Abstract
PurposeThe aim of the research was to compare the effect of two types of organic sorbents—humic acid (HA) and biochar (BC)—in sorption-desorption processes of different polar pesticides, which residues are commonly present in arable soils and are potentially harmful for the environment. It also aims to advance the understanding of behavior of both ionizable and nonionizable pesticides in the presence of BC and HA in soils.Materials and methodsThree different classes of pesticides were investigated: carbamates (carbaryl and carbofuran), phenoxyacetic acids (2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA)), and aniline derivatives (metolachlor). Investigated humic acid was extracted by Shnitzer’s method from topsoil horizon of arable Gleyic Phaeozem. Biochar was produced from wheat straw in gasification process at 550 °C, remaining 30 s in the reactor. To obtain the experimental goal structural properties of both sorbents were determined and sorption-desorption experiments conducted. To the investigated organic matter samples (HA or BC), 10 or 15 mg L−1 pesticide solutions in 10 mM CaCl2 were added and the mixtures were shaken for 24 h. Afterwards, the samples were centrifuged and supernatants analyzed by LC-MS/MS for the pesticide content. Analogous experiment was performed for desorption studies (samples refilled with 10 mM CaCl2).Results and discussionHumic acids exhibited strong affinity for the ionic substances, for which high-percentage uptake (74.6 and 67.9% initial dose of 2,4-D and MCPA, respectively) was obtained. Retention of nonionic carbamates on HA was much weaker (35.4% of carbofuran and 10.2% of carbaryl sorbed). Sorption of carbamates to BC was significantly reduced (76.4–84.3%) by the alkaline hydrolysis. Metolachlor was bound comparably strong both by HA (72.9%) and BC (70.2%), although different mechanisms governed its sorption. Noticeable desorption occurred only in the case of 2,4-D bound to HA (over 50%), whereas other studied compounds were released from HA within the range of 4.4–10.8% of the dose sorbed. Oppositely to HA, desorption of all studied pesticides from BC was completely inhibited, except for 2,4-D (3.7% desorbed).ConclusionsInvestigated humic acid has high affinity to polar, ionic pesticides of high water solubility, which are sorbed via specific interactions with HA functional groups. Studied biochar, due to its moderately hydrophobic character, preferentially attracts nonionic pesticides of relatively high logP values and low water solubility. Hydrophobic bonding is postulated as a main mechanism of their attraction to BC. Besides sorbent structural properties, pH is the main factor governing sorption equilibria in the studied mixtures.
Highlights
It has often been reported that soil organic matter (SOM) is actively involved in the binding of pesticides (Bollag et al 1992; Pignatello 1998; André et al 2005; Barriuso et al 2011; Ćwielag-Piasecka et al 2017)
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
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)
Summary
It has often been reported that soil organic matter (SOM) is actively involved in the binding of pesticides (Bollag et al 1992; Pignatello 1998; André et al 2005; Barriuso et al 2011; Ćwielag-Piasecka et al 2017). The carboxylic and phenolic hydroxyl group content, the organic free radical concentration, and the aromaticity and humification degrees appear to be the most important chemical properties of HAs affecting adsorption of many organic xenobiotics in soil, including pesticides (Jezierski et al 2000; Milori et al 2002; Gennari et al 2007; Witwicki et al 2015; Klimkowicz-Pawlas et al 2017; Schellekens et al 2017). Many of these mechanisms depend on molecular properties of pesticides. Previous research has established that the highly aliphatic, low humified HAs from organic amendments tend to bind pesticides by hydrophobic bonding, whereas the well-humified soil HAs would prefer chemical binding forms (Senesi et al 2001)
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