Production of carbonaceous material from avocado peel for its application as alternative adsorbent for dyes removal

Carolyn Palma *,Lucia LloretAntonio Puen Maira Tobar Elsa Contreras

1 Departamento de Ingeniería Química y Ambiental,Universidad Técnica Federico Santa María,Vicu?a Mackenna 3939,Santiago,Chile

2 Departamento de Ingeniería Química,Universidad de Santiago de Chile,Alameda 3363,Santiago,Chile

1.Introduction

Industrial effluents represent an important source of chemical pollutants responsible for the water quality deterioration after their discharge into the environment.Among those contaminants,the presence of colored substances in wastewaters means an important public concern due to their impacts:interference with the sunlight penetration,retardation of the photosynthetic processes and therefore,the growth inhibition of aquatic biota[1,2].Pulp and paper mills[3],tanneries[4,5],distilleries[6,7]and dyestuff and fabric dyeing plants[8,9]are the main industries generating colored wastewaters.

Various dyes are manufactured based from known carcinogens such as benzidine and other aromatic compounds[10].Moreover,certain synthetic dyes and their byproducts exhibit mutagenic and teratogenic characteristics to aquatic species[11]and can even cause negative effects in the human health such as damage to the kidney and reproductive and central nervous systems[12,13].Nevertheless,the abatement of colored pollutants by conventional technologies was proved to display in most cases a low effectiveness due to their high degradation resistance caused by their complex aromatic structures[14,15].

Thereby,advanced and specific treatments are needed for the removal of these pollutants.A variety of methods has been developed for this purpose:e.g.treatment systems based on white rot fungi and their enzymes[16,17],biological degradation by bacteria and algae[14],chemicalprocesses such as photocatalysis and oxidation by a number of agents such as hydrogen peroxide,ozone,UVirradiation and TiO2[18-20]and even the implementation of a biosorption stage was explored as pretreatment so as to reduce the pollutant concentration and effluent volume aiming to favor the later biological oxidation step[21].Anyhow,adsorption processes of organic pollutants by using solid adsorbents have received special attention due to their capacity to generate effluents with a considerable quality,the simple design and the low investment in terms of both initial cost and required land,being a potential alternative to those chemical and biological processes.In fact,adsorption onto activated carbon has been cited by US Environmental Protection Agency as the Best Available Technology to reduce the environmental pollution in water,soil or air by separation and purification processes[22,23].

The great adsorption capacity achieved with activated carbons is mainly due to their high surface area and porosity but also to the chemicalnature oftheir surface,properties which may vary depending on the carbon generation procedure.For instance,there exist different techniques for improving the porous structure and density,as chemical activation by the impregnation with dehydrating chemicals[24-26].However,the carbonization temperature is considered the most significant parameter affecting the properties of the carbonaceous materials[27].According to previous authors,high temperature leads to greater primary decomposition of biomass and/or secondary decomposition of char residue[28].For example,González et al.[29]determined that chars generally present higher ash yield and alkaline metals content when prepared above 300°C.Considering thatthe properties ofthe sorbent greatly depend on the operational conditions for its preparation besides the materialprecursor,it is essentialto carry out a characterization of the carbonaceous solid as well as to evaluate the optimal carbonization conditions aiming to facilitate its successful feasibility as contaminants adsorbent.

On the other hand,activated carbons have been traditionally prepared from coal/lignite,wood/coconut or animal bones[30,31];nonetheless,the use of low-cost and eco-friendly adsorbents,such as those based on agricultural wastes and industrial byproducts,are being investigated as alternative materials.Recently,research is focused on the use of:(1)naturarsorbents,which are naturally occurring materials,and(2)carbonaceous adsorbents,prepared from the previous ones.Regarding the natural adsorbents obtained from agro-wastes,a range of materials can be found including residual biomass of fruits,plants and cereals[22,23,30,32-37].

Among the possible residues to be used,those generated for avocado processing appear to be an interesting option because of their abundance,once avocado has become a major agro-industrial commodity reaching in 2011 a global production of about 4 million ton/year[38].The commercialization and consumption are mainly as fresh fruit,but during the lastdecades,industrialapplication was developed for the extraction of avocado oil[39],the elaboration of processed foods such as guacamole,frozen products and avocado paste[40,41]and even for the production of cosmetics,personal care and nutraceutical products using non-edible avocado oil[42].The avocado processing yields a considerable amount of byproducts,corresponding the 15%of the product to the peel,which is often not valued and conversely,many times disposed in land fills[43].The use of this residue would be an opportunity to produce eco-friendly adsorbents,but also to avoid its handling as solid waste with its concomitant valorization.

