Adsorption of Phosphate and Nitrate Using Modified Spent Coffee Ground and Its Application as an Alternative Nutrient Source for Plant Growth

Aisyah Humayro, Hiroyuki Harada^*, Kanako Naito

Faculty of Environmental Science, Prefectural University of Hiroshima, Hiroshima, Japan.

DOI: 10.4236/jacen.2021.101006                

Abstract

Phosphate (PO₄^3-) and Nitrate (NO₃^-) are two main nutrients that cause water eutrophication. In the other hand, the presence of PO₄^3- and NO₃^- is needed for plant growth. The aims of this study are to recycle Spent Coffee Ground (SCG) modified with calcium hydroxide for adsorption PO₄^3- and NO₃^-. The optimum adsorption capacity for PO₄^3- and NO₃^- is 36.74 mg/L and 20.21 mg/L, respectively. The Freundlich isotherm model was suitable for PO₄^3- and NO₃^- adsorption. The kinetic model for adsorption was linear using Pseudo-second order. The application of modified SCG after enrichment with PO₄^3- and NO₃^- for plant growth (Raphanus sativus) showed optimum growth at a dose of 0.3% with value of germination index was 203%.

Keywords

Spent Coffee Ground, Adsorption, Calsium Hydroxide, Phosphate and Nitrate, Plant Growth

1. Introduction

Water pollution containing high concentration of nutrients such as phosphorus and nitrogen is a serious concern in the environment worldwide, because both are implicated in the eutrophication of receiving water [1]. Phosphorus is released into the aquatic environment through the weathering of rocks and by various human activities [2]. Furthermore, the presence of nitrate ion in drinking water is a potential public health hazard. A high concentration of nitrate leads to production of nitrosamine, which is related to cancer [3]. On the other hand, phosphorus in the form of phosphate ( ${\text{PO}}_4^{3-}$ ) and nitrogen in the form of nitrate ( ${\text{NO}}_3^{-}$ ) are an important part of the mechanism of plant growth in agricultural sector.

Several physical-chemical and biological methods have been developed to manage the ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ in water, such as ion exchange, chemical precipitation, adsorption and bacterial assimilation method. Adsorption is one of the techniques for removal anions from aqueous solution, which is comparatively more useful and low cost using easily available materials in wastewater treatment has been widely investigated [4].

Coffee is one of the most popular drinks worldwide, a significant quantity of by-products as well (6 Mt/year). As a consequence, the coffee industry is responsible for producing large amounts of coffee residues. One of the residues of coffee produced is Spent Coffee Ground (SCG) [5]. SCG is one of the biomass that is suitable to be used as an adsorbent because it has porous characteristic on its surface. Therefore, SCG is often used as an adsorbent for removal of heavy metal or dyes [6] [7]. Based on the early research, the adsorption of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ using original SCG showed a low adsorption capacity. In this study, we improved the adsorption capacity of SCG treated using calsium hydroxide (hereafter referred to the modified SCG). The addition of calsium hydroxide to carbonization of wood biomass has been shown to adsorb more phosphorus [8]. The evaluation of modified SCG based on initial concentration, effect of pH, adsorbent dosage, isotherm and kinetic model. The final objective of this study is to make SCG as an alternative source of nutrients that contribute to plant growth.

2. Materials and Experimental Methods

2.1. Materials

The samples of SCG in this study were collected from coffee machine at convenience store in Shobara City, Hiroshima, Japan. First, SCG washed using distilled water and dried in the oven for 1 - 2 days at 60˚C. After, it was sifted using test sieve by Tokyo Screen Co., Ltd. with sieving size 425 µm. Then, SCG immersed in 0.04 M calcium hydroxide solution with ratio of 2 g of SCG in 160 mL of calcium hydroxide solution during 24 hours and dried in the oven at 60˚C. The modified SCG washed using distilled water subsequently until pH become 7.5. All chemicals were either reagent on analytical grade and purchased from Kanto Chemical Co. Inc. and Wako pure chemical corporation.

