Enhanced removal of cationic dye by eco-friendly activated biochar derived from rice straw

This report deals with converting the agriculture waste (rice straw) to environmental cleaner materials (biochar) using airless pyrolysis followed by eco-friendly activation. The biochar (p-Biochar) obtained after pyrolysis step (poorly active material) was activated using wet attrition method to give m-Biochar (highly active materials). The both p-Biochar and m-Biochar were characterized in detail and utilized for MB and CV dye removal from aqueous solution. Various parameters affecting the adsorption process such as dye concentration, adsorbent dose, contact time, temperature, NaCl dose and pH were investigated. The adsorption isotherm was well fitted using Langmuir isotherm, and the maximum adsorption capacity is 90.91 and 44.64 mg/g, for MB and CV dyes, respectively. The contact time data obtained showed that the two dyes were poorly adsorbed over p-Biochar. The equilibrium was reached quickly in 15 min for MB dye and 20 min for CV dye using the m-Biochar, and removal percent was 94.45 and 92.70% for MB and CV dyes, respectively. Moreover, the kinetic isotherm presented very well fitted by pseudo-second-order model. In addition, the adsorption percent increases with further increasing the pH value. Finally, we observed that m-Biochar highly adsorbs the MB dye compared with the CV dye over all experimental conditions.


Introduction
Dyes are considered as colored ionized aromatic organic materials. Dyes are required by numerous industries such as textile, leather, paper, plastic, and food. The effluents from these industries contain various types of contaminated, dye is one of the major involvement (Kobylewski and Jacobson 2010). The discharging of such contaminants in the environment causes a bad effect on human being, fish species, and microorganisms (Gupta 2009). Most of dyes are toxic, carcinogenic or mutagenic and can pose a hazard to health. Hence, it is very important to treat the dyed aquatic wastewater.
Several techniques were investigated to remove the dye species from the wastewater (Srinivasan and Viraraghavan 2009). Adsorption is considered a promising technique, in which the contaminants were transferred from the solvent to the solid phase. Based on this consideration, the adsorption appears to be simple to design, available, easy to operate, does not produce toxic materials and does not require high cost.
Activated carbon is the most applicable adsorbent used in industry; this is attributed to its adsorptive efficiency, high surface area, and porous structure. Nowadays, there is a great attention to prepare the low-cost activated carbon from alternative sources. Many researchers have prepared activated carbon from various waste agriculture products such as orange peel (Khaled et al. 2009), coconut tree (Senthilkumaar et al. 2006), coffee husk (Ahma and Rahman 2011), coconut husk (Foo and Hameed 2012;Aljeboree et al. 2017), pine cone (Gecgel and Kolancilar 2012), black acacia bark wastes (Da Silva et al. 2018), and rice husk (Rahman et al. 2012).
Rice straw can be considered as the most common biomass agricultural waste in the Egyptian Nile delta regions. Its production is quantified as 4 million tons per year. In the last decade, burning this in huge quantities by farmers has added an additional source of greenhouse gases (GHG) emitted from agricultural sector and caused "the black cloud," a thick black smog that can be seen over the Cairo sky every autumn. Converting this agricultural waste to biochar based on its content of 43% of cellulose, 26% of hemicellulose, and 16% of lignin (El-Masry 1983) can be one of the solutions for mitigating the emission of GHG in Egypt.
The aim of this search is to develop a way to manage the agriculture waste (rice straw) that causes environmental problems to an environmental cleaner material, used in water treatment process. The rice straw was converted into Biochar (p-Biochar) by burning in airless environment. The resulted product (p-Biochar) was activated by green method using the wet milling technique to give milled Biochar (m-Biochar). This active product was used to remove MB and CV dyes from aqueous solution.

Materials
All chemicals were of analytical grade and used as received: methylene blue (MB) and crystal violet (CV) (99.9%, Sigma-Aldrich), and rice straw (Egyptian Nile delta region).

Preparation of Biochar
The feedstock of rice straw was converted to biochar using the pyrolysis unit described by El-Gamal et al. (2017). The pyrolysis unit is equipped with two double-jacket drums. The cover of the external drum was supplied with chimney of 100 cm height. Rice straw materials were placed inside the stainless steel net in the internal jacket. Pyrolysis process lasted 2 h at 500 °C in the partial absence of oxygen. The produced biochar was then ground by porcelain mortar and sieved at 2.00 mm.

