic degradation and advanced oxidation. However, these treatment approaches have various limitations such as expensive chemical demand, high maintenance cost, inefficiency of treatment, high energy demand, fouling, high operating cost, undesirable toxic by-product formation and high chemical usage [5, 9–11]. Conversely, natural biological material-based biosorption process is a simple, eco-friendly, economically viable, effective and flexible biotechnological method, and due to these remarkable properties, it has started to attract great attention in recent years [3, 12–18].

In this regard, Pyracantha coccinea M. J. Roemer-based a novel biosorbent material was first investigated for environmentally friendly, economical, effective and sustainable treatment of malachite green micropollutant from aqueous medium in the present study. P. coccinea M. J. Roemer is an easy-care, robust shrub-shaped plant species. It is widely available in many countries around the world. In addition to the widespread use of the plant in many fields such as food, areal decoration, medicine, contaminant biomonitoring and removal, pest biocontrol and erosion prevention [19–26], if it is successful, its use in such a new field will both provide added value to it and offer an eco-sustainable solution to the micropollutant trouble. Important operating parameters affecting malachite green treatment process were optimized using the single-factor experimental approach. The biosorption kinetics, thermodynamics, isotherm, characterization and comparison studies were conducted to evaluate malachite green treatment manner of the biosorbent material.

2. Materials and methods

2.1. Preparation of P. coccinea M. J. Roemer-based biosorbent material

The fruit biomass of P. coccinea M. J. Roemer was collected from a local park in Sinop, Turkey. The raw plant biomass was thoroughly cleaned from dust and dirt by washing with tap water and distilled water. The biomass material was oven-dried at a temperature of 80 °C for about 24 h. It was crushed into powder with the help of a grinder and sieved with a 0.5 mm mesh sieve. The powdered biomass was then mixed with dilute sodium hydroxide solution (0.3 mol L⁻¹) using a magnetic shaker at room temperature for about 24 h. After the chemical treatment, the biomass was thoroughly washed with distilled water to remove remaining chemical and achieve neutral pH. Finally, the plant biomass was dried again and stored in an airtight bottle for use as biosorbent material in the study of malachite green biosorption.

2.2. Solution of malachite green micropollutant

Malachite green (CAS No: 569-64-2) was provided from a local chemical supplier in Samsun, Turkey. A stock solution of the micropollutant (1000 mg L⁻¹) was made by dissolving the required amount of malachite green in distilled water. It was then diluted to the required malachite green concentrations (5–15 mg L⁻¹) using distilled water to get the desired micropollutant-laden solution grades. Dilute hydrochloric acid (0.1 mol L⁻¹) and sodium hydroxide (0.1 mol L⁻¹) solutions were used to regulate the pH levels of the prepared malachite green-laden solution samples. The chemicals of analytical purity were used in all of the experiments in malachite green biosorption study.

2.3. Malachite green biosorption study

Malachite green biosorption practice was conducted using batch experiment technique at room temperature on an orbital shaker. A certain amount of the biosorbent material and a certain concentration of malachite green solution were added to a conical flask with a working volume of 100 mL and mixed at 150 rpm for a predetermined time. After the experiment period was completed, approximately 1.5 mL of solution sample was taken from the mixture and then the biosorbent material was separated from the solution phase. The concentration of malachite green in the sample was quantified by measuring the absorbance at 617 nm with the aid of a UV-Visible spectrophotometer device (Thermo Genesys 10 S). The following equations were used to determine the efficiency of the biosorbent material to remove malachite green from the solution phase at a specified time (t) and equilibrium (e), respectively. (1) $\begin{array}{rl}{q}_{\text{t}}=& \frac{\left(C_{\text{0}}-C_{\text{t}}\right)V}{m}\end{array}$(1) (2) $\begin{array}{rl}{q}_{\text{e}}& =\frac{\left(C_{\text{0}}-C_{\text{e}}\right)V}{m}\end{array}$(2) where q_t (mg g⁻¹), q_e (mg g⁻¹), C₀ (mg L⁻¹), C_t (mg L⁻¹), C_e (mg L⁻¹), V (L) and m (g) are the efficiency of the biosorbent at a specified time, the efficiency of the biosorbent at equilibrium, malachite green concentration at start time, malachite green concentration at a specified time, malachite green concentration at equilibrium, the solution volume and the biosorbent mass, respectively.

