Ionic adsorption and characterization of biochar from discarded potatoes

: Studies aimed at mitigating environmental impacts have received attention in recent years. The use of biochar obtained from the pyrolysis is an alternative for the recovery of organic materials that usually are directly discarded in the environment. Biochar has the potential to recover nutrients from wastewater, and in the latter case, the reuse or recycling of biochar as a controlled-release fertilizer could be an option. However, the adsorption characteristics of the ions into different biochars have not been known. The aim of this study was to evaluate the ions adsorption in biochar derived from discarded potatoes, as well as to make physical and chemical characterization of the produced materials. The biochar derived from discarded potatoes was produced by pyrolysis at 450 °C under an N2 atmosphere. In order to define its ionic profile, the adsorption of H n PO 4(3–n)– , Ca 2+ , Mg 2+ , and Al 3+ was tested in pure biochar, as well as adsorption of H n PO 4(3–n)– in biochar after it was doped with Al 3+ . The desorption of these ions was tested by washing with deionized water, and the KCl (1.0 mol·L –1 ) and Mehlich-1 extractors. In absolute values, the adsorption order of the ions was Al 3+ (in acid pH value) = Ca 2+ > Al 3+ > H n PO 4(3–n)– (biochar doped with Al 3+ ) > Mg 2+ > H n PO 4(3–n)– (biochar without doping). The adsorption of H n PO 4(3–n)– was positively influenced by the doping of biochar with Al 3+ . The biochar has preferential characteristics for cations adsorption, and the doping process can aid in adsorption of anions.


INTRODUCTION
Eutrophication of water from poor use of the soil and from agricultural systems, from animal waste, and from sewage and household garbage has become an increasing problem, given its environmental impact on aquatic flora and fauna. Biochar, the product of pyrolysis of plant and animal residues (Lehmann and Joseph 2015), is an adsorbent of various chemical elements, such as nutrients for plants, heavy metals, etc. (Cui et al. 2016;Jung et al. 2015a;Takaya et al. 2016).
Biochar has potential for safe disposal of waste. The increase in recalcitrance and the stability of this waste from pyrolysis of the product is provided by fixed carbon (Agegnehu et al. 2016;Lehmann and Joseph 2015;Maurer et al. 2017;Puga et al. 2015;Smith 2016;Teichmann 2014). In addition, the application of biochar on the soil carries other benefits, such as an increase in the availability of nutrients such as N and P (Sánchez-García et al. 2016), an increase in moisture content and in soil structure (Atkinson et al. 2010), an increase in pH (Agegnehu et al. 2016), and promotion of microbial conditions (Madari et al. 2012).
Considering potato production, the waste and residues in agriculture are huge, particularly due to losses in the field, and also the lack of marketing conditions. In addition, the potatoes without commercial value that are kept in the field after harvest can be a source of pathogens (Reddy 2017;Rossmann et al. 2018). Furthermore, the manufacture of starch https://doi.org/10.1590/1678-4499.20210313

Organic waste used
Tubers of potato (Solanum tuberosum L.), cultivar Agata, were used to produce biochar. First, the samples were washed in running water and maintained in a forced air circulation oven at 70 °C until the material achieved constant weight. After drying, the material was ground in a Wiley mill equipped with a 1.27-mm mesh sieve. The ground potato underwent a prepyrolysis process in a muffle furnace at 150 °C for 4 h to remove excess water. This prepyrolysis material was placed in a tubular oven for production of biochar, at a temperature of 450 °C for 4 h, with a 10 °C per min rise in temperature. An N 2 cylinder was used, with a flow of 100 cm 3 •min -1 and purity of 99.9%.

