Biosorption of Copper (II) Ions Using Coffee Grounds—A Case Study

Abstract

Industrial and domestic human activities have a significant impact on the environment, contributing, among other things, to the increased pollution of natural waters. The spread of heavy metals is particularly dangerous to the health and life of living organisms due to the high accumulation potential of, among others, Cr (VI), Zn (II), Cu (II), Cd (II), Fe (II), and Ni (II). In order to remove, concentrate, and/or recover ions of these metals, various physical and/or chemical methods are commonly used. In this study, spent coffee grounds (SCGs) efficiently removed copper ions from simulated aqueous solutions, especially at low metal ion concentrations. Without additional modification, coffee grounds performed comparably to traditional adsorbents like activated carbon or ion exchangers. It was found that used ground coffee grounds effectively removed Cu (II) ions at a wide range of concentrations, with the highest efficiency (over 85%) obtained for dilute solutions. On the other hand, regeneration tests performed using a 10% hydrochloric acid solution successfully restored the coffee residue adsorbent, achieving a desorption efficiency of about 35%. This method concentrated the solution and facilitated efficient metal recovery by minimizing acid usage. The sorbent used is an innovative, cheap, and easy-to-use material with high sorption capabilities.

1. Introduction

The intensive development of industry and changes taking place in the modern world have a significant impact on the natural environment, contributing to the increase in the pollution of natural waters. Particularly dangerous effects of the development of civilization and industry include the spread of heavy metals [1]. They occur mainly in industrial sewage from steelworks, electrochemical plants, and metallurgical, galvanic, and mining industries. Galvanic wastewater, which contains metal ions from which galvanic baths are prepared, is particularly dangerous for the environment and includes Cr (VI), Zn (II), Cu (II), Cd (II), Fe (II), and Ni (II) [2]. These heavy metals are particularly dangerous to the health and life of higher living organisms due to their high toxicity and ability to accumulate. Their presence disrupts the natural biological balance and self-purification processes in water reservoirs. One of these metals is copper, which is considered an element necessary for the body, but excessive doses may cause various diseases [3].

This element is particularly prone to bioaccumulation from aquatic environments. Its toxicity in water is influenced by factors such as pH levels, water hardness, dissolved oxygen concentration, the presence of chelating agents, humic acids, the amount of suspended solids, and interactions between different metals [4]. In surface waters, increased amounts of copper compounds usually come from wastewater from the metallurgical, dyeing, textile, and chemical industries [5]. It can also transfer to water from installations made of copper, brass, or bronze due to corrosion.

The requirements of environmental protection regulations indicate that the concentration of copper (II) ions cannot exceed 0.1 mg/L for wastewater from the ceramics industry and 0.5 mg/L in the case of other types of wastewater [6]. However, the content of these ions cannot exceed 2 mg/L [7] for raw water abstraction and drinking water production.

Physicochemical methods are commonly used to remove, concentrate, and/or recover various metal ions, including precipitation and co-precipitation methods [8], coagulation [9,10,11], electrochemical [12,13,14] and membrane [15,16,17] processes, adsorption [18,19,20,21], and ion exchange [22,23,24,25,26]. In recent years, biosorption has also been used, in which both dead organic matter (organic waste, the so-called biomass) and microorganisms can act as sorbents ([19] or [27,28,29,30,31,32]). Materials of this type are characterized by significant sorption capacity, which has been confirmed by numerous scientific studies. Selected examples of sorbents used to remove copper (II) ions are presented in

Table 1. Sorption capacity of selected sorbents regarding Cu²⁺ ions.