2.4. Optimization of operating parameters affecting malachite green treatment process

Important operating parameters affecting the biosorption operation of malachite green from wastewater environment, such as contact time (t: 0–360 mins, C₀: 15 mg L⁻¹, pH: 6, m: 10 mg), malachite green charge (C₀: 5–15 mg L⁻¹, t: 120 mins, pH: 6, m: 10 mg), pH (pH: 4–8, t: 120 mins, C₀: 7.5 mg L⁻¹, m: 10 mg) and amount of biosorbent (m: 10–30 mg, t: 120 mins, C₀: 12.5 mg L⁻¹, pH: 6), were optimized using the single-factor experiment approach.

2.5. Biosorption kinetics

The kinetics study for malachite green treatment process was conducted using the pseudo-first-order, pseudo-second-order and intra-particle diffusion models [27–30].

2.6. Biosorption thermodynamics

The standard Gibbs free energy change thermodynamics parameter was used to determine the feasibility and spontaneity of malachite green biosorption process [2].

2.7. Biosorption isotherm

The isotherm analysis for the biosorption operation of malachite green was performed using Freundlich, Langmuir and Dubinin-Radushkevich models [31–34].

2.8. Calculations

The mathematical calculations for malachite green biosorption study were done using the experimental data averages with the aid of the SigmaPlot 12.0 software. The agreement of the experimental data with the mathematical equations was determined by the nonlinear regression approach. The R², adjusted R² (adjR²) and RMSE were used to assess the fit quality of the mathematical equations.

2.9. SEM and FTIR experiments

The surface features and active site profile of P. coccinea M. J. Roemer-based biosorbent were studied by SEM and FTIR experiments using SEM (Zeiss Evo Ls 10) and FTIR (PerkinElmer Spectrum 400) instruments.

3. Results and discussion

3.1. Effect of contact time

The effect of contact time on malachite green biosorption process is presented in Figure 1. At first, the biosorption process was rapid and most of malachite green biosorption occurred in this stage. The biosorbent material had a large number of the available active binding sites and the empty surface at this stage, resulting in rapid and intense malachite green biosorption. Later on, the biosorption performance of the biosorbent increased to a certain extent, but its biosorption rate gradually decreased. At this stage, most of the binding sites of the biosorbent were occupied by malachite green molecules as the biosorption process progressed, resulting in a reduction in the speed of the biosorption. Eventually, the biosorption activity hardly increased and the biosorption process reached the equilibrium state as the available binding sites of the biosorbent were almost completely filled by malachite green molecules [15, 35]. Thus, the optimum contact time was determined to be 360 mins for malachite green treatment from water medium.

Figure 1. Effect of contact time.

3.2. Effect of malachite green charge

The effect of malachite green charge on the biosorption process is shown in Figure 2. The biosorption capacity of the biosorbent increased from 30.524 to 79.363 mg g⁻¹ as malachite green charge increased from 5 mg L⁻¹ to 15 mg L⁻¹. The high concentration of malachite green in the solution reduced the mass transfer resistance and increased the chance of effective collision between the biosorbent particles and malachite green molecules, resulting in increased the biosorption performance of the biosorbent. However, when the biosorption capacity of the biosorbent was calculated as a percentage, it was noticed that its biosorption yield decreased with the increase of malachite green charge (data not presented). At the low malachite green charge, the ratio of the active sites on the biosorbent material to the total malachite green molecules was high, which increased the biosorption efficiency by increasing the probability of interaction between the biosorbent particles and malachite green molecules [36]. Thence, the optimum malachite green charge was selected to be 15 mg L⁻¹.

Figure 2. Effect of malachite green charge.

3.3. Effect of pH

The effect of pH on malachite green biosorption operation is given in Figure 3. The biosorption capacity of the biosorbent increased from 24.884 to 34.107 mg g⁻¹ with the increase of pH level from 4 to 8. At the low pH level, the surface of the biosorbent became positively charged by the protonation of the active sites on the biosorbent, which reduced the biosorption efficiency of the biosorbent as it created an electrostatic repulsion effect against the positively charged malachite green molecules. At the high pH level, the positively charged active sites on the biosorbent were deprotonated and became negatively charged. These negatively charged active sites enhanced the biosorption capacity of the biosorbent material by retaining the positively charged malachite green molecules well due to the force of electrostatic attraction [37]. Therefore, the optimum level of pH was determined to be 8.

Figure 3. Effect of pH.

