The leaching behavior of organic carbon and nutrient compounds from banana peel (BP) was investigated in batch assays with respect to particle size, contact time, pH value, and temperature. The granularity, contact time, pH, and temperature caused no significant effects on the leaching of total phosphorus (TP) from the BP. The maximum leached total nitrogen (TN) content was found at pH 5.0 and 90 minutes, while no significant effects were caused by the granularity and temperature. The maximum leached total organic carbon (TOC) content was found by using a powder of 40 mesh, 150 minutes and at pH 6.0, while the temperature had no effect on the TOC leaching. The proportions of the TN, TP, and TOC contents leached from the dried BP ranged from 33.6% to 40.9%, 60.4% to 72.7%, and 8.2% to 9.9%, respectively, indicating that BP could be a potential pollution source for surface and ground water if discharged as domestic waste or reutilized without pretreatment.

NOMENCLATURE

     
  • BP

    banana peel

  •  
  • TN

    total nitrogen

  •  
  • TOC

    total organic carbon

  •  
  • TP

    total phosphorus

INTRODUCTION

Banana is one of the most consumed fruits in the world. Preliminary investigations have shown that several tons of banana peel (BP) are produced daily in market places and in household garbage (Anwar et al. 2010; Kima et al. 2015). BP, accounting for 30–40% of the total weight (Oberoi et al. 2011), has no significant industrial or commercial value (Albarelli et al. 2011), and thus represents a major agro-waste problem. BP is usually disposed of as kitchen waste together with other domestic waste, which, at present, is mostly disposed of in landfills (Hameed et al. 2008; Yan et al. 2014; Zhang et al. 2014). Thus, a suitable method of disposing of or utilizing BP has been receiving increased research interest.

BP is composed of several chemical constituents, including pectin (galacturonic acid), hemicellulose, cellulose, and lignin acid (Noeline et al. 2005). These chemical constituents bear various polar functional groups, including carboxylic and phenolic acid groups (Wang & Chen 2009; Nguyen et al. 2013), which can be involved in metal-ion complexation. Thus, the utilization of BP as a biosorbent could be economically viable. The advantage of BP as an alternative absorbent is not only its excellent sorption capacity for heavy-metal ions (Annadurai et al. 2002; Memon et al. 2008a, 2009; Nada et al. 2010), but also its low cost, and regenerative and nontoxic properties (Volesky 2007; Park et al. 2008, 2010; Febrianto et al. 2009; Liu et al. 2012).

There have been several attempts to investigate the sorption capacity of BP in recent years (Nguyen et al. 2013). Anwar et al. (2010) evaluated the efficiency of BP as an adsorbent for cadmium and lead, and successfully modeled the sorption data with three two-parameter equations. Albarelli et al. (2011) demonstrated that BP can be used successfully for the adsorption of copper ions after being subjected to supercritical fluid extraction. The results of Memon et al. (2008b) indicated that BP modified with acidic methanol (99.9% methanol with 0.1 mol/L HCl) can significantly increase its Cr(III) adsorption capacity. Achak et al. (2009) investigated the efficiency of BP as a biosorbent for the removal of phenolic compounds from olive-mill wastewater. Additionally, methylene blue could also be removed effectively from aqueous solution by adsorption using banana stalk waste (Hameed et al. 2008).

Thus, BP has become an alternative to more costly adsorbents such as activated carbon. However, some soluble organic substances and nutrient compounds in BP might also be released into solution in the sorption process, which could threaten aquatic life by increasing the total organic carbon (TOC), nitrogen (Zhou et al. 2013), and phosphorus contents in water (Feng et al. 2009). Currently, however, there is not much information available about the leaching behavior of BP. The aim of the present work is to investigate the total contents of organic substances and nutrient compounds, and, in particular, to investigate their leaching behavior from BP into solution under different environmental conditions, including granularity, contact time, pH value, and temperature.

MATERIALS AND METHODS

Sample preparation

BP was collected from a local market and carefully washed with tap water, before rinsing three times with ultrapure water (>18 MΩ). Subsequently, the peel was cut into small pieces and dried in an oven (PHL-1003, Shandong, Longkou, PR China) at 80 °C until reaching a constant weight. Before the batch experiments, the dried BP was ground to powders with granularities of 80 meshes (<200 μm), 60 meshes (200–250 μm), 40 meshes (250–400 μm), and 20 meshes (>400 μm) and then stored in a desiccator at room temperature (25 °C).

Determination of total organic content, nitrogen, and phosphorus in BP

To determine the total nitrogen and phosphorus concentrations in BP, 0.1 g of each powdered sample (80 meshes) was wet digested with 10 mL of a concentrated H2SO4–HClO4 (3:1) acid mixture in a 25-mL Teflon PFA (perfluoroalkoxy) vial. After 3 drops of HF acid were added, the mixtures were heated to a clear solution, and then further heated to near dry. The cooled residue was dissolved in 5 mL of 5% HCl, and the solution was diluted to 25 mL with ultrapure water (>18 MΩ) for the measurements. The organic carbon content of the BP was determined by loss on ignition at 450 °C for 3 hours.

