Preparation of adsorbent
The raw material used in preparing the samples of absorbent were the shells of the Brazil-nut (Bertholletia excelsa), a residual material of chestnut processing. Firstly, this material was selected, washed in running water and then dried in an oven (drying and sterilization), model 315 SE of the brand FANEM-Brazil, at a temperature of 105 °C ± 5 for 24 h. After drying, the material was placed on two ceramic vases, where were charred and calcined in the muffle-type electric furnace of QUIMIS model Q318M24-Brazil.
Production and characterization of AC2
Carbonization
Carbonization assays of AC2 were performed in 10 runs and each in duplicate, wherein the previously dried shells were stored in ceramic vessels of 10.8 cm and 11.8 cm in diameter and carbonized at 400 °C ± 1 for 3 h in a muffle furnace, with a mean heating rate of 17 °C min-1. The masses obtained at the beginning and end of the procedure were important for determining yield parameters on charcoal (ACY) and on coal volatile material (CVM) released during the carbonization according to the methodology of Ramos (2005) [10].
Thermal activation
In this step, the same ceramic vessels were used, this time with materials charred in the previous step and inserted in the muffle furnace for activation during 2 h at 800 °C ± 1, in average time of 57 min, with a mean heating rate 17 °C min-1. The mass obtained at the beginning and at end of the procedure were recorded to determine the parameters of average yield of activated charcoal (AYAC) and volatile material of activated charcoal (VMAC) released during calcination in accordance with Ramos (2005) [10].
Characterization techniques
AC2 was characterized as: moisture by ASTM D 2867-04 method, ash content by ASTM D 2866-94 method, pH by ASTM 3838-05 method, porosity in fixed bed ABNT NBR 9165-1985, bulk density according to ASTM D 2854-09 method, actual density and fixed carbon performed according to Ramos (2005) [10].
Specific surface area
The analysis of the specific surface area was obtained by the theory of multilayer in which nitrogen was used in its gaseous phase N2 at 77 K, with approximately 40 adsorption-desorption cycles. The AC2 samples were treated at the temperature of 250 °C for 2 h before each test. The obtained data were analyzed by BET (Brunauer, Emmet and Teller) method, using a porosimeter of type MICROMERITICS TRISTAR-II USA.
Porosimetry
Data of the mean diameter and total pore volume were obtained from the N2 adsorption isotherm in gas phase at 77 K with adsorption-desorption cycles by the method of Barrett, Joyner and Halenda (BJH) using a MICROMERITICS TRISTAR-II USA porosimeter.
Scanning electron microscopy (SEM) coupled with Energy dispersive X-ray spectroscopy (EDS)
The SEM analysis was performed on electron microscope, model LEO-1430-USA; conditions for images of secondary electrons (ES) with beam current of 90 μA at constant voltage of 20 kV, working distance of 15 mm; AC2 samples were coated with a thin layer of platinum in sputter Emitech K550-USA.
X-ray diffraction
The analysis for phase identification was carried out on the samples by the total powder method using an X-ray diffractometer model X’PERT PRO MPD of PANALYTICAL- USA, with a goniometer PW 3050/60 (θ-θ) with X-ray ceramic tube with copper anode Cu (Kα1 = 1.540598 Å), model PW 3373/00 with fine focusing, Ni Kβ filter, 2200 W, 40 kV and 40 mA. For samples,the following were used: a sweep angle from 5 ° to 75 °, a voltage of 40 kV and current of 30 mA, a step size of 0.0170 ° and a time/step of 10 s, and a fixed divergent and anti-scatter.
Surface functional groups
The surface chemical properties of charcoal were determined by the acidity or basicity, which can be altered when in liquid and gaseous phases if oxidising agents exist in its structure, which when treated with solutions such as nitric acid, sodium hypochlorite or hydrogen peroxide modify the nature and the amount of oxygen in the complex surface of charcoal [9]. The surface functional groups of AC2 were determined by Fourier transform infrared spectroscopy, (FTIR) and by Boehm method [19–22]. FTIR was carried out using the Thermo Scientific Nicolet apparatus, model IS10-USA in the region 4000-400 cm-1. The carboxylic groups were obtained in tests with potassium hydrogen carbonate (KHCO3). The amount of phenolic groups were found by the difference between groups found in titration tests with sodium hydroxide (NaOH) and sodium carbonate (Na2CO3), and lactonic groups by the difference of the groups found in tests with sodium carbonate (Na2CO3) and potassium hydrogen carbonate (KHCO3). To calculate the mass of acidic surface functional groups by Boehm method, Equation 1 was used [19–22]:
$$ ASFG=\frac{0,1\mathrm{x}\kern0.5em f\kern0.5em \mathrm{x}\left({T}_b-T\right)\mathrm{x}\left(50/20\right)}{w} $$
(1)
Where: Tb, the volume of HCl 0.1 mol L-1 consumed by the solution of NaOH 0.1 molL-1 for the blank experiment (mL); T, the volume of HCl 0.1 mol L-1 consumed in different filtered solutions after the time of contact with AC (mL); f, the factorization of HCl 0.1 mol L-1 and w , the mass of AC used (g).
