Role of Alumina and Montmorillonite in Changing the Sorption of Herbicides to Biochars Jianfa Li,* Saijun Li, Huaping Dong, Shengshuang Yang, Yimin Li,* and Jiaxing Zhong Department of Chemistry, Shaoxing University, Zhejiang 312000, China S Supporting Information *
ABSTRACT: The inﬂuence of biochars on the fate of herbicides in soil depends mostly on environmental factors among which the role of soil minerals is not clear. Two wood-derived biochars produced at 400 °C (BC400) and 600 °C (BC600) were treated with alumina and montmorillonite to investigate their interaction with biochars and the inﬂuence of herbicide sorption. Both minerals exhibited a pore-expanding eﬀect that was likely relative to the removal of authigenic organic matter away from the biochars’ surface. Alumina brought more remarkable pore expansion by doubling the surface area of the BC400 biochar and the mesopore area of the BC600 biochar. Consequently, more adsorption sites were accessible for herbicide molecules, which resulted in higher sorption of herbicides (acetochlor and metribuzin) to the mineral-treated biochars than to the untreated biochars. The results are useful for understanding the change of surface and sorption properties of biochars with soil applications. KEYWORDS: biochar, soil mineral, pesticide, sorption, porosity
INTRODUCTION Biochars, when used for soil improvement, will inﬂuence the environmental fate of soil-applied pesticides, because most biochars are reported to be good sorbents of pesticides.1−4 In most cases, the sorption of pesticides by biochar-amended soil has been enhanced with an increasing biochar dose,5−8 and the enhanced sorption will extend the retention of herbicides in topsoil, control their release, and reduce their leaching and potential pollution to groundwater.9−12 However, the sorption properties of biochars after their application in soil is mostly dependent on environmental factors (e.g., soil constitution, organic matter, pH, and coexisting ions). Biochars may interact with soil constituents and organisms and undergo physical, chemical, and/or biological processes (so-called “aging”), which results in a change in the compositional and surface properties of biochars.13−15 For example, the adsorption of dissolved organic matter (DOM) on the surface of crop residue burns reduced the sorption of diuron,16 and this reduction has been attributed to the obstructed pores of black carbon by DOM.17 Biochars are the residues of incomplete biomass combustion, similar to black carbon, and have also been reported to adsorb natural organic matter,18 which likely acts as pore-blocking agents or competitive adsorbates that restrict the adsorption space available to other organic compounds.19 In contrast to numerous reports on the inﬂuence of natural organic matter on the sorption properties of biochars, much less attention has been paid to the eﬀect of soil minerals, although they are major soil constituents. Larsbo et al.11 reported recently that the eﬀect of biochar amendment on sorption and leaching of pesticides was relative to the soil texture, and the formation of biochar−mineral complexes has been observed by Lin et al.20 and Qian and Chen.21 However, the inﬂuence of this complexation on the properties of biochars is not clear yet. In addition, some minerals, including clays and oxides, are important sorbents of organic matter22 and pesticides.23 Therefore, it is necessary to examine the role of © 2015 American Chemical Society
soil minerals when evaluating the environmental fate of pesticides in biochar-amended soil. In this study, we have prepared two wood biochars at 400 and 600 °C and mixed them with two typical soil minerals (aluminum oxide and montmorillonite) in an aqueous suspension. Next, the biochars were separated out from the suspension to investigate possible changes to their compositional and surface properties, as well as their ability to adsorb two common soil-applied herbicides.
