Apple peels as a potential adsorbent for removal of Cu and Cr from wastewater

Water makes up 65% of our bodies and covers up to 71% of Earth's surface. Clean water for drinking, bathing and other uses is a basic necessity of society. Contaminated water loses its aesthetic as well as economic value and it can be dangerous for the survival of aquatic creatures that rely on it. Wastewater is defined as water that carries solid or liquid waste gathered from industries, houses and institutions as well as from storm water, ground water or surface water. Wastewater contains high concentrations of organic materials, inorganic compounds, minerals, sediments, oxygen demanding waste, plant growth nutrients, pathogenic and disease producing agents. It could also include poisonous substances (Sonune & Ghate 2004). Wastewater contains toxic heavy metals. Heavy metals are described as metallic elements with a density that is higher than that of water, generally more than 5 g/cm³. There are many elements included in this category such as copper, chromium, zinc, lead, mercury, arsenic, nickel and cadmium. This can cause disorders of the joints, such as rheumatoid arthritis, as well as diseases of the nervous system, kidney, lungs and circulatory system, and fetal brain damage. It can create serious mental disorders at higher levels (Polat & Erdogan 2007).

This study focuses on the removal of copper and chromium from wastewater. Chromium is a hazardous metal that is present in waste streams. Tanning, dyeing, explosives, painting, pottery, wood processing, and the paper industry have all employed it. It is present in the form of both Cr (III) and Cr (VI). One of the most hazardous forms of chromium is hexavalent chromium (Enniya et al. 2018). When Cr (VI) is present in amounts higher than 0.05 mg/L for potable water or 0.1 mg/L in water used for various purposes, it causes health problems such as skin allergies, liver, stomach and kidney injuries, and lung cancer (Ajmani et al. 2019).

Copper is an element that occurs naturally and is found in drinking water. Stagnation of water in copper and copper alloy-containing pipes and fittings in distribution systems and domestic plumbing allows leaching and raises copper levels in the water (National Research Council 2000). Copper poisoning can result in vomiting, diarrhea, nausea and stomach cramps. Copper is more readily retained in the bodies of some newborns and children, people with liver disease, and those with Wilson's disease who are more prone to have negative health impacts such as kidney and liver damage (Demiral & Güngör 2016). In Pakistan the pollution status of various heavy metals is of great concern. Pakistan is facing different environmental and health problems due to copper and chromium pollution. The discharge of different industrial, municipal and medical waste in lakes and rivers increases this pollution (Waseem et al. 2014).

The materials used in these experiments were apple peels purchased from the local market in Bahawalpur District, Pakistan, zinc sulfate heptahydrate (ZnSO₄.7H₂O), copper sulfate pentahydrate (CuSO₄.5H₂O), potassium chromate, sulfuric acid, ammonia, and sodium hydroxide. All the solutions were prepared in distilled water.

Copper sulfate pentahydrate (CuSO₄.5H₂O) was used to make copper stock solution (1,000 ppm); 0.39 g of (CuSO₄.5H₂O) was added to a 100 ml measuring flask and diluted up to the mark with ‘double distilled water’. All the required solutions were prepared with analytical reagents and double distilled water. Potassium chromate (K₂CrO₄) stock solution was prepared by adding 0.373 g to a 100 ml measuring flask and diluting up to the mark with double distilled water to obtain 1,000 ppm (mg/L) of Cr (VI) stock solution. Synthetic samples of different concentrations of Cu and Cr (VI) were prepared from these stock solutions by appropriate dilutions.

Standard solutions (10–250 ppm) were prepared from stock solution by taking different volumes of 1,000 ppm solution and diluting up to 50 ml with distilled water to prepare 10–250 ppm (Cu, Cr) mix solution (50 ml), respectively.

Apple peels were collected from the local market and washed twice with distilled water to remove impurities. Washed apple peels were air dried, then these dried peels were ground into fine powder and passed through 40 mesh sieves and stored in plastic zipper bags. Apple peels were further used to prepare adsorbents for the removal of heavy metals (Cr, Cu) from aqueous solutions by following literature (El-Ashtoukhy et al. 2008; Yi et al. 2017).

