1. Introduction
An increase in global food production/consumption inevitably leads to increased waste generation. Food waste induces a loss of resources and has a harmful environmental impact that covers all emissions derived throughout the entire supply chain. Food loss and food waste occur at different stages of the supply chain and are caused by different reasons [
1]. Both depend on the product and region: in middle/high-income countries, it mainly occurs during the distribution and consumption phases, while in low-income countries, the same thing happens during the production and post-harvest phases. The later the product is wasted in the supply chain, the greater its environmental impact, due to the cumulative effect of all emissions during production, processing, transportation etc. [
2].
According to the FAO, in addition to roots and tubers, fruits and vegetables have the highest wastage rates of any food products. Almost half of all the fruit and vegetables produced are wasted. In Serbia, food processing and food waste management are extremely limited. According to a report by the German Agency for International Cooperation titled “Climate Sensitive Waste Management Project in Serbia” [
3], 770,000 tons of food are wasted annually and only less than 10% is recycled. The report estimates that about 90% of the total waste is still disposed of at non-sanitary municipal landfills or even worse, at illegal dumpsites. The report also suggests that providing another route for food waste before it reaches the landfills can results in savings of 580 kg CO
2 eq per ton.
According to the FAO, world fruit production has increased by 54% in the last 20 years, reaching 883 million tons, an increase of 311 million tons. Five fruit species, namely bananas and plantains (18%), watermelons (11%), apples (10%), oranges (9%) and grapes (9%), accounted for 57% of the total production [
4]. In the last two years, global apple production was about 82 million tons, with Serbia being one of the largest apple producers in South-East Europe, with an average production of 468,000 tons over the last five years [
5]. Processed apples are usually used for making beverages, and the main by-product of the processing is apple pomace, which represents 10–35% of fresh fruit. This perishable waste contains mainly skin, pulp (70–76%), seeds (2.2–3.3%), and stalks (0.4–0.9%) and is susceptible to fast fermentation due to its high moisture content, which causes serious problems [
6]. The composition of apple pomace depends of the apple variety and processing technology used [
7]. According to Bhushan et al. [
8], dried apple pomace contains 3.9–10.8% moisture, 0.5–6.1% ash, 2.94–5.67% proteins, 48–62% total carbohydrates, 36.5% insoluble fibers, 14.6% soluble fibers, 1.2–3.9% fat and 3.5–14.32% pectin.
In comparison to apples, beetroot (
Beta vulgaris L.) production and consumption are currently on a smaller scale. However, beetroot is becoming more attractive and accepted by the consumers due to the high levels of desirable compounds, such as folate, manganese, potassium, iron, vitamin C, etc., that promote human health. Since juices consumption has faced a decade-long decline in the US and Europe due to the high levels of sugar content, the still and soft drink industry had to shift towards low and no-calorie products by using vegetables such as beetroot to reduce overall sugar content [
9]. After squeezing the beetroot, pomace (15–30%) is often considered as a waste. Beetroot pomace contains significant amounts of fiber, as well as high concentrations of phenolic compounds and nitrogenous pigments called betalains. However, beetroot pomace is usually sent to landfills or rarely used as livestock feed [
10]. According to Costa et al. [
11], beetroot pomace contains 10.1% moisture, 5.62% ash, 12.64% proteins, 20.83% total carbohydrates, 45.08% insoluble fiber, 20.14% soluble fiber and 1.31% fat.
By relaying on Landfill Directive (1999/31/EC) (Council Directive 1999/31/EC, 1999), EU Waste Framework Directive (2008/98/EC) and the Packaging and Packaging Waste Directive (94/62/EC) (Directive, E.C., 1994) the European Commission pronounces a new targets to reduce landfilling by 2025 for recyclable waste (including bio-waste) in landfills, corresponding to maximum landfilling rate of 25% [
12]. In order to diminish amounts of food waste, the emission of greenhouse gases, and provide a new value of these materials, sustainable solutions needs to be applied [
13].
A number of studies have been reported for heavy metal ions removal by using different kind of food waste, such as sunflower seed husks [
14], Oleaster seed [
15], peach stone [
16], banana peel [
17], pecan nutshell [
18], corn silk [
19],
Solanum melongena leaf [
20], apple pomace [
21], apples, pears, chokeberry and rosehip pomace [
22], etc.
