Properties of the investigated sorbents

The basic characteristics of the BC and HA are summarized in Tables 2 and 3, respectively. Studied biochar was highly alkaline with the pH value of 9.9 which might be influenced by the separation of alkali salts from the organic matrix in the input material. Relatively high ash content (28.08% w/dw) may be attributed to the combustion of cellulose, hemicellulose, and removal of the volatile material at the pyrolysis temperature. The investigated wheat-straw biochar had a high specific surface area of 237.39 m2 g−1.

Calculated H/C and O/C molar ratios are the indicators of BC aromaticity and polarity, respectively. It is assumed that BCs produced at the temperature higher than 400 °C should be characterized by the H/C ratio lower than 0.5 and decrease with the raising pyrolysis temperature below 0.3, which is an indicator of highly aromatic ring systems (Cely et al. 2014). In the case of the investigated BC, produced at 550 °C, H/C ratio falls within the 0.3–0.5 range which might indicate a decreased fraction of original wheat residues. Obtained BC molar ratios (Table 2) emphasize the presence of aromatic structural features and (similar to other biochars produced at that temperature; Mayakaduwa et al. 2016) reduced content of O-containing polar functional groups on BC surface (low molar O/C ratio and polarity index). Also Monterumici et al. (2015) observed that biochar samples obtained at high temperatures (650 °C) were reach of aromatic C. Moreover, they found differences in aromatic ring condensation between biochars derived from different sources (Monterumici et al. 2015). BC hydrophobicity was measured in the water drop penetration test. WDPT result (time necessary for the drop of water to be soaked into the sorbent) was in the range of 120–180 s. It places the investigated BC in the moderately hydrophobic sorbents group. It is consistent with the literature according to which the heat treatment of the chars at temperatures between 400 and 500 °C increases BC wettability through the removal of aliphatic domains from its surface (Gray et al. 2014).

The investigation of the acidic functional groups in the HA sample revealed the presence of total acidity of 5.46 meq g−1 (Table 3). Most of this acidity (69%) is due to phenolic groups presence (3.76 meq g−1), while the carboxylic groups were responsible for the remaining 31% (1.70 meq g−1).

Elemental composition of humic acid and biochar (expressed in atomic percent) is presented in Table 4. According to the van Krevelen diagram, the investigated HA and BC samples can be classified into the same region of lignin-like compounds (H/C = 0.7–1.5, O/C = 0.1–0.67), based on their H/C and O/C atomic ratios (Rosell et al. 1989; Aranda and Oyonarte 2006; Ohno et al. 2010; Li et al. 2013b; Lu et al. 2015; Barančíková et al. 2018). This BC characteristics is in accordance with thermal resilience of lignin, as opposed to hemicellulose and cellulose that decompose at temperatures lower than 400 °C (220 to 315 °C and 315 to 400 °C, respectively) (Yang et al. 2017). Nevertheless, the pyrolysis temperature (550 °C) was not sufficiently high to place BC into the condensed aromatic ring structure (CAS) region (H/C = 0.2–0.7; O/C = 0–0.67). Obtained HA atomic ratios were typical for the humic acids extracted from topsoil horizons of Gleyic Pheaozems located in the Wroclaw district (Łabaz 2010). They are higher than in BC sample which might suggest the bigger number of oxygen-containing functional groups in HA. It is supported by the higher polarity index ((O + N)/C) and over two times higher internal oxidation degree (ω) of HA than the one calculated for BC (Table 4).

