pH-Dependent Mineral Release and Surface Properties of

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Environ. Sci. Technol. 2010, 44, 9318–9323

pH-Dependent Mineral Release and Surface Properties of Cornstraw Biochar: Agronomic Implications A. SILBER,* I. LEVKOVITCH, AND E. R. GRABER Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Center, Bet Dagan, 50250, Israel

Received April 21, 2010. Revised manuscript received October 17, 2010. Accepted November 9, 2010.

Surface charge and pH-dependent nutrient release properties of cornstraw biochar were examined to elucidate its potential agronomic benefits. Kinetics of element release was characterized by rapid H+ consumption and rapid, pH-dependent P, Ca, and Mg release, followed by zero-order H+ consumption and mineral dissolution reactions. Initial K release was not pHdependent, nor was it followed by a zero-order reaction at any pH. Rapid and constant rate P releases were significant, having the potential to substitute substantial proportions of P fertilizer. K releases were also significant and may replace conventional K fertilizers, however, not long-term plant demand. The cation exchange capacity (CEC) of the biochar leached with a mild acidic solution increased linearly from 179 to 888 mmolc (kg C)-1 over a pH range of 4-8, while the anion exchange capacity of 154 mmolc (kg C)-1 was constant over the same pH range. Since native soil organic constituents have much higher CEC values (average 2800 mmolc (kg C)-1 at pH 7), improved soil fertility as a result of enhanced cation retention by the biochar probably will be favorable only in sandy and low organic matter soils, unless surface oxidation during aging significantly increases its CEC.

Introduction When used as a soil amendment together with organic and inorganic fertilizers, different biochars have been reported to improve soil tilth, crop productivity, and nutrient availability to plants (1-5). While the specific mechanisms underlying the contribution of biochar to plant response are poorly understood, it appears that biochars may have direct effects due to their nutrient content, as well as many indirect effects, including: (i) improved retention of nutrients (3, 4); (ii) improvements in soil pH (6); (iii) increased soil cation exchange capacity (7); (iv) improved soil physical properties (1); (v) alteration of soil microbial populations and functions (5, 8); and (vi) increased crop protection from plant disease (9). These factors may also act in concert to result in improved crop performance. Different biomass feedstocks and pyrolysis conditions create biochars with different physical and chemical properties (10), giving them varying effects in the soil (1, 11). In addition, regional conditions including soil type, chemistry, and condition (depleted or healthy), temperature, and humidity also affect biochar agronomic benefits. * Corresponding author phone: 972-(0)3-968-3633; fax: 972-(0)3960-4017; e-mail: [email protected]. 9318

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Physical/chemical factors which can influence the quality of a biochar as a soil amendment include its charge, surface area, porosity, cation and anion exchange capacities, and nutrient content. Mineral contents of biochars prepared from different feedstocks are very variable (12, 13). Little P is lost during pyrolysis, but its plant availability decreases as the temperature of pyrolysis increases (14). Pyrolysis also affects the availability of base cations, with an increasing proportion of remaining K being in the exchangeable and acidextractable fractions rather than the water-soluble fraction (15). The cation exchange capacity (CEC) of biochar is affected by both feedstock and treatment temperature, with higher temperature chars having lower CEC values due to loss of functional groups (16). When biochar ages, its CEC increases due to formation of carboxylic and other oxygenated functional groups at the surface (17, 18). At the same time, aging causes a substantial reduction in anion exchange capacity (AEC) due to disappearance of surface positive charge (17, 18). Aged biochars also have lower pH and lower point of zero net charge (PZNC) (18), the pH at which the AEC equals the CEC. Direct nutrient benefits (i.e., fertilizer replacement) and improvement in nutrient availability are dependent both on pH and ionic strength of the soil solution, yet there is very little data on plant available nutrient content of biochars, and none on either of these dependencies. To determine the potential for a given biochar type to be a source of plant nutrition, it should be screened for nutrient release under different solution pHs and ionic strengths. The objectives of this study were thus to characterize cornstraw biochar for (i) the kinetics of pH-dependent release of nutritional elements; (ii) the effect of solution ionic strength on release of nutritional elements; and (iii) the characterization of biochar surface charge properties using two alternative methods: pHdependent CEC and AEC measurements, and potentiometric titration.