Leaching assays for BP

Effect of contact time on the leaching behavior of BP

Point five grams of the prepared BP (80 meshes) was weighed out and put into 21 polyethylene (100 mL) centrifuge tubes, 50 mL of ultrapure water (>18 MΩ) were added to each tube before shaking in a vapor-bathing constant temperature vibrator (HZQ-Z, Jiangsu, Jintan, PR China) at 240 rpm and 25 °C. The pH values of the liquid mixtures were all approximately 5.0. Three tubes were taken out at a series of given sampling time intervals after the initial assay time to investigate the leaching behavior of the BP as a function of contact time.

Effect of granularity on the leaching behavior of BP

To investigate the effect of particle size on the leaching behavior of BP, 0.50 g of the prepared BP samples with granularities of 20, 40, 60, and 80 mesh were weighed out and put into 12 polyethylene centrifuge tubes (100 mL). The other leaching procedures and conditions were the same as those for the contact time assay, and the sampling time was set to 90 minutes.

Effect of pH values on the leaching behavior of BP

To study the effect of pH on the leaching behavior of BP, 0.5 g of the prepared samples (80 meshes) and 50 mL of ultrapure water (>18 MΩ) were put into 18 polyethylene centrifuge tubes (100 mL). The pH values of the suspensions were adjusted to 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 using 1 mol/L NaOH and 1 mol/L HCl, and then placed into a vapor-bathing constant temperature vibrator at 240 rpm and 25 °C. After shaking for 90 minutes, the suspensions were taken out and filtered for analysis.

Effect of temperature on the leaching behavior of BP

To study the effect of temperature on the leaching behavior of BP, the temperature of the vibrator was set to 15, 25, 35, and 45 °C. The weights of the samples, sampling time, and leaching procedures were the same as those for the pH assay, and the pH values of the liquid mixtures were all approximately 5.0.

Analysis and data processing

The samples collected from each leaching assay were filtered through a 0.45-μm cellulose acetate membrane filter, and the filtrates were analyzed for total nitrogen (TN), total phosphorus (TP), and TOC. The concentrations of TN and TP in the leached liquor were measured using an ultraviolet spectrophotometer (UV-756, Shanghai, PR China) according to APHA (1998). The TOC was tested using a TOC Instrument (TOC-LCPH/CPN, Shimadzu, Japan).

All experiments were carried out in triplicate, and the averages, together with the corresponding standard deviation, are used for the discussion. All chemical reagents were of analytical reagent grade. All glassware and polyethylene tubes were soaked in 3% HCl overnight before being used in the experiments. Excel 2003 and SPSS 11.5 software packages were used to produce figures and conduct statistical analyses.

RESULTS AND DISCUSSION

Effect of contact time on the leaching behavior of BP

The total amounts of organic matter, nitrogen, and phosphorus were 886.99 ± 9.04, 7.08 ± 0.01, and 1.81 ± 0.06 mg/g in dry BP, respectively. High concentrations of the contaminants contained in BP could be released into water, and consequently pose a threat to the aquatic system.

The contact time was evaluated as an important factor affecting the leaching behavior. The effect of contact time on the leaching behavior of BP is presented in Figure 1. The TN content released from the BP ranged from 2.80 to 3.06 mg/g, accounting for 40.9 ± 1.6% of the TN content of the BP. The maximum amount of leached TN was found after 90 minutes, which was followed by a slight decrease. For the TP content, no significant differences were observed for different contact time, with the leached TP contents ranged from 1.27 to 1.33 mg/g. The percentage of TP leached was 72.7 ± 1.3% of the total in the BP, which was much higher than that of TN. The average TOC content leached from the BP was 78.25 ± 0.98 mg/g, and no significant differences were found within the contact time of 120 minutes. However, the maximum leached TOC content was found after 150 minutes (84.39 mg/g) and 180 minutes (84.39 mg/g), which was 7.16% higher than the leached TOC contents within 120 minutes. This indicates that the contact time may exert some effect on the leaching of TOC from BP, with an equilibration period of 150 minutes. The maximum amount of TOC leached from the BP accounted for 9.5% of the total organic matter, representing a much smaller proportion than those of TN or TP.

Figure 1

Effects of contact time on the leaching behavior of banana peel.

Figure 1

Effects of contact time on the leaching behavior of banana peel.