FTIR analysis revealed significant differences between BC and HA in the types and relative abundances of functional group peaks observed on the spectra (Fig. 1). As it can be seen, HA is more abundant in hydroxyl groups (vibrational band from –OH groups at 3409 cm−1) in comparison with BC. The latter exhibits only a trace band at this region corresponding probably to the stretching vibrations of –OH group of bonded water (Ahmad et al. 2012). The bands at 2922 and 2854 cm−1 observed in the HA spectrum were assigned to asymmetrical and symmetrical stretching of –CH 2 groups, respectively (Giovanela et al. 2004), whereas absorption band at 1384 cm−1 is assigned to COO or C–H bending of CH 2 group in the aliphatic chain (Banach-Szott et al. 2014). Since those three bands are not present in the spectrum of BC sample, it might suggest the absence of labile aliphatic groups on the char surface. The sharp peak present on HA spectrum at 1619 cm−1 and a very weak one on BC spectrum at about 1600 cm−1 are ascribed as stretching vibration of C=C or C=O in the aromatic ring (Fang et al. 2015). Additional weak band at 1714 cm−1, present on the HA spectrum corresponds to carboxyl stretching absorption peak (Giovanela et al. 2004). Intense, sharp band at 1035 and at 1029 cm−1 observed on BC and HA spectrum, respectively, can be assigned to C–O stretching vibrations of carboxylic groups (Trigo et al. 2014) or of alcohols on ligno-cellulosic polymers (Li et al. 2013a; Jamroz et al. 2014). BC in comparison with HA tend to contain less oxygen- and hydrogen-containing functional groups, while the presence of these groups in the HA sample (namely phenolic and carboxylic) was additionally confirmed by the titration procedure (Table 3). Thus, according to the experimental results from FTIR and elemental analysis (Table 4) BC seems to be more aromatic and less polar than the investigated HA. These findings are in agreement with the studies of Chun who discovered that under high temperature (500–700 °C) the biochar derived from wheat is well carbonized and has a relatively high surface area and low oxygen content (Chun et al. 2004; Yu et al. 2009). Also, Ahmad et al. (2013) observed that decrease of atomic ratios (H/C and O/C) in the case of high-temperature chars was attributed to the removal of H- and O-containing functional groups resulting in the formation of high aromaticity and low polarity biochars (Ahmad et al. 2013).

Fig. 1 FTIR spectra of investigated biochar (BC) and humic acid (HA) samples Full size image

Sorption of pesticides on biochar and humic acid

Percentage adsorption and desorption yields obtained in the sorption experiment for the investigated pesticides are presented in Table 5 and Fig. 2. The adsorption affinity of the humic acid for the pesticides varied greatly and decreased in the order 2,4-D > metolachlor > MCPA > carbofuran > carbaryl. Obtained data clearly show that humic acids sorb investigated phenoxyacetic acids and metolachlor more preferentially than the carbamates. What is interesting about the data in Table 5 is that the wheat-straw biochar reveals different affinity towards studied pesticides than HA. BC attracts carbamate pesticides to the highest extent (98.9 and 92.1% of carbofuran and carbaryl sorbed, respectively). Metolachlor sorption by BC (70.2%) is comparable with the one observed for HA (72.9%), whereas phenoxyacetic acids is a group of pesticides that are sorbed to the least extent among investigated compounds (sorption of approximately 40% of the dose introduced to BC sample).

Table 5 Sorption and desorption magnitude of the investigated pesticides, calculated for both sorbents (HA and BC) Full size table

Fig. 2 Comparison of the effect of different sorbent (HA and BC) on: a adsorption and b desorption of 2,4-D, MCPA, metolachlor, carbofuran and carbaryl. Desorption is expressed as a percentage of dose that desorbed from HA or BC samples. Error bars represent standard deviation of triplicate samples Full size image

Sorption of phenoxyacetic acids on HA and BC

Sorption of phenoxyacetic acids was much higher in the case of the humic acid (74.6 and 67.9% for 2,4-D and MCPA, respectively) than the studied wheat-straw biochar (Table 5). Investigated HA is abundant in oxygen containing functional groups such as carboxylic or phenolic (Fig. 1; Table 3) which may serve as a sorption site for binding of organic molecules. It may be the reason why the probability of electrostatic interactions between 2,4-D or MCPA and polar moieties on the HA surface increases, which results in higher adsorption of phenoxyacetic acids on HA. Another factor influencing their sorption might be a very low experimental pH (2.9) of the humic acid suspensions. It has been proven before that adsorption of ionic pesticides, particularly phenoxyacetic acids, may occur via formation of negative charge-assisted H-bonds in the presence of carboxylic or similar in structure groups on the sorbent surface, particularly at pH values below their pK a (Khan 1973; Senesi et al. 1986; Li et al. 2013b). Thus, the relatively high 2,4-D and MCPA uptake by HA might be attributed to the specific interaction with active HA groups (Spurlock and Biggar 1994; Chiou 2002).