Materials and Methods General. Biochar was produced at 500 °C from cornstraw using a lab-scale fluidized fast pyrolysis reactor (feedstock processing capacity: 5 kg h-1). The biochar (median grain size 0.02 mm) was rinsed with deionized water to a solution electrical conductivity (EC) of ∼1.5 dS m-1, oven-dried (40 °C), and stored under ambient conditions. A portion of the rinsed biochar was acidified with 0.1 M HCl to a solution pH ∼2, and then rinsed from excess acid to a solution pH of ∼5.5. The water-rinsed and acid-leached biochars are designated CS and CSAc, respectively. Biochar specific surface area was determined by BET-N2 adsorption. The chemical composition of the CS biochar was determined in duplicate following digestion at 120 °C in 70% HNO3 for 24 h. Element concentrations in the HNO3 solution were determined by inductively coupled plasma (ICP-AES Arcos, EOP model, Spectro Ltd.). Ash content (in triplicate) was determined by weight loss after heating to 550 °C in an oxygen atmosphere for 4 h. Total C, H, N, O, and S were determined in triplicate by element analyzer (Thermo Flash EA-1112 Elemental Analyzer). X-ray diffraction analysis was carried out on a Philips XRD diffractometer, and biochar samples (both uncoated and gold coated) were observed by scanning electron microscope (SEM) (model JSM-5410, JEOL Ltd.). Element identification on uncoated samples was by Link ISIS X-ray Analysis detector (Oxford Instruments). FTIR absorbance spectra of KBr pellets prepared with 0.6 wt % biochar were recorded between 400 and 4000 cm-1 with one 10.1021/es101283d

 2010 American Chemical Society

Published on Web 11/23/2010

hundred scans averaged with a resolution of 4 cm-1 (Bruker Tensor 27 FTIR Spectrometer). Determination of pH-Dependent Surface Properties. CEC and AEC of the CSAc biochar was determined as a function of pH at four solution pH levels. Kinetics of element release from CS biochar into 0.01 M NaCl solution was also tested at 4 solution pH levels. Potentiometric titrations and the effect of ionic strength and pH on element solubilization from CS biochar was tested at 48 h at three ionic strengths (0.001, 0.01, and 0.1 M NaCl) and 6 pH levels. All determinations were in duplicate. Full experimental details are given in the Supporting Information (SI). Statistical Analysis and Calculations. Differences between means were tested with the standard least-squares mode of ANOVA, followed by a Tukey pairwise comparison by means of the JMP software (version 7.02, SAS Institute Inc.). Differences with probability larger than 95% were taken as significant. Ion speciation in solutions was calculated with the MINEQL+ software (19). Model parameters were fitted by the NLIN procedure of SAS, by means of the DUD routine.

FIGURE 1. pH dependent CEC and AEC of acidified cornstraw biochar. Symbols represent experimental data ( SE (SE not shown when smaller than the symbol). The CEC curve was plotted according to a linear equation (SE of the fitting parameters given in parentheses): CEC (mmolC kg-1) ) 115(8.4) × pH 342(49.4).