Effect of granularity on the leaching behavior of BP

The granularity of particles plays an important role in the leaching behavior of substances due to the direct effects of the surface characteristics and agglomeration of the BP powders. The effect of granularity on the leaching behavior of BP is depicted in Figure 2. The TN content released from the BP ranged from 2.69 to 2.81 mg/g, accounting for 38.8 ± 0.7% of the TN content in the BP. It can be seen from Figure 2 that no significant differences (p > 0.05) in the leached TN contents can be identified among the various BP powders. Similarly, the leached TP content was also unaffected by the granularity of the BP powders, with the leached TP content ranging from 1.22 to 1.27 mg/g, corresponding to 69.2 ± 1.3% of TP in the BP. However, the maximum leached TOC content was found to be 100.15 mg/g for the 40-mesh BP powder, which was much higher than that of the other BP powders, indicating that the granularity had a significant effect on the leaching of TOC from BP. The leached TOC contents accounted for 9.8 ± 0.1% of the TOC in the BP powders of 20, 60, and 80 mesh, and 11.3% for BP powders of 40 mesh.

Figure 2

Effects of granularity on the leaching behavior of banana peel.

Figure 2

Effects of granularity on the leaching behavior of banana peel.

Effect of pH on the leaching behavior of BP

It is well known that the pH value affects the degree of dissociation of functional groups from the biomass surface (Deng et al. 2007). The pH value of the suspension was adjusted to 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 using 1 M NaOH and 1 M HCl. The effects of the pH value on the leaching behavior of BP are presented in Figure 3. The leached TN content increased with pH varying from 3.0 to 5.0, with a maximum leached TN content of 2.72 mg/g at pH 5.0, which was then followed by a slight decrease, as shown in Figure 3. This suggests that the pH of the solution significantly affects the amount of TN leached from BP. The TN content released from the BP ranged from 2.08 to 2.72 mg/g, accounting for 33.6 ± 3.5% of the TN content in the BP. Similar trends were also found for the TOC content leached from BP at various pH values. The maximum leached TOC content was 79.57 mg/g at pH 6.0, which accounted for 8.97% of the total organic matter in the BP. However, the pH of the solution did not have any significant (p > 0.05) effect on the leaching of TP from BP. The TP content leached from the BP ranged from 1.03 to 1.15 mg/g for pH values varying from 3.0 to 8.0, accounting for 60.4 ± 2.2% of TP in the BP.

Figure 3

Effects of pH of solution on the leaching behavior of banana peel.

Figure 3

Effects of pH of solution on the leaching behavior of banana peel.

Effect of temperature on the leaching behavior of BP

The leaching liquid mixtures were kept at 15, 25, 35, and 45 °C to investigate the effect of temperature on the leaching behavior of BP, and the results are shown in Figure 4. The TN, TP, and TOC contents released from the BP ranged from 2.83 to 2.95 mg/g, 1.25 to 1.27 mg/g, and 85.33 to 89.55 mg/g, respectively. In contrast to the effects of granularity, contact time, and pH value of the solution on the leaching behaviors of TN, TP, and TOC, the temperature of the liquid mixture can clearly be seen (Figure 4) to have no significant (p > 0.05) effect on the TN, TP, and TOC contents leached from BP. The results suggest that room temperature (25 °C) would be suitable for conducting and evaluating leaching behavior of BP. The average proportions of TN, TP, and TOC contents leached from the BP were 40.6 ± 0.7%, 69.6 ± 0.6%, and 9.9 ± 0.2%, respectively.

Figure 4

Effects of temperature on the leaching behavior of banana peel.

Figure 4

Effects of temperature on the leaching behavior of banana peel.

Thus, high amounts of the contaminants embedded in BP were found, their leaching behavior from BP into solution as a function of the granularity of powder, contact time, pH value, and temperature was investigated. The results obtained from this study indicate that BP could be a potential pollution source for surface and ground water if discharged directly as domestic waste or reutilized without pretreatment.

CONCLUSIONS

High amounts of the contaminants embedded in BP. The total amounts of organic matter, nitrogen, and phosphorus in dry BP were 886.99, 7.08, and 1.81 mg/g, respectively. TP leached from BP was not significantly affected by granularity, contact time, pH value, or solution temperature. The pH value and contact time significantly affected the leaching of TN from BP. The amount of TOC leached was mainly influenced by the granularity of the powder, contact time, and pH of the solution. The proportions of TN, TP, and TOC leached from dry BP ranged from 33.6% to 40.9%, 60.4% to 72.7%, and 8.2% to 9.9%, respectively, indicating that BP could pose a potential threat to the aquatic system if discharged directly as domestic waste or reutilized without pretreatment.

ACKNOWLEDGEMENTS

The present study was co-funded by the National Basic Research Program of China (no. 2008CB418203), the China Postdoctoral Science Foundation (no. 20080431214) and the Shandong Province Natural Science Foundation of China (no. ZR2014EEM005).

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