The main objective of this study was to explore the production of carbonaceous material from avocado peel by evaluating the carbonization temperature and time effects aiming to explore the optimal conditions to maximize the surface area of the generated adsorbent;in addition,the feasibility of the carbonized material was investigated for the removal of acid,reactive and basic dyes.

2.Materials and Methods

2.1.Agricultural residue

Avocado peel(Persea americana),which was the precursor of the carbonaceous solid to be used as adsorbent,was collected from the University dining service;two different types were used:Hass and Florentina.The raw material was pretreated by washing with deionized water and drying at room temperature(22-25°C)for 3 days,when the equilibrium moisture content was reached.Afterwards,the dried solid was crushed and sieved by using the[-14+140]mesh cut in order to obtain a solid with particle size in the range of 106-1400 μm.

The obtained material,with a relation of avocado varieties of 1.8:1(Hass:Florentina)and measured properties:moisture content 74.4%and volatile fraction 24.5%(by mass),was used for the following experimental procedures.

2.2.Dyes

The dyes used:Naphthol Blue Black(AB1),Reactive Black 5(RB5)and Basic Blue 41(BB41),were selected as adsorbates with acid,reactive and basic characteristics,respectively.Allofthem were ofanalytical grade and were supplied by Sigma-Aldrich;stock solutions were prepared in distilled water.The main properties of these compounds are presented in Table 1.

2.3.Production of carbonaceous material:optimization

A range of experiments was performed by using a muffle furnace under a self-generated atmosphere(Thermo Scientific,model F6020C-33-60).Two different samples of 20 g of dried raw residue were introduced per batch in the equipment,which was operated at a temperature increasing rate of 10 °C·min-1until reaching the selected carbonization temperature.A flux of 10 L·min-1of purified nitrogen(99.99%)was maintained,which is supposed to generate a better porosity development of the material[44].

The carbonization procedure was conducted at different temperatures(400-900°C)and operation times(30-90 min)in orderto evaluate the effectofthese variables and maximize the surface area,being this the response variable.A total of 12 experiments were carried out according to the factorial experimental design 32along with triplicate experiments of the central point.The evaluated factors and coded levels as well as the experimental design are shown in Tables 2 and 3,respectively.

Experimental results were analyzed by the use of the software Statgraphics Centurion XVI(Statsoft Inc.)in order to link the response to the selected variables and to determine the optimum conditions by maximizing the response variable;response surface methodology(RSM)was applied for this purpose.The quality of the models was expressed by the correlation coefficients,and three-dimensional response surface and Pareto charts were plotted.The evaluated response was related to the selected variables by the quadratic model shown in the following Eq.(1):

where Y is the predicted response;x1and x2are the coded levels of the factors;α0is the intercept term;α1and α2are the coefficients for linear effects; α12is the cross-coefficient;and α11and α22are the quadratic coefficients.

2.4.Surface area determination

Methylene Blue(MB,Sigma-Aldrich)adsorption was evaluated through Langmuir isotherm as a rapid and accurate method for the comparative determination of the specific surface area of the carbonaceous materials prepared under all the evaluated conditions,to be used as response variable for the optimization of the carbonization process.These experiments were performed in 250 ml Erlenmeyer flasks(working volume 100 ml)containing a solution of 50 mg·L-1of MB and different amounts of carbonized avocado peel in the range 0.01-0.6 g;these samples were maintained at room temperature and under continuous stirring(150 r·min-1)for 3 h,when the equilibrium was reached.Finally,the samples were filtered and the residual concentration ofthe dye was determined by measuring the absorbance atits maximumwavelength(664.4 nm)using a spectrophotometer(Analytikjena Specord 210).

Once the method assumes monolayer coverage of the dye onto the solid,the specific surface area(S,m2·g-1)was determined by Eq.(2):

where qmis the adsorption maxima(mg·g-1),i.e.amount of dye adsorbed per mass unit of adsorbent for forming a complete monolayer on the surface,N is the Avogadro constant(6.022×1023mol-1),AAMis the area per MB molecule(130×10-20m2)and Mw is the molecular weight of the dye(319.85 × 103mg·mol-1).