The study proposed in this paper aims to investigate the potential of apple and beetroot pomace as low-cost and effective sorbents for removing lead ions from synthetic solutions. The chemical composition of pomace makes it suitable for heavy metal binding due to the presence of functional groups that have an affinity towards cations (–CO, –COO, –OH and –NH
2) [
23]. The study examines the impact of different operating parameters, such as pH, temperature, contact time, initial lead concentration, and sorbent dosage, on the capacity of the sorbents to remove lead ions from the solution. The data obtained will be simulated using different isotherm and kinetic models to evaluate the lead sorption onto the pomace and determine the maximum capacity and nature of the sorption mechanism. This study has the potential to provide a sustainable solution for the utilization of pomace waste and contribute to the reduction of heavy metal pollution in the environment.
2. Materials and Methods
2.1. Biosorbent Preparation
The apple pomace was obtained from plant “Fruvita” (Smederevo, Serbia) and the beetroot pomace was obtained from plant “Zdravo” (Selenča, Serbia), at industrial scale level. No treatment other than squeezing of thoroughly washed apples and beetroots was used. Both pomaces were collected aseptically, after squeezing and dehydrated as described in patent [
24]. Dehydrated pomace were ground to a particle size below 300 microns. Obtained samples were marked as AP (apple) and BR (beetroot).
2.2. Metal Solution Preparation
Lead stock solution (1000 mg/L) was prepared by dissolving required mass of Pb(NO3)2·3H2O (analytical grade) in deionized water. For the experimental purposes, the initial solution was further diluted with deionized water to obtain desired concentrations.
2.3. Characterization of AP and BP
LabSwift-aw, Novasina AG, Switzerland AW-meter was used to measure the water activity of the samples at 25 °C [
24].
In order to determine mineral composition, materials were dissolved by using standardized microwave-assisted acid dissolution procedure (EPA Methods 3052) in High-performance Microwave Digestion System ETHOS UP, Milestone. After sample digestion, minerals concentrations were detected by atomic absorption spectrometry (AAS) using PerkinElmer PinAAcle 900T, USA. Analyses were performed with three replicates, where the average values are presented.
Scanning Electron Microscopy—Energy Dispersive X-ray Spectroscopy (SEM-EDX) analysis was performed with dried samples coated under vacuum with thin layer of gold and observed using a JEOL JSM-6610 LV model (JEOL Ltd., Japan).
Cation exchange capacity (CEC) of the both samples was determined by soaking 0.2 g of the sample in 100 mL of 1.0 mol/L ammonium acetate [
14]. After 120 min of shaking at 250 rpm, the suspension was filtered and supernatant with released exchangeable cations (K
+, Na
+, Ca
2+ and Mg
2+) has been analyzed on AAS.
In order to determine the surface charge of the samples, zeta potential was measured by using Zetasizer Nano Z (Malvern, UK). The samples were dispersed in distilled water which pH value was adjusted at pH 5.0. The measurements of zeta potential were repeated five times, and the average values are presented. Refractive index (RI) for both samples was 1.35.
Surface functional groups were determined by Fourier Transform Infrared Spectroscopy (FTIR-ATR mode) using Thermo Nicolet 6700 FTIR (International Equipment Trading Ltd., USA). Both samples were analyzed before and after the sorption of lead, in order to record changes induced by the metal binding, as well as material behavior during the sorption process.
2.4. Sorption Experiments
In order to investigate effect of different operating parameters onto AP and BR sorption behavior, sorption experiments were performed in a batch system. The suspensions were agitated on orbital shaker (Heidolph, Unimax, Germany) at 250 rpm. After filtration (Whatman 542) lead residual concentration in solution was determined by AAS. The amount of sorbed Pb
2+ onto sorbent (
qe (mg/g)) is calculated using the following equation:
where
V (mL) is volume of the solution,
m (g) is a weight of the sorbent,
Ci (mg/L) and
Ce (mg/L) are the initial and the equilibrium Pb
2+ concentration in solution, respectively. All experiments were conducted in triplicate, and the average value is presented.