Adsorption of P, Ca, Mg, and Al
Having obtained the potato biochar, information regarding its ionic profile was sought. To do so, the adsorptions of H n PO 4 (3-n)-, Ca 2+ , Mg 2+ , and Al 3+ (this last substance under two pH conditions: controlled in an acidic range and without control) were tested in pure biochar, as well as adsorption of H n PO 4 (3-n)in biochar doped with Al 3+ . The adsorption tests consisted of addition of separate concentrations of these elements. All analyses were made in triplicate.
For P, the method of determination of remaining P (Alvarez et al. 2000) was applied, which, in soil fertility, refers to a measure of buffering capacity or P capacity factor of soils (Novais and Smyth 1999). They are remaining-P concentrations in solution, obtained after shaking of 60 mg•L -1 of H n PO 4 (3-n)in a 10 mmol·L -1 CaCl 2 solution with the biochar sample at a biochar:solution ratio of 1:10 (w/v) for 5 min at 200 rpm.
For adsorption of Ca 2+ , Mg 2+ , and Al 3+ , the procedures were similar to those adopted for P, but these were at the concentration of 100 mg•L -1 in their chloride forms (CaCl 2 ·2H 2 O, MgCl 2 ·6H 2 O, and AlCl 3 ·6H 2 O) in aqueous solution, without the addition of CaCl 2 . In the case of Al 3+ , the procedure was carried out in two manners, under the supposed need to preserve the pH of the solution acidic to maintain the solubility of the Al3+.
For Al 3+ , in addition to the objective of determining the magnitude of its adsorption, a biochar sample was subjected to what is called doping by this cation so as to develop and/or increase the AEC of the material and make it adsorbent of phosphate. To do so, after contact of the Al 3+ solution with biochar, 0.1 mol·L -1 HCl was added until the pH stabilized in an acidic condition, with a value of around 4.5, in which Al remains in its ionic form (Al 3+ ), since raising pH above 5.2-5.5 causes its complete hydrolysis, with formation of the precipitate aluminum hydroxide (Greenland 2015). After shaking and resting, the biochar, at this stage doped, was filtered and recovered. Under this condition, its remaining P was measured, as was also done beforehand for its nondoped condition, to find how much the AEC of this material increased.
After shaking of each standard solution prepared with biochar, all were left to rest for 16 h, after which they were filtered and washed once with deionized water, using a laboratory wash bottle, to remove any excess that may have remained on the biochar structure or in the filter and which was not truly adsorbed. The concentrations of P, Ca 2+ , Mg 2+ , and Al 3+ were determined in each standard solution, in the extract after filtering, and in the deionized water after washing. The sum of the amount of the element of the extract with the filtrate indicated the amount that was not adsorbed. The difference between the initial amount placed under shaking with the biochar and the sum of the extract and of the filtrate resulted in the amount adsorbed in the biochar in weight/weight.

Desorption of P, Ca, Mg, and Al
To assess the stability of adsorption of elements in the biochar, after the biochar was washed once with deionized water, as described above, successive extractions were performed. For the cations Ca 2+ , Mg 2+ , and Al 3+ , the extractant KCl (1 mol·L -1 ) was used, whereas for H n PO 4 (3-n)-, the extractant of available P of the soil, Mehlich-1, was used. The procedures used were similar to those for determination of their exchangeable and available forms in the soil (Donagema et al. 2011;Tedesco et al. 1995).
Accordingly, after washing with deionized water, the biochar samples with the adsorbed elements, still on the filter paper so as not to lose part of the biochar sample retained on it and not measurable, were transferred to Erlenmeyer flasks and subjected to extractions and determinations of the elements in the filtrate. Further extractions were performed, similar to the first, until the amounts of the elements extracted were no longer detectable.

Control of pH of the solutions
So as to evaluate the behavior of the pH of the solutions and the buffering level of the biochar, the pH was read throughout the process. Before shaking, the pH value of the suspension (initial pH) was measured. After shaking, the suspension was left to rest for 16 h, after which the pH (final pH) was measured. After that, the solution was filtered with the use of fast filtering quantitative filter paper, followed by reading the pH of the filtrate (pH of the extract). The material was washed while still on the filter paper with deionized water, and the pH (pH after washing) was determined.

