Activation of spent coffee ground for the removal of glyphosate in aqueous solution

. Glyphosate (N-(phosphonomethyl) glycine) is one of the most widely used herbicides on agricultural land with an annual consumption is about 826 million kg in 2014, resulting in considerable amounts of water and soil pollution. Great attention has been paid to the removal of glyphosate in aqueous solution and many techniques have been developed. Among them, adsorption offers itself as a viable option for glyphosate removal and the searching of low cost adsorbent is always desired. Coffee drinking generates a massive volume of spent coffee grounds. Therefore, using spent coffee as feedstock for making adsorbent is investigated in this study. The activated carbon was prepared from spent coffee ground using chemical, Zn(CH 3 COO) 2 and H 2 O 2 , then followed by heat treatment. The coffee grounds activated carbon was characterized by scanning electron microscope, BET Surface Area Analyzer. The effects of preparation conditions of obtained spent coffee ground powder on glyphosate removal were investigated using one variable at a time method. The adsorption tests were conducted in batch mode.


Introduction
Pesticides, such as herbicides, insecticides, nematicides, molluscicides, piscicides, avicides, rodenticides, bactericides, insect repellents, animal repellents, antimicrobials, fungicides, etc., have been widely used in agriculture over the past few decades, resulting in considerable amounts of water and soil pollution [1,2]. Glyphosate (N-(phosphonomethyl) glycine) is one of the most widely used herbicides on agricultural land with an annual consumption is about 826 million kg in 2014 [3]. Studies have shown serious toxicity to humans at concentrations lower than those used in agriculture [4]. Great attention has been paid to the removal of glyphosate in an aqueous solution [5]. Membrane separation, electrolysis, photocatalytic degradation, advanced oxidative processes, microwave radiation, ozonation, sedimentation, UV irradiation, and adsorption are among the presently known techniques for eliminating glyphosate from water and effluents. Each of these processes has its own set of benefits and drawbacks, depending on considerations such as cost, effectiveness, investment, amount of time, and types of chemicals. Because glyphosate may be found in water and effluents in quantities ranging from ng/L to mg/L, adsorption offers itself as a viable option for glyphosate removal [6]. In adsorption techniques, the search for low-cost adsorbent is always desired. Coffee drinking generates a massive volume of spent coffee grounds. Therefore, using spent coffee as feedstock for making adsorbent is a low-cost and environmental friendly solution. There are several reports which utilize spent coffee ground in waste water treatment by adsorption such as [7][8][9] In this study, activated carbon was prepared from spent coffee ground using chemical and heat treatment. First, the spent coffee ground was activated by Zn(CH3COO)2. Then, the material was further chemically treated with H2O2. Finally, the adsorbent was obtained after a conventional thermal process. The activated carbon derived from spent coffee ground was characterized by SEM, BET methods. The effects of preparation conditions of obtained spent coffee ground powder on glyphosate removal were investigated. The adsorption tests were conducted in batch mode.

Adsorbent and absorbate
Spent coffee ground (SCG) was acquired at a coffee shop in Ho Chi Minh City, Vietnam. The powder was rinsed several times with distilled water, then with boiling deionized water to degrease. After that, the SCG was dried at 105 o C for 3 hours to completely remove the water content. The product obtained after drying was sieved to collect the particles with the size below 0.250 mm. After the pretreatment, the powder was soaked in Zn(CH3COO)2 solution for 24 hours. Then, the powder was filtered, rinsed with distilled water, and dried at 105 o C for 3 hours. The obtained powder was heated under reflux with H2O2 solution. Then the product was filtered, rinsed with distilled water and dried at 105 o C for 3 hours. Finally, the activated spent coffee ground was carbonized at various temperatures in the range from 200 o C to 500 o C in 1 hour to obtain activated carbon. The final product was preserved in a polyethylene bottle for the adsorption process later. In the adsorbent preparation step, the effects of some significant factors were investigated as follows. The concentration of Zn(CH3COO)2 was varied from 0.1 M to 1.3 M with a step size of 0.3 M. The volume of H2O2 solution was varied from 10 mL to 50 mL with a step size of 10 mL. The carbonization temperature was varied from 200 o C to 600 o C with a step size of 100 o C. Glyphosate solution was prepared from glyphosate powder dissolved in deionized water into the desired concentration.

Batch adsorption experiments
The adsorption experiments were carried out in a 100-mL beaker containing 10 mL of glyphosate solution 0.3 M and 0.2 g of activated spent coffee ground powder. The mixtures were continuously stirred (200 rpm) at room temperature for 30 minutes.
The concentration of the remaining glyphosate was measured using UV-Vis Spectrophotometer method. The analytical method is based on the reaction of glyphosate with ninhydrin in the presence of sodium molybdate as a catalyst. The reaction occurs in a neutral aqueous medium at 100 o C to produce a purple-colored solution which has the maximum VIS absorption at 570 nm [10].
The removal efficiency was calculated by the following equation: where H(%) was recovery efficiency, C0: initial concentration (mol/L) and Ct: concentration after the time t (mol/L).
For the evaluation of the adsorbent, the analysis of adsorption equilibrium data is crucial. The adsorption isotherm is a relationship between the concentration of glyphosate in the solution and the amount of glyphosate adsorbed onto the adsorbents. Numerous isothermal models have been extensively utilized to analyze the adsorption data. In this study, Langmuir adsorption models were used to analyze the equilibrium.
The Langmuir adsorption isotherm is as follows: In linearized form: In which, Ce is the solution equilibrium adsorbate concentration (mg/L), qm is the adsorbent capacity (mg/g), KL is the Langmuir constant (L/mg).

