Activated carbon from Diplotaxis Harra biomass: Optimization of preparation conditions and heavy metal removal

Highlights

• Diplotaxis Harra is a good precursor to produce an efficient activated carbon using in cadmium and cobalt removal.

• Carbonization temperature was more significant with a negative effect for iodine number and methylene blue index.

• Removal of cadmuim and cobalt ions were more sensitive to methylene blue index.

• The most factor influenced the cadmium and cobalt ions removal is the interaction between carbonization and activation temperatures with a positive effect.

• Optimal activated carbon characteristics were shown close than those of a commercial activated carbon.

Abstract

Diplotaxis harra biomass was used as a precursor for the preparation of activated carbons by phosphoric acid activation. The effects of several factors controlling the activation process, such as carbonization temperature (500–600 °C), activation temperature (400–500 °C), activation time (1–2 h) and impregnation ratio (g H₃PO₄/g carbon) (1.5–2) were established. In order to reduce the number of experiments, full factorial experimental design at two levels (2⁴) were carried out to achieve optimal preparation conditions and better conditions for the removal of cadmium and cobalt ions from aqueous solutions. The experimental results showed that the carbonization temperature was more significant with a negative effect for iodine number and methylene blue index. Therefore, activation temperature and activation time present a positive effect for iodine number and negative effect for methylene blue index. The impregnation ratio shows a positive impact for the both indicated responses. The removal of cadmuim and cobalt ions onto activated carbons was more sensitive to methylene blue index. Moreover, the interaction between carbonization and activation temperatures was the most influencing in cadmium and cobalt ions removal with a positive effect. Using this statistical tool, the best conditions for the removal of cadmium and cobalt by Diplotaxis Harra based activated carbons were established. The maximum iodine number and methylene blue index obtained under these experimental conditions were 1058.8 mg/g and 280.4 mg/g respectively. The high sorption capacities were 31.6 mg/g for cadmium and 25.9 mg/g for cobalt. Those characteristics were shown close than those of a commercial activated carbon used in water treatment and those reported by other researchers studying activated carbon preparation from various solid wastes.

Introduction

Heavy metal pollution of aqueous media and industrial effluents is one of the most significant environmental problems. Heavy metal contamination exists in wastewater of many industries such as metal plating, mining operations, surface finishing industry, tanneries, paper and pulp industries, fertilizer and pesticide industry, radiator manufacturing, energy and fuel production, aerospace and atomic energy installation, alloy industries and batteries industries. The presence of heavy metals, especially cadmium and cobalt ions, in the aquatic environment is of great concern as they are reported to be a source of major environmental and health hazards due to the unabated discharge of toxic effluents, their resistance to degradation, and adverse effects on both aquatic life and human consumption [1], [2].

Several methods have been reported for the removal of heavy metals from industrial effluents and wastewaters, including chemical precipitation [3], filtration [4], ion exchange [5], electrochemical treatment [6], reverse osmosis [7], solvent extraction [8] and adsorption [9]. Among these methods, sorption on activated carbon is one of the most effective, economic and simplest methods for the removal of pollutants from aqueous solutions. Therefore, activated carbons are excellent adsorbents and promising materials that are extensively used in a wide range of applications such as medical uses [10], industrial applications [11], gas storage [12], catalysis [13] and environmental pollution. The activated carbon can be produced from various fossil carbon sources such as lignite, peat and oil residues, however the depletion of these resources encourages researchers to use renewable resources from biomass as precursors for activated carbons [14]. Moreover, a number of lignocellulosic biomasses including date palm tree [15], marine red alga Pterocladia capillacea [16], hemp (Cannabis sativa L.) [17], macadamia nut shells [18], almond shell and orange peel [19], mung bean husk [20], coconut frond [21], sawdust [22], de-oiled canola meal [23], Enteromorpha prolifra [24], and macroalgae waste [25] have been tested as precursors in the production of activated carbon.

In general, The production of activated carbon consists of the pyrolysis of the precursor material followed by a controlled oxidation stage (in cases of physical activation) or the pyrolysis of the precursor material in a single step by chemical activating agents such as NaOH, KOH, K₂CO₃, ZnCl₂ or H₃PO₄. The manufacture of activated carbon by physical activation requires high temperatures (800–1000 °C), which involves high power consumption and a low yield of carbon [26]. In contrast, in the chemical activation, the carbonization temperature is ranged between 400 and 600 °C. Therefore, the power consumption is significantly reduced and the yield can be increased [27]. Depending on the conditions of the manufacturing process, the typical surface areas for activated carbon vary from 500 to 1400 m²/g, although values as high as 2500 m²/g have been reported [28].

The preparation of activated carbon is influenced by many factors including the temperature, impregnation ratio and activation time, among other factors. For this reason experimental designs have been used to control the different factors which influence and interfere in the preparation, in order to optimize experimental conditions.

In this research, activated carbon was prepared from Diplotaxis harra (DHAC) by phosphoric acid activation. The preparation conditions and the removal of cadmium and cobalt ions were simultaneously optimized using a factorial experimental design. The factors included in the experimental design were the carbonization temperature, activation temperature, activation time and impregnation ratio. Four responses are analyzed, which are; iodine number (IN), methylene blue index (MB index), cadmium and cobalt ions removal (Cd(II), Co(II)). To establish the optimal conditions for the production of DHACs, and to investigate the removal of heavy metals, a 2⁴ full factorial experimental design was used. The surface morphology of DHACs produced at the optimal conditions was investigated by scanning electron microscopy (SEM).