Table 1 Properties of the selected dyes

Table 2 Factors and levels evaluated during the experiments of avocado peel carbonization and dyes adsorption on carbonaceous material through 32 factorial designs

Table 3 Experimental design and specific surface area results obtained for the experiments of avocado peel carbonization

The parameter qmwas determined by fitting the experimental data to the linear form of Langmuir equation,as expressed in Eq.(3):

where qe(mg·g-1)is the ratio amount of dye adsorbed per amount of solid,Ce(mg·L-1)is the equilibrium concentration and Ka(L·mg-1)is the Langmuir constant.

2.5.Characterization of the carbonized avocado peel

The carbonaceous solid obtained under the optimized carbonization conditions determined by RSM was characterized in terms of the following parameters.

First,carbonization yield was calculated as the ratio between the residue mass prior carbonization and that of the carbonaceous material,both expressed on dried basis.Specific surface area was determined by MB adsorption following the method described above,which was also useful for the validation of the model after fitting the experimental data to Eq.(1),by comparing the experimental value with that predicted by the model.

BET surface area was determined for comparison with other carbonized residues or conventional activated carbons;for this,nitrogen adsorption-desorption isotherms of the sample,previously dried and outgassed,were obtained at-196°C by means of a Micromeritics ASAP 2010 sorptometer.Surface area was calculated according to Brunauer,Emmett and Teller(BET)model isotherm,for which nitrogen adsorption data within the range of relative pressure from 0.1-0.3 was utilized.Additionally,pore size distributions were obtained by the Barrett,Joyner and Halenda method.

The scanning electron microscope(SEM,Tescan Vega 3 equipment)was used to observe the surface pore structure of carbonized avocado peel.Previously to SEM determinations,the sample was coated with a thin layerofgold and mounted on a copperstab using a double stick carbon tape.

The point of zero charge(pHPZC)was measured by following the method described by Palma et al.[45].Brie fly,various suspensions of 0.1 g of the solid in different volumes(1.25,2.5,5,7.5 and 10 ml)of 0.1 mol·L-1HCl were prepared in Erlenmeyer flasks;similarly,carbonized material suspensions were prepared with the same volumes of 0.1 mol·L-1NaOH.Thereafter,5 ml of 0.1 mol·L-1KCl was added to all the samples and distilled water was incorporated until a final volume of 100 ml.Then,the flasks were incubated for 1 h and the pH(pH1)was determined by a conventionalpHmeter(Hanna Instruments).The nextstep was the addition of 5 ml of 1 M KCl,and after 1 h agitation the pH was measured(pH2).A sample with 0.1 mol·L-1KCl and distilled water was used as control experiment.Finally,the difference pH2-pH1was calculated;thus,the pointpHPZCoccurs when there was no change in the pH.

2.6.Dyes adsorption on carbonaceous material

The adsorption capacity of the carbonized avocado peelwas demonstrated by evaluating the removal of the selected dyes:AB1,RB5 and BB41,through their adsorption on the material prepared under the optimal conditions.

The experiments were performed in 250 ml Erlenmeyer flasks(working volume 100 ml)containing specific amounts of both solid and dyes,which were maintained under continuously stirring at 150 r·min-1.The contact time required for achieving the equilibrium was determined by monitoring the process through the determination of the dye residual concentration in the solution at different incubation times,until the variation among successive measurements was not significant.After this period,the suspensions were centrifuged and filtered under vacuum,and the residual concentration of each dye in the supernatant was determined by measuring the absorbance at the corresponding maximum wavelength(Table 1).Controls lacking solid were conducted to demonstrate that dye removal is caused only by interaction with the adsorbent.

Dyes removal was analyzed by performing experiments at different dyes initial concentration(10-50 mg·L-1)and solid doses(0.5-20 g·L-1)to maximize the removal percentages;as conducted for carbonization optimization,a total of 12 experiments were performed in accordance with a factorial experimental design 32.The factors and levels evaluated as well as the experimental design are shown in Tables 2 and 4,respectively.