3.4. Effect of biosorbent dosage

The effect of the biosorbent amount on malachite green biosorption capacity of the biosorbent material is shown in Figure 4. The biosorption performance of the biosorbent material decreased from 66.490 to 31.630 mg g⁻¹ as the amount of the biosorbent increased from 10 to 30 mg. At the high biosorbent amount, the overlapping of the biosorbent particles and the fact that a large number of the active binding sites remained empty caused a decrease in malachite green biosorption capacity of the biosorbent. However, when malachite green biosorption performance of the biosorbent was examined as a percentage, it was observed that the biosorption capacity of the biosorbent increased as the biosorbent amount increased (data not shown). This increase in the biosorption efficiency of the biosorbent was due to the availability of larger surface area and higher numbers of the active binding sites with rising the biosorbent amount [38]. Thus, the optimum amount of the biosorbent for malachite green biosorption was selected to be 10 mg.

Figure 4. Effect of biosorbent amount.

3.5. Kinetics study for malachite green biosorption

The biosorption kinetics study was carried out to evaluate malachite green treatment manner of the biosorbent material using the following models. (3) $\begin{array}{rl}& \text{Pseudo - first - order}:q_{\text{t}}=q_{\text{e}}(1-{\text{e}}^{-k_{1}t})\end{array}$(3) (4) $\begin{array}{rl}& \text{Pseudo - second - order}:q_{\text{t}}=\frac{k_{\text{2}}{q_{\text{e}}}^{\text{2}}t}{1+k_{\text{2}}{q}_{\text{e}}t}\end{array}$(4) (5) $\begin{array}{rl}& \text{Intra - particle diffusion}:q_{\text{t}}=C+k_{\text{p}}\sqrt{t}\end{array}$(5) where k₁ (min⁻¹), k₂ (g mg⁻¹ min⁻¹), C (mg g⁻¹) and k_p (mg g⁻¹ min^−1/2) are the pseudo-first-order rate constant, the pseudo-second-order rate constant, a parameter about the boundary layer thickness and the intra-particle diffusion rate constant, respectively. The results of kinetics study for malachite green biosorption are shown in Table 1. The biosorption kinetics for the treatment of malachite green from wastewater environment followed the pseudo-second-order model (R²: 0.996). This result indicated that the chemisorption mechanism, which included the electron exchanging or sharing between the active sites on the biosorbent and malachite green molecules, dominated the biosorption operation [39, 40].

Table 1. Results of kinetics study for malachite green biosorption.

Model	Parameter
Pseudo-first-order
	qe	k1	R^2	AdjR^2	RMSE
	mg g^−1	min^−1	–	–	–
	98.046	0.016	0.989	0.988	3.960
Pseudo-second-order
	qe	k2	R^2	AdjR^2	RMSE
	mg g^−1	g mg^−1 min^−1	–	–	–
	118.080	0.00014	0.996	0.995	2.581
Intra-particle diffusion
	C	kp	R^2	AdjR^2	RMSE
	mg g^−1	mg g^−1 min^−1/2	–	–	–
	4.443	5.775	0.960	0.957	7.636

The effect of the intra-particle diffusion on malachite green biosorption is presented in Figure 5. The movement (mass transfer) of malachite green molecules from aqueous medium onto the biosorbent particles consisted of three stages. The first stage was the external (film) diffusion, in which malachite green molecules were transported from the solution bulk phase to the outer surface of the biosorbent. The second stage was the intra-particle diffusion, in which malachite green molecules were transported from the external surface of the biosorbent into its pores. The final stage referred to the biosorption (surface reaction), in which malachite green molecules were retained by the biosorbent. Furthermore, the curve of malachite green biosorption capacity (q_t) against √t was non-linear and did not pass through the point of origin. These findings revealed that there were multiple stages (mechanisms) controlling malachite green treatment process [36, 41].

Figure 5. Effect of intra-particle diffusion on malachite green biosorption.