Characterization of the obtained powder
The morphology of the obtained materials was examined by SEM, using a JEOL JSM-7800F (JEOL Ltd., Japan). Nitrogen adsorption-desorption isotherms (BET) were performed using Quantachrome Nova 2200e to evaluate the specific surface area of the materials.

Activated product characterization
The obtained SCG after activation and heat treatment has a specific surface area of 128.687 m 2 /g, measured by BET method. This relatively high specific surface area suggests that the product could be utilized as a good adsorbent. The morphology of the raw spent coffee ground and obtained material is investigated using SEM analysis, and the results are shown in Fig. 1. From the SEM images, it can be obtained that after activation by chemicals and heat, the obtained product has a significant increase in the porous structure, resulting in a higher specific surface area. The sizes of the islands on the surface of the activated SCG are finer, which is favor for the adsorption of glyphosate by the enhancement of active sites. The BET results also confirm that after intensive treatment, the specific surface area is notably enhanced, from typically about 5 m 2 /g for raw spent coffee grounds [11].

The effect of Zn(CH3COO)2 concentration:
In order to study the effect of Zn(CH3COO)2 concentration on the removal efficiency of the obtained powder, the concentration was varied from 0.1 M to 1.3 M with the step size of 0.3 M while other parameters were kept constant. The volume of H2O2 was 40 mL, and the carbonization temperature was 500 o C. The adsorption results are shown in Fig. 2.
From the results, when the concentration of Zn(CH3COO)2 solution increases up to 1 M, the removal efficiency increases. It can be explained by when the amount of Zn(CH3COO)2 increases, the interaction between Zn(CH3COO)2 and carbon increases and causes the reaction with carbon in spent coffee ground occurs easily. Therefore, the formation of porosity increases, resulting in a better adsorption ability of the product. However, when the concentration increases higher, the removal efficiency decreases. It can be explained as follows. When the amount of Zn(CH3COO)2 is too much, the amount of carbon reacting with the activator increases significantly. Therefore, besides forming porosity, it also destroys the porous structure and reduces the adsorption capacity. The results show that at the Zn(CH3COO)2 concentration of 1M, the adsorption efficiency has the highest value, which is 66.83%.

The effect of H2O2 volume:
In order to study the effect of H2O2 volume on the removal efficiency of the obtained powder, the volume was varied from 10 mL to 50 mL with a step size of 10 mL, while other parameters were kept constant. The Zn(CH3COOH)2 concentration was obtained from the best of the previous investigation, which was 1 M. The carbonization temperature was 500 o C. The adsorption results are shown in Fig. 3. From the results, it can be obtained that when the volume of H2O2 increases, the adsorption efficiency of the product also increases. However, when the volume is higher than 30 mL, the percentage adsorption increases only slightly. During the activation process, H2O2 acts as an oxidizing agent, which results in carbon consumption and forming porous structures in the product. Therefore, the more H2O2 present, the more adsorption capacity of the product. Beyond a certain amount, the process will slowly reach equilibrium due to the limited available sites in the spent coffee ground structure. In terms of the cost consideration, it is appropriate to select the H2O2 volume of 30 mL. The corresponding removal efficiency is 67.38%.

The effect of carbonization temperature:
In order to study the effect of carbonization temperature on the removal efficiency of the obtained powder, the temperature was varied from 200 o C to 600 o C with a step size of 100 o C while other parameters were kept constant. The volume of H2O2 and the Zn(CH3COOH)2 concentrations were 30 mL and 1 M, respectively, which were obtained from the best of the previous investigation. Note that at 600 o C of calcination temperature, the spent coffee ground becomes ash in our experiment. Therefore, the result at 600 o C is not presented in Fig. 4, which shows the adsorption ability of the obtained powder.
The results show that the adsorption percentage increases when the activation temperature increases from 200 o C to 500 o C. It can be explained that when the temperature increases, it will promote the carbonization reaction. Therefore, the porosity increases, enhancing the adsorption efficiency. However, when the temperature increases further, it will completely break the structure of the spent coffee ground. At 600 o C, the structure was totally destroyed, and the powder became ash.

Adsorption isotherm
The validity of the Langmuir isotherms for glyphosate was checked by plotting Ce/qe versus Ce (Equation (3)). The results was displayed in Fig. 5. From the results, the equilibrium data were well fitted with Langmuir isotherm model, indicated by a high correlation coefficient (R 2 ) of 0.9830. The results suggest that the adsorption of glyphosate onto the activated carbon was monolayer and physical adsorption, which can be described by Langmuir model. The maximum adsorption capacity qm can be obtained from the slope of the linear regression line and was found to be 3.29 mg/g.

Conclusions
In this study, the activated carbon was successfully synthesized from the spent coffee ground with the specific surface area of 123.687 m 2 /g using chemical activation and heat treatment process. The activation agent are Zn(CH3COO)2 with the concentration of 1 M, H2O2 with the volume of 30 mL. The carbonization temperature is 500 o C. From the best synthesis condition, the removal efficiency can be achieved as high as 70%. The adsorption equilibrium were well fitted with the Langmuir isotherm model, and the maximum adsoption capacity is 3.29 mg/g. The adsorbent show a potential application in glyphosate removal. Furthermore, the adsorbent are prepared from coffee residue, which can solve problems in both solid waste from food industry and water contamination.