Article pubs.acs.org/est
Quantification of Chemical States, Dissociation Constants and Contents of Oxygen-containing Groups on the Surface of Biochars Produced at Different Temperatures Zaiming Chen,†,‡ Xin Xiao,†,§ Baoliang Chen,*,†,§ and Lizhong Zhu†,§ †
Department of Environmental Science, Zhejiang University, Hangzhou 310058, China Department of Environmental Engineering, Ningbo University, Ningbo 315211, China § Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China ‡
S Supporting Information *
ABSTRACT: Surface functional groups such as carboxyl play a vital role in the environmental applications of biochar as a soil amendment. However, the quantification of oxygen-containing groups on a biochar surface still lacks systematical investigation. In this paper, we report an integrated method combining chemical and spectroscopic techniques that were established to quantitatively identify the chemical states, dissociation constants (pKa), and contents of oxygencontaining groups on dairy manure-derived biochars prepared at 100−700 °C. Unexpectedly, the dissociation pH of carboxyl groups on the biochar surface covered a wide range of pH values (pH 2−11), due to the varied structural microenvironments and chemical states. For low temperature biochars (≤350 °C), carboxyl existed not only as hydrogen-bonded carboxyl and unbonded carboxyl groups but also formed esters at the surface of biochars. The esters consumed OH− via saponification in the alkaline pH region and enhanced the dissolution of organic matter from biochars. For high temperature biochars (≥500 °C), esters came from carboxyl were almost eliminated via carbonization (ester pyrolysis), while lactones were developed. The surface density of carboxyl groups on biochars decreased sharply with the increase of the biochar-producing temperature, but the total contents of the surface carboxyls for different biochars were comparable (with a difference pH 9 (SI Figure S-3). This decrease in the relative absorbance for Si−O−Si is most likely attributed to the dissolution of Si-containing mineral fractions, e.g., SiO2 + H2O + OH− ↔ H3SiO4− (pKa1 of H4SiO4 is 9.84).33 313
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Dissolved Organic Carbon Released from Biochars. To reveal whether esters from carboxyl and hydroxyl cleave in the alkaline region, the DOC content from biochar release under different values of solution pH was monitored (Figure 3a), and the corresponding color of the supernatant was also recorded (Figure 3b). The amount of DOC released from DM100, DM250, and DM350 increased significantly when the solution pH increased to higher than pH 7. This observation was in line with the decrease in the infrared absorption of C−O−C in the alkaline region, confirming that one part of the carboxyls formed esters on the biochar surface. The DOC content of DM100 was much higher than that of DM250, but the color of the supernatant solution at > pH 11.0 for DM100 was less intense than that for DM250. The DOC content of DM350 was comparable to that of DM250, yet the color of DM250 seemed much darker. The inconsistency of DOC content and supernatant color for a given sample (DM100, DM250, and DM350) demonstrated that the composition of DOC of DM250 and DM350 were different from the composition of DOC of DM100 because they experienced different degrees of carbonization. For DM500 and DM700, less organic matter was released into solution at pH 2.0−12.0, and the supernatant of DM500 and DM700 was almost colorless. Therefore, the dissociation of carboxyl groups in the alkaline region for DM500 would not originate from the hydrolysis of esters like DM100−DM350 but may be composed of lactones, for which the infrared absorption overlapped with COOH.25,28,29 The pKa and Contents of Biochar Surface Acid/Base Groups. Acid−base titration curves of biochars and water were determined (Figure 4a), and the proton-consumed curves for different biochars were presented in Figure 4b. Regression analysis of the proton uptake curves was performed with a proton-consumption model24,32,37 to derive the pKa value and the content of acid/base groups on biochars, and the regression parameters were presented in Table 1. The amount of proton uptake by the biochar decreased from positive (proton-consumed) to negative (proton-released or hydroxyl-consumed) when the solution pH increased from pH 2 to pH 12 (Figure 4b). As the proton-release pH region of an acid (or proton-uptake pH region of a base) was within 4 pH units, the progressive decrease of the proton uptake amount over approximately 10 pH units indicated that at least two acid/base groups existed on the surface of DM100-DM700. Further regression analysis of proton uptake curves showed that only when the number of acid/base groups increased to 4 (i.e., n = 4), the regression-derived curves exhibited a good fit for the data (the fitted curves are shown in Figure 4b with R2 = 0.997−0.999). Therefore, there were at least four major acid/base groups with different pKa on the biochar surface. In previous analysis of the acid−base titration curves of a class F coal fly ash and an activated sludge, the best curve fitting approach was found when n = 3 (i.e., three types of acid/base groups) and n = 1 (only the carboxyl group COOH),24,32 respectively. However, when using eq 1 to do regression analysis of proton uptake curves of biochars DM100-DM700, regression-derived curves with n = 3 and n = 1 deviated from the data (see SI Figure S-4, R2 = 0.641− 0.791 when n = 1, and R2 = 0.983−0.997 when n = 3). The pH region of the titration curve for class F coal fly ash and activated sludge are pH 2.0−12.0 and pH 4.0−7.5, respectively.24,32 More categories of acid/base groups derived from acid−base titration analysis of biochars DM100-DM700 than from coal fly ash and activated sludge would be attributed to a wider titration pH range and more complex surface groups on biochars.