The percentage adsorption values for biochar sorbent were 42.3 and 37.6% for 2,4-D and MCPA, respectively. This relatively low sorption extent may be partly ascribed to electrostatic attraction between herbicides and small number of polar functional groups on BC surface or to a nonspecific interaction with the sorbent. Increased pH in samples containing BC (pH of 9.6 after 24 h of equilibration) may have resulted in their decreased sorption (Fig. 2) as in the experimental conditions (mixtures containing BC) both 2,4-D and MCPA occur predominantly in anionic form. Hence, the negative charge of both BC and phenoxyacetic acids may increase, which leads to enhanced repulsion and thus the weaker adsorption in comparison with HA (Table 5). It is in agreement with some findings according to which an elevated pH of biochar may be negatively correlated with adsorption of some ionic agrochemicals, i.e., glyphosate (Herath et al. 2016) and MCPA (Trigo et al. 2016a). On the contrary, Tatarkova and Cabrera (Cabrera et al. 2011; Tatarková et al. 2013) observed an increased sorption of MCPA in the presence of biochar. Nevertheless, the effect was observed for soil amended with various biochars and was attributed to the general increase in carbon content and distinct surface affinities of the studied biochar for the herbicide.

Sorption of metolachlor on HA and BC

Both sorbents attracted metolachlor comparably strong (72.9 and 70.2% of the applied pesticide dose for humic acid and biochar, respectively). According to the literature, metolachlor adsorption mechanism to humic acids is based on two types of interactions that is ionic and hydrogen bonding (Senesi 1993). The first one involves the electrostatic interaction of cationic amine group with carboxylic or phenolic groups at the surface of humic acid. Second type of interaction, namely hydrogen bonding is also a path of metolachlor interaction with HA, predominantly at acidic conditions (Senesi 1993). The probability of the described interaction is even higher when one takes into account the fact that based on the FTIR spectrum of HA (Fig. 1) and the results of functional groups determination (Table 3), it was clearly shown that the studied humic acid is relatively abundant in polar regions.

Metolachlor sorption on BC cannot be explained by the same mechanism as above mentioned due to the moderately hydrophobic nature of BC and its lower polarity in comparison with HA. It is also connected with the chemical properties of metolachlor, which exhibits relatively high value of logP (3.4 at pH 7, Table 1) making it less polar than the other pesticides under study. According to the rule “like dissolves like,” congruent properties of both sorbent and the sorbate enables the hydrophobic effect to occur in the case of metolachlor sorption to biochar. Similar adsorption mechanism was proposed for phenylurea herbicide diuron (Fontecha-Cámara et al. 2007), which logP equals to 2.87 and is similar to the octanol-water adsorption coefficient of metolachlor. This pesticide property may also at least partially explain much lower sorption of the studied phenoxyacetic acids on BC, due to their high, negative logP values (Table 1).

Sorption of carbamates on HA and BC

Humic acid does not sorb studied carbamates as strong as the other pesticide classes under investigation. Approximately 10.2 and 35.4% of the introduced carbaryl and carbofuran dose, respectively, were efficiently sorbed by HA. This limited magnitude of sorption may be attributed to hydrophobic interactions between carbamates that can occur with nonpolar regions of HA moieties. But more importantly, recently, we have proven that the studied carbamates are capable to react with the indigenous radicals of humic acids, decreasing the semiquinone radical spin concentration (Ćwielag-Piasecka et al. 2017). According to that finding, structural differences between carbaryl and carbofuran (presence of the furanyl moiety in carbofuran) were responsible for the higher reactivity of the latter in radical reactions. This finding also supports results of the sorption experiment presented here, according to which carbofuran is 3.5 times more extensively sorbed than carbaryl (Table 5).