Table 4 Experimental design and removal percentages results obtained for the experiments of dyes adsorption on carbonaceous material

2.7.Removal percentage determination

Dyes removal yields(R,%)were determined according to Eq.(4)for all the adsorption experiments with the goal of analyzing experimental data by maximizing this variable:

where C0and Ce(mg·L-1)are the initial concentration(before adding the solid)and the concentration in the equilibrium,respectively.

3.Results and Discussion

3.1.Production and characterization of carbonaceous material from avocado peel

3.1.1.Production of carbonaceous material:optimization

The RSM is a collection of mathematical and statistical tools useful for evaluating relative significance of several variables and determining optimum conditions for desirable responses[46,47].In the current work,this methodology was applied to evaluate a totalof12 experiments performed to obtain carbonaceous materialfrompretreated avocado peel at different values of temperature and carbonization time(Table 2).Surface area was defined as response variable since the carbonization performance should be evaluated through the surface structure/porosity of the material once this determines the surface available for interaction with the contaminant molecules.The experimental results obtained through MB methodology for all the runs are shown in Table 3,where it is observed thatvalues from79.9 up to 133.7 m2·g-1were accomplished.

Moreover,the experimental data were fitted to a polynomial model aiming to establish an expression to predict the carbonization yield in terms of surface area depending on the variable coded values;the empirical expression of Eq.(5)was obtained,where x1and x2correspond to temperature and time factors:

Positive values indicate synergism whereas negative values indicate antagonism;accordingly,it can be observed that,as expected,both temperature and process time have a positive effect on the response.For instance,when the temperature is elevated from 400 to 650°C for a time of 30 min,the surface area is improved from 79.9 to 90.6 m2·g-1(Runs 1 and 3,Table 3)and until 108.7 m2·g-1for the maximum temperature tested,900°C(Run 2,Table 3).This effect was less noticeable for greater carbonization times:e.g.only a slight difference on surface area values was observed when operating for 90 min and at400 and 650°C(Runs 4 and 6,Table 3),and a temperature of 900°C was needed to increase the area from 86.8-88.0 to 115.9 m2·g-1(Run 5,Table 3).

The adequacy of the model was corroborated by determining the correlation coefficients:values for R2and adj R2(adjusted R2,which is preferred as itdoes notalways increase with the number ofparameters)of 0.886 and 0.791 were determined,respectively,which indicates a fairly good fitting and suggesting a proper use of the model to predict carbonization results within the studied factors ranges.

Pareto chart was plotted to analyze the standardized effect of each evaluated factor with a confidence level of 95%and their interdependence on the surface area of the carbonaceous material(Fig.1).As observed,temperature seems to be the variable with the major signi ficanteffect,followed by itsquadratic termand presenting both a positive effect on the surface area.On the contrary,carbonization time impact appeared to be negligible,which can be also concluded when comparing its coefficient value in Eq.(5)(1.917)with that of the temperature(17.117).In fact,scarce enhancement was observed when increasing the process time at 400 and 650 °C,being the effect greater at 900 °C:surface area is enhanced from 108.7 to 133.7 m2·g-1when extending the time from 30 up to 60 min(Runs 2 and 8,Table 3);still,the response variable is slightly negatively affected when operating at the same temperature and a period time above 60 min(Run 5,Table 3),suggesting there exists an optimum within the predetermined region of this variable.

From the analysis of the variables and their interactions by means of RSM,the surface plot depicted in Fig.2 was obtained,where the outcomes discussed above are clearly noticeable.It is evidenced the higher in fluence ofthe temperature on the response in comparison with thatof the carbonization time and moreover,a plateau at the lower values of temperature is observed:when temperature presents values in the range 400-650°C,only slight differences in the response were noted,and the impact was greater for temperatures higher than 650°C;this could be the reason for the significance of the temperature quadratic effect(Fig.1).

It is known that,although the development of specific surface area depends on various factors,temperature presents an important impact,as observed in the current research.González et al.[29]reported markedly higher surface area of carbonized oat hulls and pine bark at 500°C in comparison with those at 300°C:increments of 66 and 33 times for each precursor,respectively,were achieved.Same conclusion was stated by several previous authors working with different materials:oak wood,corn stover,switchgrass and pecan shell,among others[48,49],indicating that an increase in the process temperature enhances surface area independently on the feedstock.In addition,previous authors also highlighted the importance of residence time besides temperature on the physical characteristics of carbonized materials.Aguado et al.[50]found that carbonization of lignocellulosic waste below 400°C leads to the formation of micropores very slowly and excessively residence time was required to attain significant changes in the surface area;however,mesopores and micropores were clearly developed after only 180 s at temperatures above 450°C.