MATERIALS AND METHODS
Materials. Biochars were prepared via the thermal treatment of pine wood (Pinus radiata) shavings under oxygen-limited conditions for 4 h in a muﬄe furnace, according to previous methods.12,24 Brieﬂy, the shavings collected from a local furniture manufacturer were pretreated by a procedure of rinsing, drying, and cutting into ﬂakes of 1 cm × 1 cm in size. The wood ﬂakes were added in a ceramic pot ﬁtted with a lid, and then the pot was placed in a muﬄe furnace for pyrolysis at 400 and 600 °C, respectively. The pyrolyzed solid residues were soaked in deionized pure water (18.2 MΩ cm, with a ratio of 100 mL per gram of biochar) for 24 h to remove water-soluble matter, ﬁltered, and then dried at 70 °C. The products obtained at the thermal treatment temperatures of 400 and 600 °C are referred to hereafter as BC400 and BC600, respectively. α-Alumina (Sigma-Aldrich, 99.9% purity), a typical aluminum oxide, was used as a simulant of soil oxides according to previous studies.22,25 Calcium-saturated montmorillonite (hereafter referred to as Ca-mont) purchased from Inner Mongolia, China, was used as a representative of clay minerals in soil and is chemically composed of SiO2 (61.2%), Al2O3 (17.6%), CaO (2.83%), MgO (3.24%), Fe2O3 (4.29%), and other trace minerals. Two technical grade herbicides, metribuzin (99.0% purity) and acetochlor (96.2% purity), were kindly supplied by Hangzhou Qingfeng Agrochemicals Co. Ltd., China. Both herbicides are widely used in Received: Revised: Accepted: Published: 5740
April 1, 2015 May 28, 2015 June 2, 2015 June 2, 2015 DOI: 10.1021/acs.jafc.5b01654 J. Agric. Food Chem. 2015, 63, 5740−5746
Journal of Agricultural and Food Chemistry
method. The pore size distribution, average pore diameter (PD), cumulative surface area, and cumulative volume of the pores between diameters of 17 and 500 Å (SA17−500 Å and PV17−500 Å) were established by analyzing the N2 adsorption data using the Barrett− Joyner−Halenda (BJH) method. Infrared spectra (IR) were recorded in the 4000−400 cm−1 region on a Nexus FT-IR spectrophotometer (Nicolet) using a KBr pellet. A JSM-6360LV scanning electron microscope (SEM) (JEOL) equipped with an X-act energy-dispersive X-ray spectrometer (EDX) (Oxford) was used for the morphological survey and elemental identiﬁcation of the surface of the biochars. Sorption Experiments. The sorption isotherms of two herbicides on the biochar samples were determined by batch equilibration of the solid biochars (0.10 g for sorption of acetochlor and 0.20 g for sorption of metribuzin) in 25 mL of aqueous solutions of various initial herbicide concentrations (C0, μmol L−1), which ranged from 100 to 700 μmol L−1 for acetochlor (the saturated concentration Cs = 827 μmol L−1 (25 °C)) and from 700 to 4200 μmol L−1 for metribuzin (Cs = 5600 μmol L−1 (20 °C)), respectively. For comparison, the sorption of herbicides to the two minerals was carried at the same range of initial herbicide concentrations, but with diﬀerent doses of solid minerals (0.10 and 0.20 g for sorption of acetochlor to alumina and Ca-mont and 2.0 g for sorption of metribuzin to alumina, respectively). The sorption experiment was carried out in a thermostatic shaker bath at 25 ± 0.1 °C for 24 h according to a previous method,24 and the preliminary kinetic studies (Figure S2 in the Supporting Information) indicated that the sorption equilibrium had been reached before 24 h. The analysis of the supernatant solution was conducted in an LC-20A HPLC system (Shimadzu) equipped with an ultraviolet detector. The herbicides, acetochlor and metribuzin, were analyzed at 225 and 228 nm, respectively, with a detection limit of 0.2 μmol L−1 for both herbicides. The amount adsorbed (Qe, μmol g−1) was calculated from the diﬀerence in concentration between the initial (C0, μmol L−1) and the equilibrium (Ce, μmol L−1) solutions. Blanks without herbicide and duplicates of each sorption point were used in every experiment series.
agricultural soil, and their chemical structures and physical properties are listed in Table 1.
Table 1. Chemical Structures and Physical Properties of Herbicides
Data from http://www.guidechem.com.