3.6. Thermodynamics study for malachite green biosorption

The biosorption thermodynamics analysis was conducted to assess the feasibility and spontaneity of malachite green removal process using the standard Gibbs free energy change thermodynamics parameter by the following equations. (6) $\begin{array}{rl}& \text{Gibbs free energy}:\mathrm{\Delta }{G}^{\circ }=-R\text{Tln}{K}_{\text{D}}\end{array}$(6) (7) $\begin{array}{rl}& \text{Distribution coefficient}:K_{\text{D}}=\frac{C_{\text{eb}}}{C_{\text{e}}}\end{array}$(7) where ΔG° (kJ mol⁻¹), R (J mol⁻¹ K⁻¹), T (K), K_D and C_eb (mg L⁻¹) are the standard Gibbs free energy change, the gas constant, the temperature, the distribution coefficient and malachite green concentration on the biosorbent at equilibrium, respectively. The standard Gibbs free energy change was obtained in the range of −8.472 kJ mol⁻¹ and −7.357 kJ mol⁻¹ for malachite green biosorption process. The negative standard Gibbs free energy change indicates that the biosorption process is of a spontaneous and feasible nature [42]. Again, the negative standard Gibbs free energy change parameter between −400 kJ mol⁻¹ and −80 kJ mol⁻¹ shows a chemisorption process, and between −20 kJ mol⁻¹ and 0 kJ mol⁻¹ indicates a physisorption process [43]. Thus, for malachite green biosorption, the standard Gibbs free energy change ranging from −8.472 kJ mol⁻¹ to −7.357 kJ mol⁻¹ showed a physical, spontaneous and feasible treatment process.

3.7. Isotherm study for malachite green biosorption

The biosorption isotherm study was performed to evaluate malachite green treatment behaviour of the biosorbent using the following equations. (8) $\begin{array}{rl}& \text{Freundlich}:q_{\text{e}}=K_{\text{F}}{C_{\text{e}}}^{\frac{1}{n_{\text{F}}}}\end{array}$(8) (9) $\begin{array}{rl}& \text{Langmuir}:q_{\text{e}}=\frac{q_{\text{m}}{K}_{\text{L}}{C}_{\text{e}}}{\text{1 + }{K}_{\text{L}}{C}_{\text{e }}}\end{array}$(9) (10) $\begin{array}{rl}& \text{Equilibrium factor}:R_{\text{L}}=\frac{1}{1+K_{\text{L}}{C}_0}\end{array}$(10) (11) $\begin{array}{rl}& \text{Dubinin - Radushkevich}:q_{\text{e}}=q_{\text{m}}{\text{e}}^{-B{ϵ}^{\text{2}}}\end{array}$(11) (12) $\begin{array}{rl}& \text{Polanyi potential}:\mathcal{E}=R\text{Tln(}1+\frac{1}{C_{\text{e}}})\end{array}$(12) (13) $\begin{array}{rl}& \text{Mean free energy}:E=\frac{1}{\sqrt{2B}}\end{array}$(13) where K_F (mg g⁻¹ (L mg⁻¹)^1/n_F), n_F, q_m (mg g⁻¹), K_L (L mg⁻¹), R_L, B (mol² kJ⁻²), $\mathcal{E}$ and E (kJ mol⁻¹) are a parameter about malachite green biosorption capacity of the biosorbent material, a parameter about the biosorption intensity, the maximum malachite green biosorption efficiency, a parameter about the biosorption energy, the equilibrium (separation) factor, a parameter about the energy of biosorption, the Polanyi potential and the mean free energy, respectively. The results of isotherm study for malachite green biosorption are listed in Table 2. For the treatment of malachite green from aquatic medium, the biosorption isotherm was well described by Freundlich model with an R² of 0.991. This result indicated that a multi-layer biosorption mechanism operated between malachite green molecules and the active binding sites on the heterogeneous surface of biosorbent material [44]. Further, n_F, R_L and E parameters for malachite green biosorption were calculated as 1.414, 0.335–0.602 and 0.990 kJ mol⁻¹, respectively. n_F parameter (Freundlich isotherm model) between 0 and 10 shows a favourable biosorption process [37]. Again, R_L parameter (Langmuir isotherm model) between 0 and 1 also indicates a favourable biosorption operation [9]. On the other hand, E parameter (Dubinin-Radushkevich isotherm model) between 0 and 8 kJ mol⁻¹ refers a physisorption process, and between 8 and 16 kJ mol⁻¹ refers a chemisorption process [35]. These findings indicated a physical and favourable biosorption process for malachite green treatment.

Table 2. Results of isotherm study for malachite green biosorption.