On the other hand,Girgis et al.[31]indicated that peanut hull carbonization at temperatures higher than 900°C promotes a loss in the total surface area and an increase in non-microporous surface area;this was assumed to be caused by the fact that pore widening takes place as a result of wall burning between micropores.Also,Ioannidou and Zabaniotou[27]reported temperatures above certain value might induce material structure shrinkage,leading a reduction in the surface area and pore volume.Anyhow,these effects were not observed in this work,being the highest temperature evaluated,900°C,an optimal condition.An analogous effect may occur when exceeding specific carbonization times.Forinstance,the surface areasofadsorbents produced from corn hulls and corn stover were appreciably greater after 1 h than after 2 h of thermal treatment because of the rate of pore destruction due to the adsorbent structure collapse[27];a similar situation seems to be occurred here once moderate surface area decline was observed at high carbonization times(Fig.2).

Fig.1.Pareto chart obtained for the optimization of avocado peel carbonization.Black bars:synergistic effect;white bars:antagonistic effect.

Fig.2.Response surface plot obtained for the optimization of avocado peel carbonization.

Finally,the following optimal carbonization conditions were found according to the data analysis:+1 and+0.16 for the temperature and carbonization time respectively,which means 900°C and 65 min,in order to attain the theoretical maximum response:123.1 m2·g-1of carbonaceous material surface area.In order to validate the model,an additional experimental run was carried out under those optimized conditions:a surface area of 136.6 m2·g-1was determined,which is quite close to the predicted value with an error of less than 10%.This confirms the reliability of the analysis and the model to predict and optimize the carbonization yield of avocado peel.

3.1.2.Characterization of the avocado peel carbonized under optimal conditions

Prior to characterization of the carbonized residue obtained under optimum conditions(900°C and 65 min),the carbonization yield was calculated to express the amount of dried adsorbent which can be produced per mass of avocado peel:a value of about 30%was determined,quite close to the yield attained after the generation of activated carbon from coconut shells(36.9%)[51].

From the application of BET method for the nitrogen adsorption isotherm analysis,a value of BET surface area of 87.52 m2·g-1was obtained.Elizalde-González et al.[30],who investigated the use of avocado kernel seed for the adsorption of colored substances,reported relative specific surface area in the same magnitude order and also proved the augmentation of its value with the carbonization temperature:surface area was 53 m2·g-1for the raw material while values of 227 and up to 452 m2·g-1were determined after carbonization at 800 and 1000°C,respectively.

Pore size distribution of the carbonaceous material is shown in Fig.3(a):cumulative pore volume percentage is represented as well as relative pore volume(insert).Aheterogeneoussize distribution wasobserved,being the most frequent width(25%)the one corresponding to pore radius between approximately 1 and 3 nm.In accordance with the classification adopted by IUPAC[52],adsorbent pores are classified as:(1)micropores(＜2 nm),which can be divided into ultramicropores(width less than 0.5 nm)and supermicropores(width from 1 to 2 nm),(2)mesopores(from 2 to 50 nm)and(3)macropores(＞50 nm).Hence,the carbonaceous material contains 4%micropores,74%mesopores and 22%macropores.SEMimages revealed thatthe carbonized avocado peel particles are of irregular shape and present a rough and highly porous surface(Fig.3(b)),which might promote the adherence of the substrates.Similar results were found by Wang et al.[51]:activated carbon from coconut shells was obtained with a micropore,mesopore and macropore volumes of 37.06,62.85 and 0.07%,respectively.The authors highlighted that while gas-adsorbing carbons usually have more micropores,liquid-adsorbing carbons should present significant mesopores due to the generally larger size of liquid molecules;moreover,the generation of mesoporous carbons would facilitate the access of the adsorbate molecules to the interior of the material particles[53].