Material	Dose, g/dm3	Co*, mg/dm3	pH	qmax*, mg/g	X*, %	Ref.
Aquatic plants Myriophyllum spicatum	0.5	1	4			[33]
Untreated coffee residues (UCRs) and treated coffee residues (TCRs)	1	0–150	5.0	UCR: 49.34 CR: 56.90	UCR: 70 TCR: 76	[34] [35]
Coffee ground: particle size A > 200 µm and B < 200 µm	0.5	0.5–3	-	-	A: 71.8–85.2 B: 77.8–97.2	[36]
Active carbon from grape pomace	2	10–100	5.0	37.2	-	[37]
Yarrowia lipolytica 70,562 living mass	-	-	6.4	25	99.74	[19]
Activated carbon from wine production waste	0.25	5–20	5.0	22.0	-	[38]
Natural and pretreated maize husk	0.5	100	5.5	11.5–35.7	74–92	[27]
Raw spruce sawdust	10	10	6.3	-	85	[28]
Nasiona Azadirachta indica	5	10–300	5.5	11.5	ok. 57 for 100 mg/dm3	[39]
Active carbon Norit SX2	2	10–10,000	4.0	35.1	5–99	[20]
Rice husk	2	10–10,000	4.0	41.1	5–65	[40]
Azadirachta indica (neem leaf) powder	0.5	100	7.0	-	58.8	[29]
Carnauba straw (CS) and cashew leaf (CL) powders	0.5	50	6.0	9.51 (CS) 1.73 (CL)	79.33 (CS) 19.15 (CL)	[30]
Watermelon peels biochar-S	-	200	5.6	151.52	78.33	[31]
Caryocar coriaceum WITTM. bark (CCB)	0.05	20–500	5.5	32.4	85	[32]

.

The selection of the pollutant removal method is influenced by the type and composition of the sewage, the form and concentration of the contaminants, and the desired level of purification. The efficiency and economic aspect of the method used are also important. Increasingly strict environmental protection regulations lead to the search for innovative and cheap techniques for removing heavy metals from industrial wastewater. One of the most effective methods of eliminating toxic metals from aqueous solutions is adsorption. The most recent and commonly used sorbents are activated carbons of various varieties. Adsorption is an excellent alternative to other techniques used in water and wastewater treatment due to its convenience, ease of use, simplicity of reactor design, and relatively low overall costs. The aim of many studies is to demonstrate that activated carbons can be effectively replaced with equally effective sorbents or, more precisely, biosorbents. They may be waste materials from agricultural or food production.

As Obar et al. [41] points out, approximately 1.6 billion tons of food waste is produced every year. This number is likely to increase to 2.1 billion tons by 2030, which will create real problems for solid waste management. Of the bulk of food, beverage, and used coffee ground (SCG) waste, almost 15 million tonnes on a wet-weight basis ends up in garbage. If such waste became a raw material to be used as a sorption material, then its management would be consistent with the rules of the Circular Economy. However, a necessary condition is that biomass sorbents, also called biochars, are not toxic to the environment and have high sorption capacity. A good example of such waste material is coffee grounds [42].

It is worth pointing out that coffee is currently the second most frequently consumed agrifood product after tea [43] and, therefore, an important drink consumed all over the world. Coffee is also a traditional product produced in countries such as Brazil, Colombia, and Vietnam, which generated total revenues of approximately USD 11 billion in 2019 [44]. Recently, global coffee consumption has increased by approximately 2% annually, reaching almost 10 million tons [45] (International Coffee Organization); almost 50% of global coffee production is processed into instant coffee, which results in high waste production.

With the world population expected to increase by 25% by 2050, from 7.7 to 9.7 billion people [46] (United Nations), coffee is likely to play an even greater role in the coming decades. Therefore, such significant coffee consumption already leads and will in the future lead to the generation of very large amounts of solid coffee waste (i.e., residues remaining after the extraction of coffee solution from coffee beans), such as pulp, husks, coffee beans, and spent grounds. The wide possibilities of using these wastes, especially spent coffee grounds (SCGs), which are rich in beneficial ingredients and are considered products with high added value, are presented in Figure 1.

SCG can, therefore, be widely used in the fields of food, pharmaceuticals, cosmetics, energy, and materials, both after appropriate disposal and specific surface transformation [47,48].

Figure 1. Possibilities of managing spent coffee grounds (SPGs) based on Atabani et al. [48].

Figure 1. Possibilities of managing spent coffee grounds (SPGs) based on Atabani et al. [48].

It should be noted here that the 2030 Agenda for Sustainable Development adopted by the United Nations, which is implemented by the member states of the European Union (EU), indicates 17 sustainable development goals and 169 tasks related to them, in three dimensions—economic, social, and environmental. The presented study is part of activities to improve water quality, which are to include, among others, the pursuit of reducing pollution, doubling the amount of treated sewage, and significantly increasing the recycling and reuse of materials (e.g., waste as raw materials) on a global scale. This is to ensure that all people have access to drinking water and for living purposes. The listed tasks are defined in goal number six of the agenda.