2.2. Experiment

2.2.1. Adsorption Study

Evaluation of adsorption characteristics ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ in this study was conduct using batch experiments. Stock solution of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ (having concentration of 2000 mg/L) were prepared by dissolving 1 mol/L nitric acid and 0.5 mol/L phosphoric acid in distilled water. Some various concentrations (10 - 500 mg/L) were prepared from which working solution. About 0.2 g of adsorbent was added to 16 mL of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ solution. The mixture was subsequently placed in temperature controllable magnetic stirrer for the adsorption process begin. The effect of initial pH was adjusted from pH range 1 - 9 by hydrochloric acid and potassium hydroxide solution. The ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ were determined by standard method (Japan Industrial Standard method JIS KO 102). Adsorption capacity (q) was determined using the following equations:

$q=\left(C_o-C_e\right)V/m$ (1)

where q (mg/g) represents the adsorption capacity, C_eand C_o (mg/L) stand for equilibrium and initial concentrations, respectively, V (L) is volume of the solution and m (g) is the mass of adsorbent.

The effect of adsorbent dosage needed for pot treatment experiment, the aims to determine the best composition of modified SCG in adsorbing ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$. Various amount of adsorbent (0.1, 0.2, and 0.6 g) conducted to 16 mL ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ solution.

2.2.2. Desorption Study

Desorption experiments of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ were conducted after the completion of the adsorption experiments. After the supernatant was separated with the adsorbent, the adsorbent immersed in 0.1 - 0.5 M sodium hydroxide for 24 hours. The desorption amount and the rate of desorption were calculated as follow:

$q_{des}=\left(C_1\times V\right)/m$ (2)

$D\left(\%\right)=\left(q_{des}/q\right)100\%$ (3)

where D is the rate of desorption, q_des(mg/g) is the desorption amount of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$, V (L) is the volume of desorption solution, C₁(mg/L) represents the ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ concentration of desorption supernatant and m (g) is the mass of adsorbent.

2.2.3. Application for Plant Growth

From the best treatment of adsorbent dosage, 0.1%, 0.2%, and 0.3% of the fertilizer from modified SCG enrichment with ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ mixed with 150 g of soil in polybag (diameter 8 cm). The plant seeds used were Raphanussativus. Observation of plant height and leaf width was carried out for 10 days.

The acute toxicity of the examined fertilizer was determined based on a plant’s germination test. It was made with the fertilizer for modified SCG after adsorption process (0.1%, 0.2%, and 0.3%) mixed distilled water in ration 1:10 and put in a homogenizer for about 1 hour. Then, take the supernatant and put 5 mL in Petri dishes with 5 seed Raphanussativus, which were then incubated in the dark at room temperature (25˚C ± 0.5˚C) for 72 hours. The germination index was calculated from equation GI = G_e/G_k × 100, where G_e and G_k are the numbers of germinated seeds in the experimental and control series [9].

3. Result and Discussion

3.1. Characteristics of the Modified SCG

Figure 1 shows SEM images of original and modified SCG. SEM was used to observe the morphology of the adsorbent. The porosity of the modified SCG was relatively more formed than the original SCG. The addition of calcium hydroxide gives advantage to SCG for adsorb more ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$.

FTIR was used to analyze the functional groups on the adsorbent before and after modified. The spectra are shown in Figure 2, both of original and modified SCG have peak between 3600 and 3200 cm⁻¹ is related to the hydroxyl groups of O-H stretching vibration. The hydroxyl groups causes an increase in the

Figure 1. Scanning Electron Microscopy (SEM) images of (a) original and (b) modified Spent Coffee Ground.

Figure 2. FTIR analysis of original and modified SCG.

electrostatic adsorption of anions, such as nitrate [10]. The two sharp bands at 2923 and 2852 cm⁻¹ are attributed to C-H stretching vibration. The original SCG has peak between 1700 and 1600 cm⁻¹ is highly associated with chlorogenic acids and caffeine [11]. However, in modified SCG the peak has shifted become 1567 cm⁻¹ due to carboxylate groups [12].