Preparation of milled Biochar (m-Biochar)
Milled Biochar (m-Biochar) was prepared by charging of 7.0 g of pre-source Biochar (p-Biochar) in a laboratory scaled ball mill. Steel balls were used as milling media, steel vial was used as reactor, and the ball mill was adjusted at 400 rpm on a "continuous mode". A milling period of 8 h, media weight of 113 g and water volume of 50 ml were taken as milling conditions. The produced m-Biochar was filtrated and dried at 50 °C for 24 h.

Characterization
The characterization of the p-Biochar and m-Biochar was carried out by a scanning electron microscope (SEM) (JSM-636 OLA, Jeol, Japan) and Fourier transform infrared spectroscopy (FT-IR) (8400 s, Shimadzu, Japan) covering the range from 400 to 4000 cm −1 . IR spectra of solid samples were detected using the KBr disk method. Porosity and pore size distribution of p-Biochar and m-Biochar samples were measured using a mercury porosimeter (Pore sizer 9320, micrometrics, USA). The surface areas were calculated by Brunauer-Emmett-Teller (BET) equations at 77 ± 1 K. The measurements were performed by a Quanta chrome NOVA automated gas sorption system using N 2 gas as the adsorbate.

Adsorption process
The dye adsorption by m-Biochar was recorded and studied by detecting the variation of the color of the MB and CV dyes in an aqueous solution, under continuous stirring. We carried out the experiments by placing 50 ml of the dye solution (MB and CV) in a 100-ml glass beaker at various initial dye concentrations (20, 40, 60, 80, and 100 ppm). Different amounts of m-Biochar (0.010-0.050 mg) and the effect of contact time were demonstrated at time intervals (0.5-45 min). The salinity was tested by adding NaCl to the dye solution in the range of 0-0.5 g, at temperatures of 25, 35, 45 and 55 °C and pH of 1. 5,3.5,5,7,9.5,11.5 for MB and (2,3.5,5,7,9.5) for CV. Finally, 1.5 ml of the suspension was removed and centrifuged. After phase separation, 1.0 ml of the centrifuged solution was diluted to 5 ml (for MB) and 10 ml (for CV) by distill water. MB and CV concentrations were determined at 662 and 590 nm, respectively.
The dye removal efficiency (%R) is defined as: where C o and C t are the initial concentration and the concentration of dye at time t, respectively.

SEM-EDX analysis
The surface morphology of p-Biochar and m-Biochar at various magnifications was investigated using SEM, as shown in Fig. 1. At low magnification, p-Biochar seems like the grooves-like structure, and as the magnification power increases, we observe the presence of grains in the surface of the grooves, as presented in Fig. 1a-g. On contrast, the grains possess different morphological structures which are scientifically seen by further magnification, as shown in Fig. 1d-g. As a result of the milling step, a great variation in the morphological structure of the modified biochar (m-Biochar) is observed, as shown in Fig. 1h-m. The m-Biochar preformed as fine powder free from grooves, as shown in Fig. 1h-m. Furthermore, the extra magnification shows that the size of the scanned m-Biochar material highly reduced from compared with p-Biochar (Fig. 1m).
EDX analysis of origin ash straw, p-Biochar and m-Biochar is shown in Fig. 2a-c. The origin rice straw composed of mainly organic material (as proved from the high content of C and O) and different levels of inorganic materials such as Si, K, Cl, Ca, Mg, Na and S as shown in Fig. 2a. Carbonization of rice straw in inert atmosphere to produce p-Biochar was confirmed by increasing the carbon content in the produced p-Biochar, as seen in Fig. 2b. On the other hand, there is an increase in the O content in the m-Biochar sample obtained from the wet attrition of the p-Biochar, as presented in Fig. 2c. Consequently, as expected the hydrophilicity and water dispersibility of m-Biochar will be enhanced compared with p-Biochar, as explained in Fig. 2d.

FTIR
FTIR spectra required determining various surface function groups for p-Biochar and m-Biochar, as pointed in Fig. 3. The shown absorption peak at 3437 cm −1 corresponds to O-H stretching vibration of the adsorbed water, the peak at 2931 cm −1 reveals C-H stretching, the peak at 1627 cm −1 corresponds to the C=C stretching (Kamal et al. 2017), 1477 cm −1 is due to bending vibration of CH 2 (Da Silva et al. 2018), and 1101 cm −1 is related to stretching C-O bonds. FTIR spectrum of m-Biochar shows the characteristic peaks obtained in p-Biochar with slightly shifted and more intense peaks. Moreover, after the treatment of the dyed water, the FTIR spectrum of the m-Biochar was highly alternated where the bands at 1095 cm −1 , 1460 cm −1 and 1624 cm −1 , before the adsorption shifted to 1099 cm −1 , 1440 cm −1 and 1599 cm −1 , after the adsorption, and became more intense as in Fig. 3. Additionally, two new bands appeared after the adsorption of MB dye 1332 cm −1 and 1390 cm −1 which may be related to the dye species. This affirms the adsorption of the dye and incorporation of the surface active groups in the adsorption process.