This manuscript presents an assessment of the possibility of using unprocessed spent coffee grounds after brewing in an espresso machine as a biosorbent for the removal of Cu²⁺ ions from aqueous solutions.

In this research, we decided to use raw coffee grounds, not subjected to additional processing, because previous studies indicated a slight difference in the degree of purification of the tested solutions from copper ions (about 8%) and in the values of the maximum sorption capacity of raw and processed coffee grounds (approximately 8 mg/g) [35].

The main objective of this work is to quantify the degree of removal of copper (II) ions from simulated aqueous solutions in relation to the initial content of these ions in a wide range of concentrations for the tested samples. Sorption processes were interpreted based on the Langmuir and Freundlich adsorption model. In addition, the possibility of regenerating the used sorbent and recovering copper from it was also checked. A thesis was put forward that unprocessed spent coffee grounds can be a cheap and easy-to-use substitute for commonly used but expensive activated carbons. However, as a waste material, it can be a desirable sorption raw material, contributing to the innovative use of its natural properties.

2. Materials and Methods

2.1. Sorbent Preparation

This research used 0.5 g of raw (unmodified) spent coffee grounds after brewing in an espresso machine, which were then dried at room temperature (air-dry state) and sifted into powder with a grain diameter ranging from 0.8 to 1.0 mm. The physical properties of untreated coffee grounds were assumed to be as follows [49]: moisture 1.71%; organic compounds 97.13%; mineral compounds 1.16%; surface area BET 298.60 m²/g; pH_zc (pH of zero charge) 5.70; moisture 1.71%; organic compounds 97.13%; mineral compounds 1.16%; surface area BET 298.60 m²/g; and pH_zc (pH of zero charge) 5.70.

2.2. Adsorption and Desorption Test

Sorption was carried out in 250 cm³ conical flasks using 100 cm³ of copper (II) ion solutions with initial concentrations in the range 10–1000 mg/dm³. Copper ion solutions were prepared from hydrated copper (II) nitrate, [Cu(NO₃)₂·3H₂O], from ACROS ORGANICS. The pH of the tested solutions was equal to 4.0 (±0.1). A total of 0.02 M HNO₃ was used to correct the pH. The applied experimental conditions were established according to the previous studies for natural sorbents [50]. This was confirmed by other researchers, who indicated a pH in the range of 4–5 as optimal for analogous measurements [32,51,52]. On the other hand, at higher pH values (above 6.0), copper (II) ions precipitate from the solution in the form of copper (II) hydroxide.

The model solutions together with a portion of the sorbent were shaken for 60 min in a laboratory shaker (type SK-0330 PRO by Dragon Lab Instruments, manufacturer Merazet S.A., Poznan, Poland) at a speed of 200 rpm. The length of the process was determined based on previous research carried out for natural sorbents [50]. It was assumed that 60 min was the time to achieve sorption equilibrium. Sorption was carried out at room temperature (approximately 22 °C). After the process was completed, the suspension was separated by gravity filtration to separate solid particles.

The sorbent regeneration tests were carried out with 50 cm³ of water acidified with hydrochloric acid (HCl) with a pH of approximately 4.5 and with 50 cm³ of 10% HCl acid solution. For this purpose, coffee grounds were used in the process of removing Cu²⁺ ions from solutions with the highest tested concentration (1000 mg/dm³). The choice of reagent and regeneration conditions was dictated by previous studies. The regeneration time was 30 min, and the shaking speed was 200 rpm.

The final concentration of copper (II) ions in model solutions and after the sorption process was determined with the cuprizone method using a UV–VIS spectrophotometer Cadas 200 Dr. Lange. The standard curve method was used. The analysis was conducted in an ammonia-citrate medium with a pH range from 8.0 to 9.5. The absorbance of the solutions was measured at a wavelength of 600 nm [53]. The data from absorbance measurements for samples and model solutions (simulated solutions), as well as the standard curve, are included as Supplementary Materials.

2.3. Batch Test for Cooper (Cu²⁺) Adsorption Studies

The purification level of the studied solutions from copper (II) ions, $X$, %, was determined using Formula (1):

$$X={\displaystyle \frac{C_0-C_e}{c_0}}·100\%$$

(1)

where C₀ and C_e are the initial and equilibrium concentrations of Cu²⁺ ions in solutions, mg/L.