The nitrogen flow was turned on and nitrogen was allowed to pass through the vessel for 3 minutes at 200 kPa. After shutting off the nitrogen flow, the second hole was also quickly sealed with mud to prevent any air from entering the vessel during the carbonization process. The charring time was 4 hours and the process was carried out over a low flame. The charcoal so obtained was finely ground in a pestle and mortar and passed through a sieve of mesh size 0.6 mm to obtain a uniform particle size.

Apple peels (25 g) were added to a 500 ml conical flask and distilled water (250 ml) was added. Solution of zinc sulfate (0.25 M) was added in a dropping funnel. The assembly was set to add this zinc sulfate to the mixture of apple peels in the flask containing the separating funnel drop wise to mix. Stirring was done at 50 °C. After complete addition the pH of the solution was maintained to 10 by the drop wise addition of ammonium hydroxide (4 M), via the dropping funnel. Further stirring was done for 1 hour at 70 °C.

Adsorbents are characterized by different instruments such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and energy dispersive X-ray (EDX). FTIR was used for the identification of functional groups in apple peels. It also provides quantitative information. SEM is used to study the surface of solid objects. It was used to obtain information about the composition and surface topography of apple samples. An energy dispersive X-ray analyzer (EDX or EDA) was used to check the elemental composition of all adsorbents and to provide quantitative information. An atomic absorption spectrophotometer measures metals concentrations from different solutions and was used to check the Cu/Cr concentrations of metal solutions by using apple peels, apple peel charcoal and treated apple peels as adsorbents.

Kinetic studies of each adsorbent were carried out by varying the contact time (15–90) minutes. Mix solutions of 50 ml (Cr, Cu) were prepared by keeping the constant initial concentration of 10 ppm. Each reaction was carried out at 150 rpm at constant temperature 35 °C. Pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic models were applied.

Adsorption isotherms were studied by varying the initial Cu/Cr concentration (10–250) ppm. Cu/Cr mix solutions of 50 mL were prepared by adding constant adsorbent dose 0.3 g; reactions were carried out at 150 rpm at constant temperature of 35 °C. Freundlich and Langmuir isotherm models were applied to check their adsorption capacities.

Adsorption capacity of each adsorbent was studied in this parameter by varying the contact time (15–90 minutes). Reactions were carried out at constant adsorbent dose and metal initial concentration of 0.3 g and 10 ppm, respectively. With the increase in contact duration the adsorption capacity of adsorbents increases up to 60 minutes. The greater the contact time, the higher the number of chances that the adsorbent surface must absorb the adsorbate molecules. After 60 minutes adsorption capacity of adsorbents decreases.

The intercept and slope of a plot of log q_e vs. log C_e can be used to calculate K_f and n.

Table 3 shows the adsorption capacities of different adsorbents that were investigated in literature. For example, corn stalks-derived ACs had maximum adsorption efficiency Q_max of almost 89.5 mg/g for Cr (Zhao et al. 2020), while hazelnut shell activated carbon had Q_max 58.27 mg/g for Cu (Samet & Valiyaveettil 2018). As studied in literature, removal efficiencies of different adsorbents increased by chemical modification or by preparing activated charcoals rather than using simple agricultural wastes (Yi et al. 2017).

This study focuses on the removal of toxic metals from wastewater by using agricultural waste. In this study apple peels are used as adsorbents to remove heavy metals Cu/Cr from wastewater. Apple peels were used in three different ways as unmodified AP, APC and TAP. The coating of zinc sulfate on the surface of apple peels after chemical treatment causes the increase in the removal capacity of Cu and Cr. Adsorbents were characterized with FTIR, scanning electron microscope, and energy dispersive X-ray analysis. The effect of time, metal initial concentration, temperature, adsorbent dosage and pH were studied by batch sorption studies. Experimental data shows increase in uptake of metals with increase in concentration of metals. Solutions kinetic studies were carried out by taking into consideration pseudo-first-order, pseudo-second-order and intraparticle diffusion. Both Freundlich and Langmuir isotherms were applied. This study showed better adsorption capacities for chemically modified adsorbents. It is concluded that unmodified apple peels, modified apple peels and apple charcoal are effective low-cost adsorbents for removal of Cu and Cr from water.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.