Surface area and porosity
The surface area pore diameters of p-Biochar and m-Biochar are illustrated in Fig. 4a. The p-Biochar possesses high surface area (223.4 m 2 /g) and large wide distribution in pore size within the range of 5-150 µm. However, after the wet attrition process the surface area and the pore size of the released biochar (m-Biochar) were dramatically reduced to 104 m 2 /g and 0.5-5.0 µm, respectively (Fig. 4a). This affirms the formation of oxygenated active groups not only on the m-Biochar surface but also inside the pores structure (Cao et al. 2016;Zhao et al. 2017), which was confirmed also by EDS analysis (Fig. 2), and FTIR spectra (Fig. 3). From the previous results, we can conclude that the wet attrition technique is a available tool for green activation process.

Thermal analysis
The thermal behavior of p-Biochar and m-Biochar shows exothermic characteristics as presented in Fig. 4b. The exothermic peaks were located at 265 °C for p-Biochar and 225 °C for m-Biochar. Moreover, the DSC curve indicated that the heat liberated in the case of p-Biochar suggests to be higher than that the heat liberated from m-Biochar. This maybe reveals the formation of O-functional groups and reduction in the biochar size upon the wet milling process (Zhang et al. 2009;Zheng et al. 2014).

Effect of initial dye concentration
The effect of initial concentration of MB and CV dyes on its removal percent %R aqueous solution is illustrated in Fig. 5a. It is observed that the %R decreases with the further increase in the initial concentration of MB and CV dyes. This phenomenon is: as the dye concentration increases, a 1 3 45 Page 4 of 11 Fig. 1 The SEM images at various magnifications of the p-Biochar (a-g) and m-Biochar (h-m) large number of the dye species compete on a limited number of available active sites. By fixation of the adsorbent dose, the active sites available for the adsorption process are limited. Consequently, using low concentration of the dye solution, the number of the active sites is sufficient for removal of all dye molecules. However, this limited number of active sites is available for the definite number of the dye species in the solution (Chatterjee et al. 2005;Chiou and Li 2003).

Adsorption isotherm
Adsorption isotherm model was utilized to study the relation among the quantity of adsorbed adsorbate (mg) on the surface of adsorbent (g). Langmuir and Freundlich isotherms are the most famous models used for the description of the adsorption isotherm. Langmuir isotherm This model suggests monolayer adsorption of adsorbate species on equally energetic active sites of adsorbate, and its mathematical expression is represented in Eq. 2.
where C e is the equilibrium concentration (mg/L), q e is the amount adsorbed at equilibrium time (mg/g), q o is the maximum dye species uptake per unit mass of adsorbent (mg/g), which is related to adsorption capacity and b is Langmuir constant (L/mol).
The equilibrium parameter, R L , is a dimensionless constant which characteristic Langmuir isotherm, can be expressed by Eq. 3.
( where C o is the initial dye concentration. The nature of the adsorption process can be either unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L < 1), or irreversible (R L = 0).
(3) R L = 1 1 + bC o Freundlich isotherm Freundlich model assumes that the adsorbate species were adsorbed on hetero-energetic active sites of the adsorbent and could be presented by Eq. 4. K f (mg/g) and n are Freundlich constants incorporating all factors affecting the adsorption process such as adsorption capacity and intensity of adsorption.
The experimental parameters resulting from linear plotting of Langmuir and Freundlich model (Fig. 5c) were calculated and are listed in Table 1. On comparing the values of correlation coefficient R 2 of the two models, it can be concluded that the adsorption of MB and CV dyes on the surface of m-Biochar follows Langmuir model; this means the adsorption process takes place on a homogenous active sites as a monolayer. Moreover, R L values (Fig. 5d) lay between 0.0 and 1.0 proving a favorable adsorption of MB and CV over m-Biochar.

Effect of adsorbent dosage
The effect of solid dosage in the range of 0.01-0.05 g on the removal percent for both MB and CV dye was demonstrated in Fig. 6. This figure shows that the removal percent increases from 34.70 to 95.06% for MB and from 29.32 to 95.13% for CV when the adsorbent dose increases from 0.01 to 0.05 g; this increase in %R can be explained by improving adsorbent surface area and rising in the available active sites for the adsorption (Da Silva et al. 2018).