The sorption capacity was calculated using Formula (2):

$$q_e={\displaystyle \frac{V\left(C_0-C_e\right)}{m}}$$

(2)

where q_e is the amount of adsorbed ions per unit mass of the sorbent in the equilibrium state, mg/g; V is the volume of the solution, dm³; C₀ and C_e are the initial and equilibrium concentrations of the tested ions in the solution, mg/dm³; and m is the amount of dry mass of the sorbent, g.

The two most popular models of adsorption isotherms—Langmuir and Freundlich—were used to describe the sorption processes.

Langmuir theory assumes that the adsorbent surface is homogeneous and has a fixed number of active places. A molecule adsorbed on the surface cannot move freely. Side interactions are irrelevant. Monolayer adsorption occurs, i.e., each active site is occupied by only one molecule. The Langmuir model is described by Equation (3):

$$Q={\displaystyle \frac{q_{max}·b·C_e}{\left(1+b·C_e\right)}}$$

(3)

where Q is the amount of adsorbate per unit mass of the adsorbent in the equilibrium state, mg/g; C_e is the equilibrium concentration of the adsorbate in the solution, mg/dm³; q_max is the maximum sorption capacity of the adsorbent, mg/g; and b is the affinity between the adsorbent and the removed ions, dm³/mg.

The Langmuir constants b and q_max were determined using the linear form of the Langmuir isotherm Equation (4):

$${\displaystyle \frac{1}{Q}}={\displaystyle \frac{1}{q_{max}·b}}·\left({\displaystyle \frac{1}{C_e}}+b\right).$$

(4)

In the case of the Langmuir equation, a dimensionless partition factor, R_L, was evaluated according to Formula (5) [54]:

$$R_L={\displaystyle \frac{1}{\left(1+b·C_o\right)}}$$

(5)

where C_o represents the highest initial concentration of the adsorbate in the solution, measured in mg/L. It is linked to the shape and determines the intensity of the adsorption process according to the following relationships:

• R_L > 1—adsorption is weak;
• R_L = 1 linear course hinge on the adsorbate concentration;
• 0 < R_L < 1—intense course;
• R_L = 0—the process is irreversible.

The Freundlich isotherm is an empirical equation that effectively describes adsorption on energetically heterogeneous surfaces and microporous adsorbents. Freundlich’s theory posits that once the adsorbent surface is fully covered with adsorbate molecules, their quantity cannot exceed the number of available active sites on the surface. This equation accurately represents the reversible adsorption of dilute solutions and is expressed by the relation in Equation (6):

$$Q=K·c_e^{1/n}$$

(6)

where Q is the amount of adsorbate per unit of mass of adsorbent at equilibrium, mg/g; C_e the equilibrium concentration of the adsorbate in the solution, mg/dm³; the constant K determines the sorption capacity of the tested sorbents at the equilibrium concentration of Cu²⁺ ions in the solution, dm³/mg; and the parameter 1/n is a measure of surface heterogeneity.

The Freundlich constants K and 1/n were determined using the linear form of the Freundlich isotherm Equation (7):

$$logQ=logK+{\displaystyle \frac{1}{n}}·log{C}_e$$

(7)

It should be noted that the value of the K constant varies within a wide range, depending on the type of adsorbent and the adsorbed substance. The Freundlich isotherm also differs from the Langmuir isotherm in its limited range of applications. It cannot be used for the rectilinear part of the isotherm occurring at low pressures, because then a value n = 1 would have to be assumed, nor for high pressures, because the curve increases indefinitely while the area has a finite value, i.e., a state of saturation must occur.

3. Results and Discussion

A graphical presentation of sorption testing results in model solutions with an initial pH of 4.0 is shown in Figure 2.

The best and comparable sorption capacities of spent coffee grounds were observed for the lowest concentration values: 10, 25, and 50 mg/dm³ (Figure 2). The degree of Cu²⁺ ion release for these samples was over 85%. It was noticed that as the concentration increased, the sorption gradually decreased, reaching a value of approximately 15% for a Cu²⁺ ion content of 1000 mg/dm³. The obtained test results interpreted in accordance with the Langmuir and Freundlich equations are presented in Figure 3 and in linear form are shown in Figure 4 and Figure 5. The values of the coefficients q_max and b of the equation are listed in

Table 2. Langmuir adsorption isotherm coefficients for Cu²⁺ ions on spent coffee grounds in solutions with pH 4.0.