Figure 5. PH dependent mole fractions of carboxyl and esters (COOH + COOC) on the surface of DM100 (a), DM250 (b), and DM500 (c). The mole fractions derived from the ATR-FTIR absorption intensity ratio are shown as black points. The estimated mole fractions of (COOH + COOC) using parameters derived from acid−base titration are shown in red curves. The corresponding data for DM350 are shown in SI Figure S-5.
Contribution of Biochar Surface CGs in Acid−Base Titration. Acid−base titration quantified the proton exchange between the biochar and water, while the ATR-FTIR spectra revealed the chemical states of surface acid/base groups on biochars directly. These states are essentially relevant. Table 1 lists the regression-derived pKa of acid/base groups (n = 4) on biochar surfaces. For DM100−DM350, all pKa values (1.44−10.5) were located in the dissociation pH region of carboxyl groups (pH 1.5−11.5) as reflected from the ATR-FTIR analysis. The pKa values of Group I ranged 1.44−2.11, approximately 4 pH units smaller than that of Group II (pKa values ranging from 5.15 to 6.11). Because the dissociation pH region of an acid or a base covers 4 pH units at best, these two groups are primarily composed of carboxyl groups because if one of these groups is not carboxyl, the dissociation pH region of carboxyl will be discrete and contradict the continuous dissociation of carboxyl 314
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Figure 6. Schematic of the pH-dependent dissociation of acid/base groups on the biochar surface and the environmental implications.
For DM500, the pKa values of four derived acid/base groups are also located in the dissociation region of carboxyl groups. Similar to low temperature biochar, the pKa of Group I is approximately 4 units smaller than the pKa of Group II, suggesting that both groups are composed of carboxyl groups. The pKa of Group III is 8.26 ± 0.40, comparable with the pKa of DM100−DM350 (pKa of Group III ranged from 7.36 to 8.62), which is located between the pKa of NaHCO3 (6.37) and the pKa of Na2CO3 (10.25).37 The pKa of Group IV is slightly higher than the pKa of Na2CO3 and smaller than the pKa of NaOH (pKa =15.74).37 NaHCO3 neutralizes CGs on the carbon surface, Na2CO3 neutralizes carboxyls and lactones, and NaOH titrates carboxyls, lactones, and phenolic −OH.25,37 Correspondingly, Group III on DM500 is most likely lactones, and Group IV may be composed of phenolic −OH and or Si-containing mineral fractions (e.g., SiO2). Figure 5c shows that the mole percentage change of undissociated carboxyl with pH by setting Group I, Group II, and Group III as CGs matches the results from ATR-FTIR, suggesting that Group I, Group II, and Group III are composed mainly of CGs. For DM700, the carboxyl group is unable to be probed by ATR-FTIR. The four acid/base groups on DM700 are assigned the same as DM500 based on their approximate pKa (the pKa of DM700 is approximately the same as the pKa of DM500). Variation and Functions of Acid/Base Groups on Biochars Produced at Different Pyrolytic Temperatures. Table 1 summarizes the category of acid/base groups corresponding to these four groups in biochars DM100−DM700. The pKa of biochars for each group decreased slightly from DM100 to DM250 and then gradually increased from DM250 and DM350, to DM500 and further to DM700. The contents of each group on different biochars were comparable and their difference was less than 3-fold. However, when they were normalized according to biochar surface area, the surface density of CGs on biochars exhibited a sharp decrease from DM100 to DM700 (Table 1). In preparing biochars by slow pyrolysis, the oxygen-containing groups were gradually eliminated with the increase of pyrolytic temperature.16,19,20,30,31 The current study supported a recognition that the increase in surface areas of biochars with an increase of pyrolytic temperatures produces high-temperature biochars having comparable content of CGs to low-temperature biochars. The major