The mechanism controlling the compounds adsorption on lignocellulosic wastes is dificult to predict because of their diverse origin;the physicochemical characterization of the sorbent surface,specifically the acid-base behavior,might be useful to understand the process involved:e.g.complexation reactions,ionic exchange and electrostatic interactions,among others[54,55].For this,the knowledge of pHPZCwould permit to hypothesize about the interaction between the functional groups on the carbonaceous material with ionic species in solution during the contaminants adsorption.In this case,the zero-point charge of the avocado peel carbonized at 900°C for 65 min resulted to be of 8.6,as observed in Fig.4.When the pH of a solution is higher than the value of pHPZC,the surface of the biosorbent has a negative charge since the acid groups are deprotonated and therefore could preferably interact with cationic species.Under these conditions,the carbonaceous material from avocado peel could be an interesting strategy to uptake basic dyes from aqueous solutions[22,45].

Agro-industrial wastes have reported to predominantly present an acidic character due to the high cellulose content and thus,because of the hydrogen atoms of the hydroxyl groups acting as electron acceptors[56].For example,orange peel surface was proved to present acid behavior[22],caused by the carboxyl and hydroxyl groups provided by the high concentration of cellulose,hemicellulose,pectin and protein[57].This outcome could be also expected for the avocado peel considering its content in phenolic hydroxyl groups[58],but a high pHPZCvalue was achieved.This might be caused by the formation of basic groups derived from the decomposition of acidic groups during the carbonization process at high temperature[59].Indeed,Dávila-Jiménez et al.[60]reported similar zero charge results for adsorbents produced by carbonization of mango seeds at 600°C(pHPZC8.8),while the value increased to 9.6 after carbonization at 1000°C.

Fig.3.Pore size distribution(a)and SEM image(b)of the carbonaceous material obtained under optimal conditions.

Fig.4.Determination of the pHPZC of the carbonaceous material obtained under optimal conditions.

3.2.Evaluation of dyes adsorption on carbonaceous material

The feasibility of the carbonized avocado peel under optimal conditions was studied through the adsorption of AB1,RB5 and BB41,by the evaluation of dyes and solid initial concentrations(Table 2),obtaining the removal percentages shown in Table 4.Moreover,the experimental data was fitted to the polynomial model(Eq.(1))and the statistical significance was investigated by ANOVA;the results are shown in Table 5.Correlation coefficients R2and adj R2were:0.953 and 0.913,0.944 and 0.897,and 0.983 and 0.968 for AB1,RB5 and BB41,respectively.

The regression coefficients and the relationship between each factor can be considered statistically significant for p-values below 0.05,with 95%of confidence interval;accordingly,adsorbent dose resulted to be the major contributor for all the tested colored compounds,presenting a positive effect on the response(values for α2,Table 5):e.g.removal percentages of 5.37,6.63 and 64.50%were achieved for AB1,RB5 and BB41,respectively,for an initial dye concentration of 10 mg·L-1and a solid dose of 0.5 g·L-1(run 1,Table 4),and these yields were greatly improved up to 78.99%-100%and 92.63%-99.31%when increasing theadsorbent concentration until 10.25 and 20 g·L-1,respectively(runs 7 and 4,Table 4).

Table 5 Regression coefficients and corresponding p-values obtained for the optimization of dyes adsorption on carbonaceous material

On the other hand,dye concentration was the second variable,and the single along with the solid dose,affecting the removal percentages in the case of AB1 and RB5;its effect was negative on the response variable(values for α1,Table 5),probably due to saturation of the adsorbent.These findings suggest the need of operating at the following extreme evaluated levels:minimum dye concentration(10 mg·L-1)and maximum solid dose(20 g·L-1),with the goal of maximizing the removal percentages of both acid and reactive dyes.Nevertheless,the quadratic effect of the adsorbent concentration was the second effect impacting on the removalofthe basic dye,BB41,being negative the corresponding regression coefficient(value for α22,Table 5).This could mean there exists a plateau for high solid doses and therefore an optimum within the evaluated range.In fact,a considerable improvement on the removal of BB41 was attained when increasing solid dose from 0.5 to 10.25 g·L-1,e.g.from 33.31 up to 100%at 50 mg·L-1of the dye(Runs 2 and 8,Table 4),butno differences were observed for greater adsorbent content since the adsorption of the dye was already completed(Run 5,Table 4).This occurs due to the low ratio of the initialnumberof dye molecules to the available surface area and subsequently,the fractional adsorption become independent on the solid dosage[61].These outcomes were corroborated by the determination of the optimal conditions by the analysis ofthe experimentaldata through RSM:complete removal of AB1 and RB5 would be reached when working atconcentrations 10 mg·L-1and 20 g·L-1of dye and solid,respectively(coded values-1 and+1),while the same initial concentration of BB41 could be removed by using 13.4 g·L-1of carbonaceous material(coded levels-1 and+0.32).