Model	Parameter
Freundlich
	KF	nF	R^2	AdjR^2	RMSE
	mg g^−1 (L mg^−1)^1/nF	–	–	–	–
	31.511	1.414	0.991	0.983	4.093
Langmuir
	qm	RL	R^2	AdjR^2	RMSE
	mg g^−1	–	–	–	–
	117.745	0.335–0.602	0.978	0.956	6.487
Dubinin-Radushkevich
	qm	E	R^2	AdjR^2	RMSE
	mg g^−1	kJ mol^−1	–	–	–
	101.108	0.990	0.862	0.725	16.242

3.8. SEM and FTIR studies

The surface features and active site profile of biosorbent material were investigated by SEM and FTIR experiments to characterize the interaction mechanism between malachite green molecules and the biosorbent surface. Figure 6 presents the image of SEM for the biosorbent (a) and malachite green-laden biosorbent (b). SEM study showed an irregular and uneven biosorbent surface morphology. Such a surface structure provided the biosorbent material with sufficient space for the biosorption of a large number of malachite green molecules [16, 30, 45, 46]. The change in surface morphology of the biosorbent after malachite green biosorption resulted from the binding of malachite green molecules to the surface of the biosorbent. Figure 7 displays the spectrum of FTIR for the biosorbent (blue) and malachite green-loaded biosorbent (red). The main FTIR spectrum bands observed on the biosorbent material were at the positions of 3333, 2922, 2854, 2378, 2349, 2309, 1730 cm⁻¹, 1603, 1508, 1417, 1317, 1259, 1028, 894, 648 and 599 cm⁻¹. These FTIR bands showed the presence of chemical groups such as O–H, C–C, N–H, C–H, C=O, C–N, C=C, C–O and C–O–C, which are in the structure of biological macromolecules in the biosorbent material [3, 9, 13, 35, 36, 47]. FTIR characterization study indicated that the biosorbent surface was rich in active site. The change in FTIR spectrum of the biosorbent after malachite green biosorption showed that this rich active site composition played an important role in malachite green treatment operation.

Figure 6. SEM image for biosorbent (a) and malachite green-laden biosorbent (b).

Figure 7. FTIR spectrum for biosorbent (blue) and malachite green-loaded biosorbent (red).

3.9. Performance comparison study for malachite green biosorption

Table 3 presents a literature review comparing the performance of various biosorbent materials for malachite green biosorption from aqueous medium. The biosorbent exhibited a good biosorption performance compared to its competitors, with a capacity of 117.745 mg g⁻¹. The comparison study indicated that the biosorbent material could be a promising candidate for the effective, economical, eco-friendly and sustainable treatment of malachite green micropollutant from wastewater medium.

Table 3. Performance comparison study for malachite green biosorption.

	Biosorption
Biosorbent	performance (mg g^−1)	References
PAN/SiO2 (ES) nanofiber	16.8	[48]
Pistacia vera L. shell–based active carbon	90.91	[3]
γ-Fe2O3/MWCNTs/Cellulose nanomaterial	47.61	[49]
Almond peel waste	34.60	[50]
Bauhinia variagata fruit	19.70	[51]
Nano-magnetic Bauhinia variagata fruit	30.1	[51]
Chinese fan palm fruit biochar	21.41	[6]
Cotton stalk biochar	35–40	[17]
Zostera marina waste	103.834	[52]
Spirulina platensis	103.09	[44]
Recombinant gellan gum-coated MgO nanoparticle	92.5	[37]
Brick sand blended fly ash activated carbon	5.88	[2]
Pyracantha coccinea M. J. Roemer	117.745	Present study

4. Conclusions

The treatment of malachite green, a model micropollutant with a wide range of uses, was first investigated using a cost-efficient and environmentally friendly biosorbent material obtained from P. coccinea M. J. Roemer in this study. The maximum biosorption efficiency was obtained at contact time of 360 mins, malachite green charge of 15 mg L⁻¹, pH of 8 and biosorbent amount of 10 mg. The biosorption kinetics followed the pseudo-second-order model (R²: 0.996), while the biosorption isotherm followed Freundlich model (R²: 0.991). Thermodynamics study showed a physical, spontaneous and feasible treatment process. The SEM and FTIR experiments study indicated that the biosorbent had an irregular and uneven surface morphology rich in active site. The biosorbent material exhibited a good biosorption performance compared to its competitors. Finally, the present study showed that P. coccinea M. J. Roemer-based natural material could be a promising biosorbent for the low-cost, effective and environmentally friendly treatment of malachite green-contaminated wastewater environment.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All data generated or analysed during the study are included in the article.