Total concentrations of the elements in the material
Nitric perchloric digestion of the prepyrolysis potato and of the biochar were carried out to determine the total concentrations of P, Ca, Mg, and Al in this raw material, as well as the influence of pyrolysis on these concentrations.

Characterization of the materials
Samples of the prepyrolysis potato, of the pure biochar, and of the biochars after being treated with H n PO 4 (3-n)-, Ca 2+ , Mg 2+ , and Al 3+ were characterized physically and chemically by means of the techniques of X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and scanning electron microscopy (SEM).
The XRD analyses were made on Shimadzu XRD-6000 equipment, using a graphite crystal as monochromator to select the radiation of the Cu-Kα1 with λ = 1.5406 Å and a step of 0.02° s -1 , with scanning angle (2θ) from 4 to 70°.
The ATR-FTIR analyses were performed in a Jasco spectrometer, model FT/IR-4100. The spectra were obtained in a wavelength range from 4000 to 400 cm -1 , resolution of 4 cm -1 and 256 scans.
For SEM analyses, the samples were held on double-sided thermal conductive adhesive tape and covered with a fine gold film. The morphology of the materials was analyzed using a scanning electron microscope (SEM) Zeiss EVO 50.

Statistical analysis
The pH values and the concentrations of the elements in the solutions, the amounts adsorbed, and the total concentrations of the elements in the pre-and postpyrolysis potato underwent descriptive statistical analysis, in which the measure of mean dispersion adopted was the standard error. Adsorption of H n PO 4 (3-n)-(doped and nondoped biochar) and Al 3+ (with and without correction of pH) were compared by the F test at the level of 1% significance. The SPEED Stat software was used 4 . The results of XRD and ATR-FTIR were plotted on the OriginPro 8.5 software.

RESULTS AND DISCUSSION
Potato biochar exhibits preferential adsorption of the cations (Ca 2+ , Mg 2+ , and Al 3+ ) in relation to H n PO 4 (3-n)- (Table 1), confirming statements in diverse studies that biochar represents, in general, a large anion (Cui et al. 2016;Jing et al. 2015). The much lower retention of H n PO 4 (3-n)-(0.14 mg•g -1 or 0.014 % in w/w) suggests the involvement of physical adsorption of H n PO 4 (3-n)as a consequence of diffusion of this element in micropores and fractures that exist in this material, as cited in the literature (Atkinson et al. 2010;Lehmann et al. 2006;Sizmur et al. 2017). Certainly, a similar mechanism occurred to the cations under study by the effect added to their adsorptions in the electronegative environment that exists in these empty spaces. The adsorption found for Al 3+ with the initial pH value adjusted to 4.5 (Table 1), of 0.90 mg•g -1 , indicates the greater adsorption for this cation, similar to that of Ca 2+ and, finally, Mg 2+ , approximating the classic lyotropic series of sequential priority of cation adsorption: higher energy of adsorption for cations of higher valence, followed by those of the same valence, and, for these, as a priority, by those of greater ionic radius and, consequently, smaller hydrated radius (Novais and Mello 2007), explaining lower adsorption of Mg 2+ . Thus, this result suggests the predominant involvement of ionic and noncovalent bonds as a mechanism of adsorption of these cations in the biochar, which also explains the opposite sequential results regarding the desorption of these same cations (Table 2). The studies on desorption of cations and of H n PO 4 (3-n)- (Table 2) indicate nondesorbed amounts retained, a probable consequence of the chemical and physical retention of Ca 2+, Mg 2+ , and Al 3+ and, in the case of P, probably only of physical retention of P. Higher desorption of Mg 2+ is observed in comparison to the other cations (Table 2), which is due to its greater hydrated radius, and thus lower binding energy. When the pH of the Al 3+ solution was controlled, the KCl extractant was less efficient in extracting this cation from the surface of the biochar. For P, the successive extractions practically did not remove it, reinforcing the idea of diffusion of this element in micropores and fractures that exist in this material and not of chemical adsorption on the surface (Atkinson et al. 2010;Lehmann et al. 2006;Sizmur et al. 2017).
The values of adsorption found for Al 3+ (0.90 mg•g -1 ) ( Table 1) in the biochar doping process are much, much smaller than those found by Novais et al. (2018a): 701.6 mg•g -1 in chicken manure biochar and 758.9 mg•g -1 in sugarcane biomass biochar. The probable reason for this is the difference between the material of origin used for production of the biochars and the procedure adopted for determination of maximum adsorption capacity of Al 3+ (MACAl) by those authors. Regarding the raw material used in biochar production, the literature abundantly shows expressive differences in ionic adsorption among materials as a variable of their origin Yao et al. 2013;Zhang and Gao 2013).

