Activated carbon from jackfruit peel waste by H₃PO₄ chemical activation: Pore structure and surface chemistry characterization

Abstract

The effects of activation temperature and impregnation ratio on the pore structure and surface chemistry of activated carbons derived from jackfruit peel with chemical activation method using phosphoric acid as activating agent were studied. Activated carbons with well-developed pore sizes were produced at activation temperatures of 450 and 550 °C. The BET surface areas and total pore volumes of the carbons produced at these temperatures are in the range of 907–1260 m²/g and 0.525–0.733 cm³/g, respectively.

Introduction

Activated carbons are the most versatile and commonly used adsorbents because of their extremely high surface areas and micropore volumes [1], large adsorption capacities, fast adsorption kinetics, and relative ease of regeneration [2]. They are produced from a variety of carbonaceous source materials. The choice of precursor is largely dependent on its availability, cost, and purity, but the manufacturing process and intended application of the product are also important considerations [3].

Precursors used for the production of activated carbons are organic materials that are rich in carbon, such as coal, lignite, and wood. Although coal is the most commonly used precursor, agricultural waste in certain condition is a better choice [1]. Activated charcoal produced from residues would reduce the pressure on forests since wood is also commonly used for this purpose [4]. Many agricultural by-products such as coconut shell [2], [4], [5], grain sorghum [6], coffee bean husks [7], rubber wood sawdust [8], chestnut wood [9], and fruit stones [10], have been discovered to be suitable precursors for activated carbon due to their high carbon and low ash contents. Agricultural wastes are considered to be a very important feedstock because of especially two facts: they are renewable sources and low cost materials [3].

Jackfruit is commonly used in South and Southeast Asian cuisines. Jackfruit was originally from India and spread out into tropic regions, including Indonesia. Jackfruit is very popular to Indonesian people. It can be eaten unripe or ripe, and cooked or uncooked. Moreover, there is no in-season or out-of-season for jackfruit. Therefore, it can be harvested all year long. These things indicate high demand of jackfruit in Indonesia and automatically result in high output of jackfruit peel waste.

Recent data given by Indonesian Statistical Center Bureau shows that there is increasing production of jackfruit; calculated as variety of nangka cempedak. The values were 537, 186; 694, 654; and 710, 795 tonnes by the year of 2002, 2003, and 2004, respectively. Although only production of one variety of jackfruit was revealed, it leads to a fact that Indonesia has abundant source of jackfruit, since besides nangka cempedak Indonesia also has numerous excellent jackfruit varieties. Jackfruit peel wastes have no economic value and in fact often create a serious problem of disposal for local environments. Conversion of jackfruit peel into activated carbon would increase its economic value, help reduce the cost of waste disposal, and provide a potentially inexpensive raw material for commercial activated carbon. In this research, raw material for activated carbon was obtained by collecting jackfruit peel of one variety of jackfruit, i.e. nangka kunir, from local fruit stores at Malang, East Java. The selection of nangka kunir is due to its high availability of the variety in East Java.

The activation of precursor can be carried out in two different methods, which are physical and chemical activation. In this study jackfruit peel was activated with chemical activation method using phosphoric acid as an activating agent. The advantages of chemical activation are low energy cost, since chemical activation usually takes place at a temperature lower than that used in physical activation, and yields of chemical activation are higher than physical one [11], [12]. Chemical activation also has better development of a porous structure [13].

Phosphoric acid and zinc chloride are used for the activation of lignocellulosic materials, which have not been carbonized previously; whereas metal compounds such as potassium hydroxide are used for the activation of coal precursors or chars. When compared to zinc chloride, phosphoric acid is the most preferred because of the environmental disadvantages associate with zinc chloride. Problem of corrosion and inefficient chemical recovery is also associates with it. Moreover, the carbons obtained using zinc chloride cannot be used in pharmaceutical and food industries as it may contaminate the product [8]. Although potassium hydroxide develops large microporosity, yield of activated carbon impregnated by potassium hydroxide is lower than those activated with zinc chloride or phosphoric acid, and at high temperature, i.e. >  ±650 °C, the carbon content is less than fixed carbon in initial precursor. The presence of metallic potassium will intercalate to the carbon matrix [12], yielding lower yield of activated carbon, less then the carbon content of the raw material.

It has been found that both the surface area and the nature of porosity are greatly influenced by the processing modes. In general, activation using phosphoric acid can be classified either as single-stage or two-stage activation process carried out either in inert medium or self-generated atmosphere. The activation process is also carried out with effort made towards developing a high surface area carbon with desired pore size by optimizing the process parameters; such as the activation time, activation temperature, and impregnation ratio. Study of various parameters by Ahmadpour and Do [14] revealed that the most important variable to porosity of activated carbon development is the ratio of the chemical agent to the precursor. The other operation variables with a direct effect on the porosity development are activation temperature and method of mixing. Nevertheless, it has been found that ordinary (sample—activating agent mixture) impregnation-method is the best method of mixing compared with physical and acid washed method [14].

According to the knowledge of the authors, there is only one publication about the usage of jackfruit peel as precursor for activated carbon production which was reported by Inbaraj and Sulochana [15]. They used sulphuric acid as activating agent and emphasized on its application on Cd (II) adsorption. Therefore, in this present study, one of well-known activating agent, i.e. phosphoric acid, was used to activate jackfruit peel.

The adsorption behavior of activated carbon is determined not only by their porous structures but also by the chemical nature of its surface. The porous structure of carbon determines its adsorption capacity, while its surface chemical groups affect its interaction with polar and non-polar adsorbates [16]. It indicates that surface chemistry has a role in adsorption process. A thorough knowledge of activated carbon surface chemistry enables preparation of adsorbent with appropriate characteristic for specific application. Thus, in this research, pore characteristic and surface chemistry are worth to be investigated by several means of characterization method because of their important roles in adsorption.