With respect to the effect of dyes initial concentration on the adsorption efficiency,it is noteworthy that despite the negative impact on removal percentages,the amount of dye adsorbed per unit mass of carbonaceous material(adsorption capacity)increases with the substrate initialconcentration.As example:when using the maximumlevelofsolid dosage(20 g·L-1),the adsorption of BB41 varied from 0.48 to 1.49 and 2.5 mg ofdye adsorbed per g ofcarbonized avocado peelas its initialconcentration was augmented from 10 to 30 and 50 mg·L-1,respectively(data not shown).This increase is caused by the decline in resistance to the solute uptake from dye solution,once the initial concentration provides an important driving force to overcome the mass transfer resistance between the aqueous and solid phases[62,63].

When comparing the average of removal yields reached within the evaluated region,it can be concluded the suitability of carbonized avocado peel to remove basic substances,followed by the acid and the reactive ones:85.83,47.24 and 43.03%for BB41,AB1 and RB5,respectively.Similar results were found by Ho and McKay[64],who achieved much higher adsorption capacity of basic dye(Basic Blue 69)on sawdust in comparison with that of acid dye(Acid Blue 25).This might be explained by the fact that the number of negatively charged sites was boosted by increasing the pH system,favoring the adsorption of the cationic compound due to electrostatic attractions[65].Similarly,removal yields of basic dyes on cellulosic materials would be preferred once these adsorbents became negatively charged when immersed in water[66].Additionally,the different results may be also correlated with the molecular size,indicating congruently that adsorbed amount is high when the molecular volume is small:the adsorbent would act as a molecular sieve,whereby the dye would penetrate the mesopores preferentially to micropores[60].

In this research,the use ofan agro-waste is proposed for the removal ofcontaminants by adsorption as an alternative to conventional activated carbons considering that their use is sometimes restricted in view of the high costs,upgraded by the fact that these materials become exhausted after their use and the regeneration processes might add considerable costs[67,68].With this goal of preparing novel adsorbents from agro-wastes,it could be possible to use either the raw material or its carbonaceous form.However,the applicability of the materials in their original state has been considered to be constrained by their relatively small surface area with inadequate pore size distribution and leaching of organic chemicals in the process stream[69],which results in lower adsorption capacities[30].This was proved here by comparing the removal results presented above with those attained for the raw avocado peel prior to carbonization with similar particle diameter and under conditions at the evaluated central point(dye 30 mg·L-1and solid 10.25 g·L-1):AB1 and BB41 were removed by 30 and 80%(data not shown),yields notably lower than those found for avocado peel carbonized at 900°C(Runs 9-12,Table 4).

Various previous authors also reported promising adsorption results of different dyes by using carbonaceous materials generated by simple carbonization procedures.For instance,Elizalde-González and Hernández-Montoya[32-34]investigated the use of seeds of guava and orange to produce carbonaceous solids at different operational temperatures for their use to remove acid,monoazo and anthraquinone dyes.Dávila-Jiménez et al.[60]proved the feasibility of carbonized mango seeds at 1000°C for the removal of a range of dyes(Acid Blue 80,Acid Blue 324 and Acid Red 1,etc.),attaining adsorption yields much higher(about 100%)than those for the raw material(0-50%).Nonetheless,no previous reports exist on using avocado peel as dyes adsorbent prior to the current research.

4.Conclusions

Dyes removalby adsorption on carbonaceous materialfrom avocado peel is presented as a promising technology thanks to the low cost and wide availability of the precursor besides the straightforward generation,the satisfactory determined characteristics and the proved adsorption capacity of the produced adsorbent.Thinking on the subsequent work,attention should be given to adsorption of dyes from mixtures considering the variety of substances in contaminated waters as well as to the feasibility of the system to treat real effluents from industries,for which results presented here might be considered as a basis for the further development of the technology.

Acknowledgments

The authors are most gratefulfor financialsupport from the Dirección Generalde Investigación y Postgrado(DGIP,Project271459),Universidad Técnica Federico Santa María.

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