Effect of the initial pH value of lead solution was studied in the range between pH 2.0 and 6.0 by adding 0.1 mol/L HNO3 or 0.1 mol/L KOH. In order to prevent metal hydroxide precipitation pH values higher than 6 weren’t included in investigations. Initial lead concentration in solution was 200 mg/L. The mixture was shaken on orbital shaker at 250 rpm for 120 min. The solid/liquid ratio was 2 g/L. After filtration the residual metal concentration was determined by using AAS. The initial and the final pH values of the solution were measured by pH meter (Hach Sension+ MM340 GLP). Effect of sorbent concentration was performed by varying concentrations from 2 to 12 g/L. In this set of experiments initial metal concentration was 200 mg/L, while initial pH of solution was set to 5.0. Contact time between solid and liquid phase was 120 min. Effect of contact time on sorption capacity, was investigated in range from 2 to 180 min. Effect of initial lead concentration was studied in range from 40 to 400 mg/L, while all other parameters were the same as it was described previously. Investigations of the temperature effects on Pb2+ sorption onto both sorbents were conducted in the range from 293 K to 323 K, under the following operational conditions: Ci = 200 mg/L, pH = 5, solid/liquid ratio 2 g/L, contact time 120 min and stirring speed 250 rpm. Obtained experimental data were used in thermodynamic studies.
2.5. Kinetic and Isotherm Studies
In order to get more profound explanation of bonding mechanism, experimental data were fitted by different kinetic and isotherm models listed in
Table S1.
2.6. Thermodynamic Study
Sorption of divalent cations onto the solid sorbent surface can be described as reversible heterogeneous sorption process [
25]. In order to conclude whether this process is spontaneous or not, it’s thermodynamic consideration is necessary to be done. An indicator of chemical reaction spontaneity is the Gibb’s free energy change, Δ
G0, including both changes in enthalpy (Δ
H0) and entropy (Δ
S0) of the sorption process. The sorption process free energy is related to the equilibrium constant, and can be described by the following equation:
where
R represents universal gas constant (8.314 J/mol/K) and
T is temperature in K, while
Ke0 (dimensionless) represents the thermodynamic equilibrium constant of adsorption.
After rearranging, Equation (2) becomes:
Constant
Ke0 might be calculated from distribution constant (
Kd), which represents distribution of metal ions between solid and liquid phase after equilibrium, defined as:
where
Ce (mg/L) is the equilibrium concentration and
qe (mg/g) is the sorption capacity at equilibrium.
In order to transform
Kd to dimensionless
Ke0, it is necessary to multiply this constant by 1000 [
26,
27].
The fundamental thermodynamic relation of three thermodynamic parameters (Δ
G0, Δ
H0 and Δ
S0) is described as:
Assuming that the changes in Δ
H0 and Δ
S0 with temperatures are negligible, after substituting Equation (3) into Equation (5), the linear (Equation (6)) form of the well-known van’t Hoff equation is achieved.
In this paper, thermodynamic study was conducted in temperature range from 293 K to 323 K under operational conditions previously determined. Parameters ΔH0 and ΔS0 are determined from the slope and the intercept of the plot of ln Ke0 vs. 1/T, and were used, along with standard Gibbs free energy change (∆G°), to explain the sorption behaviour.
4. Conclusions
This study was focused on sorption performance of dried apple and beetroot pomace toward lead ions from aqueous solution. Both materials showed excellent sorption abilities and experimental data obtained by using the AP in sorption experiments were well-fitted to Redlich-Peterson, Sips and Langmuir isotherm models, indicating monolayer sorption. In case of the BR the situation was different and correlation coefficients of all models were very similar suggesting that sorption mechanism is much more complex. The maximum sorption capacities from Sips are 31.7 and 79.8 mg/g for AP and BR, respectively.
Cation exchange capacity (CEC) for AP and BR is 24.75 and 95.85 meq/100 g, respectively. BR has significantly higher CEC, due to the presence of potassium ions which are dominant ions in the exchangeable position. The study of sorption mechanism confirmed that ion exchange mechanism plays an important role during the process of sorption, and in case of the AP it was observed that the hydrogen ions are more exchanged with lead ions along with the rise of metal solution concentration.
The obtained maximum sorption capacity of AP is two times higher in comparison to maximum sorption capacity obtained by other researchers. The way of sample preparation could contribute to preservation of functional groups which have major role in metal binding.