3.2. Adsorption Phosphate ad Nitrate

Based on Figure 3 and Figure 4, the figures show that the adsorption capacity of the modified SCG increases than original SCG. The adsorption capacity for ${\text{PO}}_4^{3-}$ is 36.74 mg/g and for ${\text{NO}}_3^{-}$ is 20.21 mg/g, respectively. The pH of the solution is an important variable in the adsorption process because it affects the surface charge of the adsorbent and also the chemical speciation of the adsorbate [13]. The effect of pH is showed in Figure 5, in the case of ${\text{PO}}_4^{3-}$ the optimum adsorption occurs at pH 3 and pH 1 - 3 for ${\text{NO}}_3^{-}$. The modified SCG surface would be positively charged when the solution pH was lower, which enhanced

Figure 3. Adsorption of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ by the original SCG (C_o 10 - 500 mg/L, initial pH solution 3 - 4, ratio liquid/solid 0.1 L/10g, 30˚C).

Figure 4. Adsorption of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ by (a) the modified SCG (C_o 10 - 500 mg/L, initial pH solution 3 - 4, ratio liquid/solid 0.016 L/0.2g, 30˚C).

Figure 5. The effect of initial pH for adsorption ${\text{PO}}_4^{3-}$ and (b) ${\text{NO}}_3^{-}$.

the adsorption possibility for the negatively charged ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$.

Figure 6 shows that the increasing mass of adsorbent doesn’t increase the adsorption capacity of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$. Furthermore, the addition of calcium hydroxide has an important role in the adsorption process of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ The less amount of adsorbent, the more active sites are formed.

3.3. Adsorption Isotherm Model

Adsorption isotherm model provide information about the capacity of the adsorbent and the solute-sorbent interaction. In the present work, the Langmuir and Freundlich isotherms were used to analyze the experimental equilibrium data. Table 1 shows the data of (a) Langmuir and (b) Freundlich isotherm models. Figure 7 shows the best fit of adsorption ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$.

The Langmuir model is indicating that there is monolayer coverage of adsorbate on a homogeneous adsorbent surface. The Langmuir isotherm equation is [14]:

$C_e/q_e=1/q_m{K}_L+C_e/q_m$ (4)

whereq_m (mg/g) is the maximum of the adsorption capacity in monolayer condition, q_e(mg/g) is equilibrium capacity, K_L (L/mg) and C_e(mg/L) is equilibrium concentration. The applicable of isotherm equation is compared on the basis of

Figure 6. The effect of adsorbent dosage with adsorption capacity.

Figure 7. Freundlich isotherm model of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$.

Table 1. Isotherm models of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ adsorption.

correlation coefficients (R²). However, R² of Langmuir isotherm model was lower than that of the Freundlich isotherm.

The Freundlich isotherm model is based in assumption that the heterogeneity of the adsorbent material and multi-layer coverage of the adsorbate. The Freundlich isotherm equation is:

$\log q=\log K_L+1/n\log C_e$ (5)

where K_f(L/mg) is the Freundlich isotherm, n is the adsorption intensity, C_e(mg/L) is equilibrium concentration, and q_e (mg/g) is the equilibrium capacity.

The best fit of adsorption ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ were obtained by the Freundlich with R² values was 0.98 for ${\text{PO}}_4^{3-}$ and 0.97 for ${\text{NO}}_3^{-}$. The data was indicating that adsorption of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ onto the adsorption are not monolayer adsorption. The addition of calcium hydroxide played important role in adsorption ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$, which is indicating not only physical adsorption but also chemical adsorption.