Treatment of Biochars with Soil Minerals. The biochars obtained after thermal treatment (at 400 and 600 °C) were treated in an aqueous suspension of alumina or Ca-mont at a ratio of 10 g of mineral per gram of biochar sample. First, 1 g of biochar sample was mixed with mineral powder and then soaked in 100 mL of deionized pure water. The suspension was mixed with gentle stirring at room temperature for 24 h and placed in a thermostatic bath at 25 ± 0.1 °C with shaking for an additional 6 days at a rotation speed of 125 rpm. The pH during the treatment was monitored, and ﬁnal pH values in the suspension close to neutral (7.0−7.8) were observed. The biochar pieces were separated by ﬁltration through a sieve with a pore diameter of 0.15 mm, and the pieces caught on the sieve were washed with deionized pure water to remove the surface-adhered minerals. Finally, the treated biochar products were oven-dried at 70 °C and are referred to hereafter as Al-BC400, Al-BC600, Mo-BC400, and Mo-BC600, according to the fresh biochars (BC400 or BC600) that were treated with either alumina (Al-) or Ca-mont (Mo-). All of the biochar samples were ground to Al−OH2+ groups,25 which probably interact with those oxygen-containing organic matter by the reaction
characteristics of the treated biochars may be corrected by deducting the fractions that should be attributed to the residual minerals. We observe from the corrected data (as shown in parentheses) that the micropore area (SAmicro) of the BC400 biochar was enhanced by 2.4 times after treatment with alumina, and its mesopore area (SA17−500 Å) was doubled (p < 0.05). In our treatment of the biochars with alumina, neither thermal nor chemical processes were involved, so the porous structure would have been produced during the pyrolysis of the wood biomass. During the cooling process after thermal treatment, some micropores should be blocked by condensed noncarbonized organic matter, the byproducts of wood pyrolysis at relatively low temperature.26−28 Most likely, the alumina removed some of these organic blocks from the biochar pores during the mineral treatment. However, only a slight increase in the micropore area (SAmicro) was observed after alumina treatment for the higher temperature biochar (BC600 vs Al-BC600) (p < 0.10), and the increased surface area was mainly attributed to the mesopores, as suggested in the corrected data in parentheses in Table 3 and pore size distribution shown in Figure 2. In contrast to the
Figure 2. BJH pore size distribution (17−3000 Å diameter) of various biochars.
biochar obtained at lower temperature (400 °C), the microporous structures were well developed during pyrolysis at the higher temperature (600 °C), and the alumina further expanded some micropores into mesopores by digging the condensed organic matter out from the micropores. The decrease of micropore area after pore expansion should be compensated by the new opening of some blocked micropores by alumna treatment, resulting in the lesser increase of SAmicro for the higher temperature biochar. According to the pore size distribution shown in Figure 2 and Figure S6 in the Supporting Information, the fraction of macropores (>500 Å) for the alumina-treated biochars (Al-BC400 and Al-BC600) should be attributed to the residual alumina. In conclusion, the alumina expanded the pores, and the contribution of mesopores (17− 500 Å) to the total pore volume and pore area was increased for the higher temperature biochar (BC600), whereas both micropores and mesopores contributed to the enlarged pore volume and pore area for the lower temperature biochar (BC400). The pore-expanding eﬀect was also observed in the claytreated biochars (Mo-BC400 and Mo-BC600). For the lower temperature (BC400) biochar, expansion of the surface area by
>Al−OH 2+ + HO−R → >Al−OH 2−O−R + H+
The release of hydrogen protons is conﬁrmed by the falling pH during the treatment of biochars with alumina and Ca-mont, speciﬁcally in the ﬁrst 3 days (Figure S8 in the Supporting Information). The latter mineral may possibly interact with biochars in a similar way as alumina, as surface bound >Al− OH2+ groups might also exist on the clay if its high content of aluminum oxide were considered. The pore-expanding eﬀect of soil minerals on the biochars may explain the change in surface properties of biochars with soil applications, particularly the increased surface area during aging in soil.35,36 Trigo et al.35 suggested that the increase resulted from the elimination of the organic ﬁlm from the biochar surface, which is in good agreement with our research results. 5743
DOI: 10.1021/acs.jafc.5b01654 J. Agric. Food Chem. 2015, 63, 5740−5746
Journal of Agricultural and Food Chemistry
Figure 3. Sorption isotherms of acetochlor to various biochars, alumina, and clay at 25 °C.