On the contrary, calculated sorption of the carbamates on biochar was nearly total (92.1 and 98.9% of the introduced dose sorbed for carbaryl and carbofuran, respectively). It is in agreement with other studies which showed that biochar amendment to soil enhances the sorption of carbaryl (Ren et al. 2016) and carbofuran (Mayakaduwa et al. 2017). Hydrophobic effect together with van der Waals interactions may explain sorption of carbamates to BC due to their nonionic nature and relatively high logP values (Table 1). Recently, it was also postulated that the ester functional groups of carbaryl (of electron-withdrawing nature), makes the associated aromatic ring an electron acceptor thus it is expected to interact with the aromatic carbons on the BC surface through π-π interactions (Ren et al. 2016). Nevertheless, one has to bear in mind that carbamates are very prone to chemical hydrolysis, which is enhanced at pH above 7 (Ren et al. 2016). Since the pH in the experimental conditions was estimated to be even higher (pH 9.6 in the sample containing 100 mg of BC and 10 mL of CaCl 2 ), there are two scenarios of BC influence on pesticides fate that need to be taken into account when considering the extent of carbamates sorption. First one assumes enhance in hydrolysis by the catalytic effect of pH, whereas the second—reduction of carbamates hydrolysis by enhanced sorption.

In the studied BC mixtures, the pH value of suspensions containing BC was very alkaline. To confirm the effect of elevated pH on carbamates degradation, their hydrolysis (in concentrations used in the sorption experiment) was examined in a background solution with pH adjusted to 9.6. After 24 h, both pesticides were decomposed to the extent of 76.4 and 84.3% in the case of carbaryl and carbofuran, respectively. It is in good agreement with other studies where the decomposition of one of the carbamates, namely carbaryl at pH of 9.1 and in the presence of 500 mg of biochar pyrolized at 700 °C was equal to 86.5% (Zhang et al. 2013b). Indeed, biochars produced at higher temperatures (about 700 °C) may enhance the chemical hydrolysis of carbaryl in soil due to their strong liming effect. However, in the presence of biochar, hydrolysis of the carbamate may also be lowered due to the reduced availability of sorbed pesticides for the chemical reaction. Based on that, it can be assumed that both hydrolysis and sorption contribute to the obtained high value of carbamates uptake on BC in our study. Hydrolysis process presumably prevails in the suspensions of carbamates with the studied BC. Taking that into account, we can estimate the effective sorption magnitude of the carbamates to be much lower (approximately 15.7 and 14.6% for carbaryl and carbofuran, respectively) than the one calculated based only on the sorption experiment results (Table 5).

Desorption of pesticides under study from HA and BC

Although the adsorption capability of investigated humic acid or biochar is considered to be a key factor that controls the environmental fate of pesticides, desorption of pesticides from the sorbents should be carefully investigated due to its association with the bioavailability and efficacy of pesticides (Khorram et al. 2016). Generally, pesticides desorb more readily from low temperature chars (Li et al. 2013a) while high temperature ones exhibit a substantial desorption hysteresis (Ahmad et al. 2012). Results of our study (Table 5) suggest that the wheat-straw biochar under investigation exhibits no significant desorptive properties. Studied biochar suspensions with adsorbed pesticides were subjected to desorption procedure four times and each time there was no pesticide release observed except for the first desorption cycle where 2,4-D was quantitatively estimated in the solution (Table 5). However, a correction should be made for the studied carbamates, where hydrolysis may contribute to the vastly reduced concentration of the insecticides after desorption. The inhibitory effect has been observed in other studies of polar organic compounds, suggesting that various pyrolytic carbon materials significantly reduces pesticide bioavailability and hence risk of its leaching (Kookana 2010).

Noticeable desorption occurred only in the case of 2,4-D bound to HA (over 50%), indicating its weak interaction with humic acid. Other studied compounds were released from HA within the range of 4.4–10.8% of the sorbed pesticide dose. These findings are in agreement with the recent work of Ozbay et al. (2017), who attributed the rapid 2,4-D desorption to soft carbon fraction of SOM (humic/fluvic acid and lipids) (Ozbay et al. 2017).