Comparison of the magnitude of adsorption of H n PO 4
(3-n)as a consequence of doping of biochar shows that doping leads to an increase in adsorption of H n PO 4 (3-n)- (Table 3), which corroborates that observed by Novais et al. (2018a). However, there is a big difference in adsorption of H n PO 4 (3-n)between the two biochars doped with Al 3+ , 0.56 and 701.5 mg•g -1 , in this study and in Novais et al. (2018a), respectively. In their study, Novais et al. (2018a) used concentrations of Al 3+ that ranged from 0 to 3000 mg•L -1 , so as to have MACAl by the Langmuir isotherm. The present study used only 100 mg•L -1 . This low concentration used was for the purpose of simply assessing the hypothesis of cationic adsorption prevailing over anionic adsorption. Therefore, in the study of Novais et al. (2018a), the biochar was doped with the maximum amount of Al 3+ obtained by means of the isotherm and, in our study, with a concentration of only 100 mg•L -1 . Whereas in the former study the development of positive charges for adsorption of H n PO 4 (3-n)was maximum, in the latter study, it was comparatively minimum. (3-n)without and with doping with Al 3+ , as well as adsorption of Al 3+ , without and with correction of pH of the solution, in potato biochar.

Adsorption of H n PO 4 (3-n)-(mg•g -1 ) Adsorption of Al 3+ (mg•g -1 )
H n PO 4 (3-n)-(biochar without doping) 0.14 b Al 3+ (pH without correction) 0.76 a H n PO 4 (3-n)-(biochar with doping) 0.56 a Al 3+ (corrected pH = 4.5) 0.90 a Means followed by the same letter in the column do not differ by the F test at the level of 1% probability. Source: Elaborated by the authors.
From the lack of statistical difference between the values found for the concentration of Al 3+ in the extract with pH not corrected and that with pH of 4.5, it can be questioned whether there is the need to stabilize initial pH in the doping process in an acidic range, as suggested by Novais et al. (2018a), and not alkaline, like the pH originally found in the biochar (Table 4). (3-n)-, Ca 2+ , Mg 2+ , and Al 3+ solutions* subjected to shaking in contact with potato biochar.
The significant adsorption of Al 3+ in the biochar, as also observed in other studies (Jung et al. 2015b;Zhang and Gao 2013), indicates that it can be successfully used in doping processes so as to develop its capacity to adsorb anions, and particularly those responsible for water eutrophication, such as phosphate and nitrate (Dodds et al. 2009;Howarth and Marino 2006). Thus, with these anions adsorbed, it can constitute a slow-release fertilizer for agriculture, as well as deepen the concept of reuse or recycling of nutrients for agricultural production (Fang et al. 2015;Sizmur et al. 2017).
The pH values of the solutions throughout the adsorption process are shown in Table 4. It can be seen that biochar is a material with very high basic buffering, just as cited in the literature ). Regardless of the solution prepared and the stage of adsorption, the biochar maintained the pH value of the medium around 9.0. Even in the case in which the pH of the Al 3+ solution was previously corrected to 4.5, after the shaking process, it returned to 9.0.