Effect of contact time
The adsorption process is highly affected by the time spent where the adsorbent and adsorbate are in contact. The adsorption performance of MB and CV dyes on p-Biochar  Fig. 7a. It is obvious that at utilizing p-Biochar the removal percent for both two dyes is very weak. On the other hand, the %R is enhanced in the case of presence of m-Biochar. Where 84.13% and 73.81% removed from MB and CV dye aqueous solution in 0.5 min, respectively. Moreover, the equilibrium was reached for MB (94.45%) in 15 min and CV (92.70%) in 20 min. This behavior can be explained on basis that the wet attrition makes reduction in the particle size (SEM images) and adds extra O-active groups (FTIR and porosity sections), which leads to increase the activity, hydrophilicity and the water dispersibility of m-Biochar compared with p-Biochar.

Adsorption kinetics
The relationship between the adsorption rate of the adsorbate and the adsorption time is explained by using adsorption kinetics. Two well-known kinetic models, pseudo-first-order presented by Lagergren equation (Lagergren 1898) Eq. 5.  where q e (mg/g) is the amount of sorption at equilibrium time, q t (mg/g) is amount of sorption at time, and K ads (min −1 ) is the rate constant of pseudo-first-order sorption.
where C o is the initial concentration (mg/L), C e is the dye concentration at equilibrium time intervals (mg/L), V is the volume of dye solution (mL), and w is the mass of adsorbent (g) where C t is the dye concentration at different time intervals (mg/L).
(5) log(q e − q t ) = log q e + K ads t 2.303 And pseudo-second-order expressed by Ho equation (Ho and McKay 1999) the equation developed in a linear form as Eq. 8: where K 2 (g/mg min) is the rate constant of reaction.
The various parameters result from analyzing the kinetic models; pseudo-first order (Fig. 7b) pseudo-second order (Fig. 7c) were calculated and are recorded in Table 2. Based on the parameter values obtained from different models, it is clearly that the pseudo-second-order kinetic model gives high coefficient factor (R 2 ) values i.e., 0.999 (for MB) and 0.995 (for CV), and the calculated q e is equal to 72.99 (for MB) and 52.63(for CV) which is more closer to the experimental value, q e , of 72.48 and 46.38 for MB and CV, respectively. This makes the adsorption of both dyes on the m-AS follow the pseudo-second-order kinetic model.

Effect of NaCl dose and temperature
The effect of NaCl dose in the range of 0.00-0.50 g and temperature in the range of 25-55 °C on the removal percent of MB and CV dyes was studied as shown in Fig. 8a-b. Clearly, the increase in the dose of the NaCl or the temperature resulted in a slight variation in the %R for both MB and CV dyes.

Effect of initial pH
The variation in the initial pH of the solution is strongly influenced on the nature of adsorbent active sites and adsorbate species. As expected, the removal percent %R of MB and CV from the aqueous solution was alerted with any variation of the solution pH, as shown in Fig. 8c. We can see the adsorption increases by increasing the pH values. MB and CV are cationic dyes, i.e., having positive charge; in this trend, at low values of pH, H-ions compete with the dye species to be adsorbed on adsorption active site; this leads to decreasing the removal percent of both dyes. Increasing the pH values makes the competition weak which enhances the adsorption percent (Shoukat et al. 2017).

Comparison of dye removal with different adsorbents reported in literature
Several reports have been developed for preparation adsorbents from various agriculture waste materials and studying its activity toward removal of MB and CV dyes from aqueous solution.
The adsorption capacities of these adsorbents were recorded and compared with m-Biochar prepared in this study (Table 3). The data presented that the adsorbent used in this study shows adsorption capacity higher than that of the other adsorbents.

Conclusion
We successfully prepared an economic, efficient, and green activation adsorbent material derived from a waste agriculture product. The prepared adsorbent was used to treat the dyed waste effluents. The experimental results obtained estimated that the adsorbent used (m-Biochar) is an efficient adsorbent for removal of MB and CV dyes from aqueous solution. According to Langmuir isotherm, the maximum adsorption capacity of MB is 90.91 mg/g and CV is 44.64 mg/g, and the two dyes were adsorbed as a monolayer on the adsorbent. The kinetic adsorption data of both used dyes flow the pseudo-second-order model. The solution pH highly controls the adsorption of the two dyes. Moreover, the removal percent of the two dyes is not influenced by the rising in the temperature or the NaCl dose. Finally, this research exhibits an excellent environmental solution. Where, m-Biochar prepared from rice straw using simple, economic preparation routs. On the other hand, the resulted adsorbent is used to remove the cationic dyes from aqueous solutions. the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.