3.4. Adsorption Kinetic Model

Figure 8 shows the equilibrium time for ${\text{PO}}_4^{3-}$ adsorption is 240 minutes and the optimum time for ${\text{NO}}_3^{-}$ is 90 minutes. The kinetic model is used to determine the speed of the adsorption process and the stages that control the adsorption process. The kinetic data obtained is in the form of adsorption capacity, which can be obtained through modeling using pseudo-first order and pseudo-second order models. To find out the appropriate kinetic model for the adsorption system of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$, it is necessary to describe the relationship between the concentration of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ adsorbed and equilibrium with time. The kinetics of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ adsorption by modified SCG was investigated by pseudo-first order and pseudo-second order models [15], were determined using the following equations:

$\ln \left(q_e-q_t\right)=\ln q_t-k_{1}t$ (6)

$t/q_t=\left(1/k_2{q}_e^2\right)+t/q_e$ (7)

where k₁ is the pseudo-first order adsorption rate constant, k₂is the pseudo- second order rate constant, q_e(mg/g) is the adsorption capacity at equilibrium, and q_t(mg/g) is the adsorption capacity at t (minute). Figure 9 shows that both adsorption data is more linear using the pseudo-second order adsorption

Figure 8. Equilibrium time of ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ adsorption.

kinetic model, evidenced by the R² value of 0.9791 for ${\text{PO}}_4^{3-}$ and 0.9347 for ${\text{NO}}_3^{-}$.

3.4. Desorption Studies

Adsorption by chemical bonding or ion exchange or combination of both, then desorption can be effected by stronger desorbent like acid or alkali solution. If the adsorption is by physical bonding then the loosely bound metal ion can be easily desorbed with distilled water in most of the cases. The result of ${\text{PO}}_4^{3-}$ desorption rate using sodium hydroxide solution showed results reaching 24.35%, while the optimum value for desorption of ${\text{NO}}_3^{-}$ reached 39.39%. These results indicate that the adsorption on the adsorbent modified SCG is not completely reversible and bonding between it and adsorbed ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ is likely to be be strong. Thus, it is relatively difficult to desorb ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ from the modified SCG [16].

3.5. Application for Plant Growth

The application of modified SCG after enrichment with ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ for plant growth (Raphanussativus) can be seen in Figure 10 and Figure 11. The dose of 0.3% was a significant growth in the plant height and leaf width. However, seeds can grow on the third day. It is estimated because of the chemical bonds between ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ in the active site of modified SCG are quite strong. Therefore, nutrients need time to release into the soil. It is linear with a desorption

Figure 9. Pseudo-second order of (a) ${\text{PO}}_4^{3-}$ and (b) ${\text{NO}}_3^{-}$.

Figure 10. The effect of modified SCG enrichment on stem height of Raphanussativus.

Figure 11. The effect of modified SCG enrichment on leaf width of Raphanussativus.

Table 2. Germination Index (GI) of Raphanussativus.

rate of only 26% - 30%.

The original SCG is characterized by a strong phytotoxic impact, such as caffeine content [17]. Caffeine (1.8 mg/g) present in SCG may serve as a chemical defense mechanism in some plants [5]. FTIR data on the original SCG indicated that there was a peak associated with caffeine content. However, the addition of sodium hydroxide has the effect of lowering caffeine levels in SCG. The effect of toxicity can also be seen from the germination rate test. The result of germination rates in Table 2 shows that even though the small dose of fertilizer can contribution to plant growth significantly.

4. Conclusion

The optimum adsorption capacity at equilibrium (q) of ${\text{PO}}_4^{3-}$ was 36.74 mg/g when pH and contact time of ${\text{PO}}_4^{3-}$ solution were 3 and 240 minutes. The optimum adsorption capacity at equilibrium adsorption capacity at equilibrium of ${\text{NO}}_3^{-}$ was 20.21 mg/g when pH and contact time were 1 - 3 and 90 minutes, respectively. The best fit of the experimental data ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ were obtained by the Freundlich isotherm and pseudo-second order. The desorption rate of the modified SCG ranged from 24% - 39%. It can be concluded that the modified SCG can be used as an adsorbent to remove ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ in wastewater. The fertilizer from modified SCG after enrichment with ${\text{PO}}_4^{3-}$ and ${\text{NO}}_3^{-}$ can contribute to plant growth even though in small dose, so it can use as an alternative source of nutrients that contribute to plant growth.

Acknowledgements

This research was supported by Prefectural University of Hiroshima. Also, I would thank to Kumahira scholarship for financial funding this study.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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