Influence of pH on the adsorption of copper (II)
To determine the influence of pH in solution in the adsorption process, the copper (II) solution was previously adjusted with standard solutions of HCl and NaOH 0.1 mol.L-1, and the measured pHs into a potentiometer of countertop HANNA in the ranges of: 3.4; 4.01; 5.09 and 6.01. The experiments were performed in duplicate for each pH range, keeping constants the initial concentration (50 mg L-1 of Cu II), volume of the solution (100 mL), solution temperature (27.2 °C), particle diameter (0.595 < D < 1.19 mm), mass in grams of the adsorbent (1.0 g) and agitation frequency (150 rpm). Then the samples were filtered and stored for determining the final concentration by atomic absorption.
Influence of CA2 particle diameter in adsorption of copper (II)
To determine the effect of CA2 particle diameter, granulometric studies were performed using sieves with mesh 14, 28 and 48 (1.19 mm, 0.595 mm and 0.297 mm) according to the Brazilian Association of Technical Norms (ABNT, Associação Brasileira de Normas Técnicas). Assays were performed in duplicate where the particle size varied within a range of 0.595 to 1.19 mm (D > 1.19 mm; 0.595 < D < 1.19 mm and D < 0.595 mm). For this study, initial concentration (50 mg.L-1 of Cu II), volume of the solution (100 mL), solution pH (5.47), solution temperature (27.2 °C), mass in grams of the adsorbent (1.0 g), contact time (60 min) and agitation frequency (150 rpm) remained constant. The samples were then filtered and stored for determining the final concentration by atomic absorption.
Influence of the contact time of CA2 in solution in the adsorption of copper (II)
For studying the contact times of CA2 assays were performed in duplicate and adsorption times evaluated at 1, 2, 5, 8, 10, 20, 30, 60, 90 and 120 min. The initial concentration (50 mg.L-1 of Cu II), volume of the solution (100 mL), solution pH (5.47), Solution temperature (27.2 °C), particle diameter (0.595 < D < 1.19 mm), mass in grams of the adsorbent (1.0 g) and agitation frequency (150 rpm) were maintained as constants. The samples were then filtered and stored for determining the final concentration by atomic absorption.
Influence of the equilibrium concentration
To study the equilibrium concentration in the adsorption process using CA2 as the adsorbent to remove Cu (II) of aqueous solution, the adsorption assays were performed in duplicate. For these experiments, 1.0 g of charcoal with diameter 0.595 < D < 1.19 mm were used, which were placed in Erlenmeyer of 250 mL containing 100 mL of aqueous solution of Cu (II) at the initial concentrations 5, 10, 20, 30, 50, 100, 150 and 200 mg.L-1 with pH equal to 5.46, temperature 27.2 °C and contact time of 60 min. The flasks were closed with a plastic film and excited at a frequency of 150 rpm. After said time, the samples were filtered for the determination of final concentration by atomic absorption. With equilibrium concentration data from all the tests it was possible to calculate the percentage of removal of Cu (II) by Equation (2):
$$ \mathrm{R}\left(\%\right)=\left[\frac{{\mathrm{C}}_{\mathrm{i}}\ \hbox{-}\ {\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{i}}}\right]\mathrm{x}100 $$
(2)
Where Ci: initial concentration of Cu (II) (mg.L-1) and Ce: final concentration or equilibrium concentration Cu (II) (mg.L-1).
With regard to the study of the amount of copper (II) adsorbed by mass of charcoal at equilibrium, the equation (3) was used:
$$ \mathrm{Q}\mathrm{e}=\frac{\left(\mathrm{Ci}\ \hbox{-}\ \mathrm{C}\mathrm{e}\right)\ \mathrm{x}\ \mathrm{V}}{\mathrm{M}} $$
(3)
Where Qe: amount of Cu (II) adsorbed (mg adsorbate/g adsorbent);
Ci: initial concentration of Cu (II) (mg L-1);
Ce: final concentration or equilibrium concentration of Cu (II) (mg L-1);
V: volume of the solution (L) and M: mass of charcoal CA2 (g).