groups as revealed by ATR-FTIR. The pKa of Group I is low (pKa = 1.44−1.89) and approximate to the pKa of chemicals having carboxyl groups in close proximity to one or more hydroxyl and/or carboxyl groups (pKa ranges 0.95−2.85 as sample chemicals listed in SI Table S-1). The presence of hydroxyl and/or carboxyl groups weakens the proton affinity of the carboxylate group (low pKa) by stabilizing carboxylate ion via the formation of a hydrogen bond.26,38 For example, the pKa decreases from 4.19 for benzoic acid to 2.91 for 2-hydroxybenzoic acid and to 1.05 for 2,6-dihydroxybenzoic acid (SI Table S-1). Accordingly, Group I is most likely to be hydrogen-bonded carboxyl. The pKa values of Group III locate at approximately pH 7, where the dissolved organic carbon from biochars begin to increase when solution pH increases, while the pKa values of Group IV (ranged 10.1−10.5) are at the center of the pH region where dissolved organic carbon and Si-containing fractions significantly release from biochars. Thus, the Group IV of low-temperature biochars is attributed mainly to the esters from carboxyl, the cleavage of which induces release of organic matter from biochars. The Group III may be composed of lactones and/or phenolic −OH, the dissociation of which has little effect on organic matter dissolution. To summarize, Group I, Group II, Group III, and Group IV on DM100, DM250, and DM350 are hydrogen-bonded carboxyl, carboxyl, lactones, and/or phenolic −OH, and esters and Si-containing minerals, respectively. For DM100, DM250, and DM350, the change of percentage of COOH and COOC with solution pH estimated by ATRFTIR were compared to the results calculated by parameters derived from acid−base titration, shown in Figure 5(a, b) and SI Figure S-5. Considering that infrared absorption from vas of COOH (and COOC) at 1690−1750 cm−1 was only slightly disturbed by other functional groups, the intensity ratio of vas of COOH (and COOC) to vas of CH2 was used to calculate mole percent change of COOH and COOC with solution pH (ratio at the lowest pH was set as 1). The result from acid−base titration was calculated by assuming that Group I, Group II, Group III, and Group IV were CGs. The percentage change of carboxyl in acid form (i.e., the sum of COOH and COOC) from acid−base titration matches the results from ATR-FTIR, confirming that the four groups on DM100, DM250, and DM350 are primarily from CGs. 315
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chemical states of CGs and their dissociation properties were revealed for the first time in the current study. Figure 6 depicts a schematic diagram for the dissociation of acid/base groups on biochar surfaces and their related functions. The protonated biochars favor inorganic anion adsorption and ionizable organic chemical sorption, while the deprotonated biochars favor cationic nutrient retention, heavy metal immobilization, and the release of dissolved materials. For low temperature biochars (i.e., DM100, DM250, and DM350), the acid/base group dissociation directly controls the pH buffering properties of biochars. The resulting surface charges regulate biochars in nutrient retention, sorption/ immobilization of hazardous pollutants and biochar particle dispersing properties. Meanwhile, dissociation of acid/base groups affects carbon and silica biogeochemical cycling by regulating the release of organic matter from the cleavage of esters and dissolution of the Si-containing minerals. For high temperature biochars (i.e., DM500 and DM700), the effect of acid/base dissociation on organic matter dissolution is eliminated, but other functions are similar. CGs are the major acid/ base groups on biochar surfaces. In field applications, such abundant CGs are worthy of concern in terms of multiple functions of biochars, such as soil pH adjustment, soil nutrient retention, and toxic metals immobilization (see the discussion on the implication for the role of CGs on biochar functions in SI).
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ASSOCIATED CONTENT
* Supporting Information S
Biochar preparation, acid−base titration, proton consumption model, and implication for the role of CGs on biochar functions were detailed. The pKa values of typical carboxlic acids are presented in Table S-1. ATR-FTIR of biochars and biochars of DM350 and DM700 buffered to different pH are presented in Figure S-1. Infrared absorption of the carboxyl in different chemical states are presented in Figure S-2. Normalized intensity of infrared absorption for −OH, CH2 (νs), aromatic CC (or CO), and Si−O−Si is presented in Figure S-3. The proton uptake curves with n = 1−3 are presented in Figure S-4. The pH dependent mole fractions of carboxyl and esters on DM350 are presented in Figure S-5. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone/fax: 0086-571-8898-2587; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Science Foundation for Distinguished Young Scholars of China (Grant 21425730), the National High-Tech Research and Development Program of China (No. 2012AA06A203), the National Natural Science Foundation of China (Grants 21277120 and 41071210), and the Fundamental Research Funds for the Central Universities.
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