Langmuir Isotherm				Freundlich Isotherm
qmax [mg/g]	b [L/mg]	RL	R	K [L/mg]	n	1/n	R
25.3165	0.0678	0.0145	0.9924	3.9001	3.2594	0.3068	0.9862

.

The linear form of the Langmuir isotherm (Figure 4) was utilized to determine the monolayer capacity, representing the amount of adsorbate that forms a monomolecular layer on the surface of the adsorbent. The maximum adsorption state, which corresponds to all active sites being occupied, is defined by the isotherm parameter qmax. For spent coffee grounds (SCGs) as the sorbent, the qmax value is 25.32 mg/g (Table 2). Additionally, the b parameter, which indicates the sorbent’s affinity for Cu²⁺ ions, was calculated. A higher b value suggests greater affinity and a steeper Langmuir isotherm before reaching the plateau. In this case, the b parameter is 0.0678. Generally, an effective sorbent should have high values for both the q_max and b constants. A dimensionless partition factor, R_L, was defined to assess the suitability of spent coffee grounds for removing Cu²⁺ ions. The R_L values fall within the range of 0 < R_L < 1, confirming that the ion exchange process is favorable. Based on the data shown in Figure 3, it can be observed that the Freundlich isotherm fits the experimental data better than the Langmuir isotherm.

This means that copper (II) ions removed from the adsorption layer exhibit a specific mobility under the experimental conditions. Therefore, the adsorption sites with the highest adsorption energy are occupied first, while the sites with lower energy are occupied second. Additionally, the change in the heat of adsorption may result from the interaction between the adsorbed particles.

The graph of the relation log Q = f (log C_e) (Figure 5) is a straight line, which easily enabled the determination of the constants K and 1/n of the Freundlich equation and the description of experimental systems using these parameters (Figure 3 and Table 2). The K constant determines the sorption capacity of the tested sorbent at the equilibrium concentration of Cu²⁺ ions in the solution. The parameter 1/n is a measure of surface heterogeneity. The closer the value of this constant is to zero, the greater the energy heterogeneity of the adsorbent surface [54].

The analysis of the data indicates that for the tested SCGs, the 1/n constant is approximately 0.31 (Table 2). This suggests moderate energy heterogeneity within the sorption system. The 1/n exponent also provides insight into the effectiveness of Cu²⁺ ion removal from the aqueous phase using the tested sorbent. When 1/n equals 1, the isotherm is linear, indicating that the free enthalpy of the process remains constant across the entire concentration range. When 1/n is less than 1, the isotherm is concave, implying that as more molecules occupy the surface, the free enthalpy increases, thereby enhancing further adsorption [55]. In this case, the value of 1/n suggests a moderate intensity in the removal of Cu²⁺ ions using spent coffee grounds, as 1/n is less than 1 (Table 2).

Although the aim of this work was not to indicate the mechanism of the adsorption process being studied, it should be pointed out that to determine the detailed mechanism, additional studies are needed, in particular, to determine the degree of development of the specific surface of spent coffee grounds and the accessibility of its internal fragments for copper (II) ions. The presence of functional groups on the surface of the grounds capable of exchanging or complexing copper may also be of significant importance.

This analysis will be performed for our sorbent. We can compare our results with the results of other researcher (Table 1), taking into account in terms of the applied methodology and the conditions of the process, which is usually sufficient in relation to conducting sorption under technological conditions.

Therefore, comparing the obtained quantitative and qualitative results of sorption using unmodified SCG, carried out in the previously described chemical and dynamic conditions and for a similar dose, it is worth noting that the determined q_max value is comparable to the results of copper (II) ion sorption using activated carbon from wine production waste, active carbon, or natural and pretreated from grape pomace. The sorption capacity of coffee waste, including SCGs, increases strongly after appropriate chemical or temperature modification. The effectiveness of modified SCGs is directly comparable to the effectiveness obtained using commercial activated carbons.