Table 4. Fitting Results of the Sorption Data with the Combined Adsorption/Partition Model biochar/herbicide a
BC400/Atl Al-BC400/Atl Mo-BC400/Atl BC600/Atl Al-BC600/Atl Mo-BC600/Atl alumina/Atl Ca-mont/Atl BC400/Mtba Al-BC400/Mtb BC600/Mtb Al-BC600/Mtb alumina/Mtb
QA (μmol g−1) 26.8 75.2 26.0 43.7 114 52.9 46.5 20.1 63.2 153 90.8 184 3.70
(81.8)b (27.3) (130) (57.1)
QA/Qe at Ce/Cs = 0.3 (%)
QA/SABET (μmol m−2)
QA/SA17−500 Å (μmol m−2)
0.0399 0.0273 0.0286 0.0380 0.0262 0.0306 0.0282 0.0230 0.0165 0.0196 0.0120 0.0233 0.00165
0.994 0.985 0.996 0.979 0.995 0.973 0.972 0.994 0.992 0.974 0.982 0.989 0.998
73.0 91.7 78.6 82.3 94.6 87.5
0.221 (0.299) (0.193) 0.102 (0.246) (0.124) 0.722 0.364 0.522 (0.684) 0.211 (0.426) 0.0575
0.924 (1.36) (0.914) 0.448 (0.687) (0.470) 1.08 0.625 2.18 (3.12) 0.930 (1.19) 0.0862
69.5 82.3 81.8 82.4
Atl and Mtb refer to acetochlor and metribuzin, respectively. bData in parentheses are corrected values by deducting the contribution of residual minerals in the treated biochars.
Changed Sorption of Herbicides to Mineral-Treated Biochars. Alumina-treated biochars (Al-BC600 and AlBC400) exhibit an increase in sorption for the herbicide acetochlor as shown in Figure 3. For the same equilibrium concentration of the acetochlor solution, the amount of herbicide that adsorbed on both alumina-treated biochars was 2 times higher than on the untreated biochars (BC600 and BC400). In contrast, sorption of the acetochlor in the claytreated biochar (Mo-BC600) was slightly increased, and the sorption even decreased for the clay-treated lower temperature biochar (Mo-BC400). To understand why there was a diﬀerence, the sorption data for various biochars were analyzed with the combined adsorption and partition model (eq 2)2,37 in which QA represents the amount of surface adsorption and KP is the partition coeﬃcient. On the basis of the isotherm shape shown in Figure 3, a linear regression was conducted at a concentration of Ce/Cs > 0.1, and the ﬁtting results with correlation coeﬃcients R > 0.97 are shown in Table 4. Thus, the relative contribution of surface adsorption (QA/Qe, %) to the total sorption at each equilibrium concentration was calculated, and QA/Qe (%) values at Ce/Cs = 0.3 (a middle concentration) for various biochars are shown in Table 4. Q total = Q e = Q A + KPCe
BC600) were increased in comparison to the untreated biochars, which resulted from the expanded pore areas of the alumina-treated biochars (Table 3). As acetochlor is a pesticide composed of aromatic structure, the π−π interactions between herbicide molecules and turbostratic graphene crystallites in biochars may play an important role for the surface adsorption.38,39 The removal of oxygen-containing organic matter away from the biochar makes it easier for acetochlor molecules to access these adsorption sites in the expanded pores. Therefore, it is understandable that the surface adsorption (QA) to the Ca-mont-treated BC600 biochar (MoBC600) was also improved, corresponding to its expanded mesopore area. However, the Ca-mont treatment did not increase the surface adsorption of acetochlor to the BC400 biochar, although the BC400 biochar’s micropore area was enlarged. This may be related to the inaccessibility of micropores to the herbicide molecules to allow surface adsorption. According to previous studies,40,41 the pore size of the carbons should be large enough (with diameter >1.7 times the molecule’s second widest dimension) for adsorption to occur, and most herbicides have a molecular weight >200 g mol−1 and an estimated length >10 Å.42 Therefore, a signiﬁcant fraction of micropores (