ABSTRACT

This work was aimed to investigate the effect of helium degassing of cattle-manure-compost (CMC) based activated carbons on the adsorptive removal of copper ions from aqueous solution. Degassing temperatures were 500˚C, 800˚C and 1000˚C. Activated carbons were characterized according to surface chemistry and pore structures. Adsorption of copper ions was carried out using the conventional bottle-point technique to which the equilibrium data were correlated to Langmuir and Freundlich models. Results indicated that the uptake of copper ions could be well characterized by Langmuir model. It was found that the adsorption of copper ions decreased with significant decrease in surface area as a result of helium degassing at higher temperature. The increase of electron density on graphene layers offered higher affinity towards copper ions at lower equilibrium concentration. It was inferred that copper ions favorably adsorbed on mesopores at lower equilibrium concentration and switched to micropores at higher equilibrium concentration.

1. Introduction

Heavy metals are toxic, not biodegradable and can easily enter the food chain because of high solubility in water. Excessive consumption of metal ions-contaminated water can cause acute illnesses such as diarrhea, nausea, brain disorders, liver and renal dysfunctions, and cancers [1].

Adsorption is a preferred choice amongst other physico-chemical techniques of heavy metals remediation [2]. It can be described as a mass transfer process by which a desired substance is transferred from the liquid phase to the surface of a solid adsorbent, and becomes bound by a physical or chemical attraction [3]. The most widely used adsorbent is activated carbon, a complex and heterogeneous material with unique adsorption characteristics. Adsorption of metal ions onto activated carbon is commonly influenced by surface area, pore volume and surface functionalities.

However, commercial activated carbon suffers from a number of drawbacks. The precursors like coal and petroleum pitch are not renewable, while regeneration of spent activated carbon is relatively expensive. Global demand of activated carbon is forecasted to be 5% - 10% a year [4], and steep increase of price would also be anticipated.

This recent scenario has brought about searches for new alternatives that are abundantly available and low cost [5]. An interesting material under this category is cattle-manure-compost (CMC) [6], a residue of temperature-phased anaerobic digestion for methane generation. Such waste can become a source of pollution and threat to public health if they are not properly handled and disposed. Therefore, converting CMC into activated carbon is a good strategy to overcome the aforesaid problems and could also be used to mitigate other pollutions [7].

Conversion of CMC into activated carbon was first described by Qian and co-workers [6]. It was reported that the nitrogen content is nearly 6 times higher than that of commercial Filtrasorb 400 (F400, Calgon Mitsubishi Chemical Corporation) [8]. Our previous work [8] suggested that the selective removal of copper ions over lead ions was partly due to the nitrogen-rich surface of CMC-activated carbons. The present work was aimed to investigate the effect of helium degassing of CMC-activated carbons on adsorptive removal of copper ions.

2. Materials and Methods

2.1. Preparation of Activated Carbon

Cattle-manure-compost (CMC) was obtained from JFE Corporation, Japan. Pre-treatment steps were described in detail elsewhere [6]. ZnCl₂, the activating agent, was dissolved and impregnated into CMC at weight ratios of 0.5 and 1.5. One-step activation was progressed at 500˚C for 1 h under the flow of N₂. The resultant products were then treated in 3M hydrochloric acid and concentrated hydrofluoric acid. Thereafter, they were washed with de-ionized water until the solution pH became constant. Degassing procedure was progressed at temperatures of 500, 800 and 1000˚C under the flow of high purity helium gas for 1 h. The activated carbons were designated as CxTy where C is activated carbon with x referring to impregnation ratio, while T represents helium degassing at y temperature. Thus, C1T5 refers to impregnation ratio of 0.5 and degassing temperature of 500˚C, while C2T10 indicates impregnation ratio of 1.5 and 1000˚C degassing temperature.

2.2. Characterization of Activated Carbon

Surface functionalities were estimated using methods described by Boehm [9]. Batches of 300 mg carbon were brought into contact with 15 mL solutions of 0.1 M NaHCO₃, 0.05 M Na₂CO₃, 0.1 M NaOH and 0.1 M HCl, and were allowed to equilibrate at 25˚C for 48 hrs. The aliquots were back-titrated with either 0.05 M HCl for surface acidic functional groups or 0.1 M NaOH for basic groups, at which the neutralization points were observed using universal pH indicators.

Batches of 100 mg carbon were brought into contact with 50 ml of 0.1 M NaCl at different initial pH, and were allowed to equilibrate at 25˚C for 24 hrs. The value of pH_PZC was determined when the equilibrium pH is equal to the initial pH [10].

Pore characteristics were determined using Beckman Coulter SA3100 surface area analyzer (USA) at 77 K, while elemental compositions were measured using a Perkin-Elmer PE2400 microanalyzer.

2.3. Adsorption Studies

Analytical-reagent grade chemicals were purchased from Kanto Chemical Co., Inc. Stock solutions of model metal ions-contaminated water were prepared by dissolving desired weight of CuCl₂∙2H₂O in de-ionized water. Adsorption of metal ions was carried out at 25˚C for 48 h in a water bath orbital shaker. Fixed amount of activated carbon was added to the conical flasks containing 50 ml of simulated wastewater with known concentration. The values of initial solution pH were measured as 5.2 ± 0.3. The amount of metal ions adsorbed, q_e (mmol/g) was calculated as q_e = (C₀ – C_e) × (V/m), where C₀ and C_e are respectively the initial and equilibrium concentrations in mmol/L, V (L) is the volume of solution and m (g) is the mass of carbon. Concentration of copper ions was measured using atomic absorption spectroscopy (Rigaku novAA 300).

3. Results and Discussion

3.1. Characteristics of Activated Carbon

Results of Boehm titration and elemental analysis are shown in Table 1. At 500˚C of helium degassing, carboxylic groups of CMC-activated carbons were readily liberated and acidic groups were dominated by phenolic groups.

The concentration of acidic groups gradually decreased with the increase of degassing temperature to 1000˚C. The surface of activated carbons became more basic as the degassing temperature increases. This was well tallied with the values of pH_PZC. In the absence of acidic groups, basic groups are mainly attributed to electron-donating character and delocalized π-electrons on graphene layers that could behave as Lewis bases.

The increase of degassing temperature also results in the decrease of nitrogen content to about 65%. The leftover nitrogen moieties may consist of quaternary nitrogen, pyridinic and pyrrolic, while the fate of nitrogen functional groups in the non-cyclized structure will be the same as that of acidic functional groups. In general, both C1 and C2 activated carbons series show identical trend of chemical properties with respect to helium degassing temperatures.

Table 2 displays the physical characteristics of CMCactivated carbons. Slight decrease in yield implies the