The total concentrations of P, Ca, Mg, and Al were determined before and after pyrolysis of the tubers (Table 5). The P and Mg concentrations in prepyrolysis are similar to those found by Fernandes et al. (2011), whereas the Ca concentration is much greater. The Al concentration was not detectable. After pyrolysis, the nutrient concentrations increased, and it was also possible to detect Al concentrations (Table 5). These initial concentrations and increases were the consequence of the effect of concentration given the loss of weight of material in the pyrolysis process, as well as of the fertilization profile of the potato crop, especially in regard to the high demand for P (Fernandes et al. 2015).  Note. # = peaks of amylopectin chains; • = peaks of graphite structure; ○ = peak of amorphous structure; * = peaks in reference to the Al of the sample holder. Source: Elaborated by the authors.
Diffraction peaks in the 2θ region from 15° to 25° (#) (Fig. 1a) are related to the structural organization of amylopectin chains (Gallant et al. 1997). This is part of starch composition, which is the form of sugar reserves in potato tubers (Zhao et al. 2018). The wide peak (2θ = 15°), the strong peak (2θ = 17°), and the double peak (2θ = 22-24°) are patterns of type B starch (Hizukuri et al. 2006). The basal spacing (distance between the amylopectin chains) calculated by Bragg's Law (nλ = 2d_ hkl senθ) is 3.20 Å. The diffraction peaks 2θ = 28°, 30-32°, and 42° (•) (Fig. 1b) are characteristics of the graphite structure (Stanjek and Häusler 2004;Sun et al. 2008). These are repeated in the biochar after contact with the ions in a less intense way (Figs. 1c-h). The wide flattened peak in the 2θ position between 5° and 30° characterizes the amorphous structure of the samples (º) (Figs. 1b-h). Amorphous materials do not present a diffraction pattern of narrow peaks as in crystalline materials, but rather wide peaks, due to the irregular arrangement of the atoms (Chauhan and Chauhan 2014). The peaks 2θ = 44° and 65°, demarcated by *, are in reference to the Al of the sample holder.
The spectra of ATR-FTIR for PP, PB, PB-P, PB-Ca, PB-Mg, PB-Al, PB-Al 4.5 , and PB-Al+P are shown in Fig. 2. The band from 3600 to 3000 cm -1 (Fig. 2a) is attributed to stretching of the O-H bonds of the hydroxyl groups and also of H 2 O molecules (Castro et al. 2018a;2020a;2020b). These groups are present in cellulose, hemicellulose, and lignin (Zugernmaier 2008). The band of 1700 cm -1 , present in all samples (Figs. 2a-h), refers to the double C=O bonds, of vibration of aromatic groups (Ferreira et al. 2015). After calcination of the material, this band decreases in intensity and undergoes small variations of displacement. The intense band at 1000 cm -1 (Fig. 2a) refers to the stretching of C-O and the vibration of C-H of the cellulose present in the potato tuber (Ferreira et al. 2015). The representative images from SEM are shown in Fig. 3. A complex, porous structure of material in pre-and postpyrolysis is observed (Figs. 3a-d). The addition of the anion (H n PO 4 (3-n)-) makes the structure a little porous (Figs. 3e and f). The addition of the cations (Ca 2+ , Mg 2+ , and Al 3+ ) to the biochar makes its structure more porous (Figs. 3g, i, k, m and o). The micropores present have a diameter of 20 nm, and after the addition of cations, they undergo a slight increase.
The results obtained are promising, in spite of the reduced values of adsorption of the ions studied. Biochar can be a viable alternative for getting around environmental and agronomic problems related to losses of nutrients in the system. Deeper studies are needed in relation to the diverse raw materials that can be used, as well as other possible processes of doping that may make optimization of the biochars feasible.