A very important factor influencing the effectiveness of adsorption is the condition of the adsorbent surface. The morphology of coffee adsorbents depends on the method of processing the beans before adsorption. Azouaou et al. indicated that ground coffee has a low-porous and homogeneous structure with deep pores [51] (Figure 6a,b). However, unprocessed coffee material has a jagged surface with many cavities in the form of channels, which results in a small sorption surface (Figure 6c,d).

Therefore, the sorption capacity of spent coffee grounds was compared with other copper sorbents, tested in similar conditions, such as activated carbons and organic waste (Table 1) [20]. It was found that the tested sorbent can successfully compete with these materials.

Desorption Test

Regeneration tests of the tested sorbent were carried out with 50 cm³ of water acidified with hydrochloric acid (HCl) with a pH of approximately 4.5 and with 50 cm³ of 10% HCl acid solution. Reducing the volume of the regeneration solution compared to the volume of the solution subjected to the sorption process (200 cm³) allowed the solution to be concentrated and facilitated the recovery of copper (II) ions. The efficiency of the desorption of Cu²⁺ ions from coffee grounds, carried out for the solution with the highest tested concentration (1000 mg/dm³) using water with a small addition of HCl acid with a pH of approximately 4.5, was only 2%. A much higher desorption efficiency of about 35% was obtained using 10% hydrochloric acid solution. The obtained results indicate the impossibility of the simple regeneration of the tested sorbent and the existence of strong interactions between its surface and copper (II) ions.

Based on our own knowledge and results, as well as a review of studies by other researchers, we can indicate that the phenomenon of physical adsorption occurs because the desorption process with clean water or acidified water as a solvent is possible, however, with a very low effect for the sample with a high concentration of copper ions. On the other hand, the leaching of copper (II) ions in a strongly acidic environment can occur on the basis of ion exchange. Based on partial results of studies already performed for ad-sorption systems consisting of copper solution and SCGs, we can make the assumption that copper (II) ions can be bound in chelate compounds on the adsorbent surface and, therefore, chemisorption would also occur on this surface.

4. Conclusions

Based on the results obtained, it was found that ground coffee grounds after brewing in an espresso machine effectively removed copper (II) ions from model aqueous solutions, especially in the range of low concentrations of metal ions, achieving sorption process efficiency values in the range of 85–89%. Compared to typical and commercially used sorbents, such as activated carbon or ion exchangers, these are promising results, especially since the sorption material was not subject to any additional processes to modify its structure and properties.

The results realized after the regeneration process showed that the tested sorbent can be regenerated with 10% hydrochloric acid solution. The efficiency of the desorption process was approximately 35%. Reducing the volume of acid used for regeneration compared to the volume of the solution subjected to sorption allowed the solution to be concentrated and facilitated metal recovery.

Although the mechanism of the copper ion sorption process was not the subject of research, it results from the determined values of adsorption isotherms (a specially the R_L parameter); this indicates that, in addition to the mechanisms of physical and probably chemical sorption, there is also the phenomenon of ion exchange.

It can also be concluded that waste material such as spent coffee grounds can be used as a sorbent. It can effectively replace synthetic sorption materials in the processes of removing inorganic pollutants from raw water. The advantages of this biosorbent include easy access, the low cost of acquisition, and no need to use special equipment.

This verifies the thesis put forward—this natural sorbent can be a cheap and easy-to-use replacement for commonly used but expensive activated carbons. This approach may help to reduce the deposition of this waste or eliminate it from circulation.

It is worth pointing out that the natural sorption capacity of a selected biowaste (this also applies to SCGs) can be additionally increased by modifying its surface using chemical or physical methods, transforming it into a highly effective sorbent. This occurs because its surface can be directionally activated to produce appropriate active centers capable of binding Cu²⁺ ions or other heavy metals. Such analyses will be the subject of a separate study.

Additionally, studies are planned to assess the effect of the amount of SCGs used for sorption on the efficiency of the process for various concentrations of copper (II) ions, to indicate the optimal dose range application of the sorbent while taking into account hydrodynamic conditions during the adsorption. In addition, we plan to examine the effect of the mechanical mixing process (mixing time, mixing intensity, and type of stirrer used) on the efficiency of the copper ions adsorption process using SCGs.