Results and Discussion General Physical and Chemical Characterization. The specific surface area (SSA) of the CS and CSAc biochars were 3.0 and 23.4 m2 g-1, respectively. The surface area of the CS material is the same as that reported for biochar made via fast pyrolysis from corn stover (20). The higher SSA of the CSAc material is presumed to be related to etching of submicrometer structures during the acidification, leaching, and rinsing procedure. Concentrations of nutritional elements in the CS biochar (determined in the HNO3 digestion; SI Table S-1) are generally two to three times higher than typical values in cornstraw (21), which accords with the common observation that pyrolysis concentrates many elements in the solid biochar phase (15). Ash content decreased from 32.5 ( 0.4 to 20.1 ( 0.1% during the acidification procedure, leading accordingly to increased CHNO concentrations in the acidified sample (SI Table S-1). However, atomic ratios, O/C, H/C, C/N, and H/O (0.16 ( 0.02, 0.42 ( 0.03, 26.90 ( 9.50, 2.59 ( 0.42, respectively) of the acidified sample (CSAC) did not differ significantly from those of the original CS sample (0.21 ( 0.04, 0.46 ( 0.08, 40.93 ( 5.72, and 2.25 ( 0.55, respectively) (SI Table S-1), and were similar to those of biochars made from corn stover (20, 22). The two biochars have a typical plant cell structure, and the differences between them in the SEM were negligible (SI Figure S-1). Electron probe analyses illustrated diffusive distribution of all the elements, and no minerals or clear arrangement of elements were observed. The XRD diffractogram of the CS biochar showed typical diffraction peaks of sylvite (KCl) and quartz, and probably calcite or dolomite minerals (SI Figure S-2). Only small differences in the FTIR spectra between the acidified and original biochars (CS and CSAC) were observed (SI Figure S-3). In the acidified biochar (CSAC), there is a small increase in the aromatic carbonyl/ carboxyl CdO stretching band at 1700 cm-1, and a decrease in the aromatic CdC ring stretching band at about 1400 cm-1 as compared with the original biochar (CS; assignments after (23)). The band at 1600 cm-1 is assigned to aromatic CdC stretching. The broad band at 1300-1000 cm-1 (with peak at 1080 cm-1) is assigned to asymmetric stretching mode of the SisOsSi moiety, and the two small bands at about 800 and 450 cm-1 are also assigned to SisOsSi vibrations (24).Silica makes up a major part of the mineral portion of cornstraw and cornstraw biochar (20). The prominent band in both samples with peak absorbance at 3400 cm-1 represents sOH stretching. pH-Dependent CEC and AEC. The CEC was found to be strongly pH-dependent, increasing by 115 mmolc kg-1 for

FIGURE 2. Net H+ added vs time for intact CS biochar. Symbols represent experimental data, and curves were fitted according to eq 2. The fitted RH values (mmolc kg-1 h-1; t > 24 h) were significant at P e 0.05. The pH values of CS suspensions prior to H+ addition (ZPT) was 8.9; constant ionic strength (0.01 M NaCl), biochar:solution ratio of 1:50. every unit increase of pH (Figure 1). In contrast, the AEC was not affected by pH, maintaining an average value of 95 ((11) mmolc kg-1 over a pH range of 4-8 (Figure 1). Transforming the CEC data to units of carbon yields CEC ranging from 179 to 888 mmolc (kg C)-1 (between pH 4.1 and 7.9, respectively) with a slope of 187 mmolc (kg C)-1 C per pH unit. These values correspond to literature reports for native and aged black carbons (18, 25). By extrapolating the pH-CEC regression line to low pHs, we calculated the point of zero net charge (PZNC; pH at which CEC ) AEC) of the CSAc biochar to be at a pH of 2.2. While this PZNC differs considerably from that reported for fresh black carbon (pH 7.0), the PZNC of the fresh carbon shifted to acidic pHs (pH 3.4 and 2.7) during subsequent to aging over 12 months at 30 and 70 °C, respectively (18, 25). pH-Dependent Proton Consumption Rates and Rate of Element Release. The CS suspensions consumed acid rapidly during the first 24 h of the experiment, then more slowly for the remaining 505 h (Figure 2). After about 24 h, the net proton consumption (NPC) became linear with time (t > 24 h) to conform to eq 1: NPC ) RH × t

(1)

where RH is the slope. Values of RH were determined from eq 1 for the NPC vs t relationships presented in Figure 2. A plot of log (RH) vs pH gave a significant straight line with a negative slope. Releases of P, Ca, and Mg into solution resembled that of proton consumption, that is, rapid element-detachment VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effects of pH and time on P, K, Ca, and Mg release from intact CS biochar. Symbols represent experimental data ( SE (SE not shown when smaller than the symbol), constant ionic strength (0.01 M NaCl), biochar:solution ratio of 1:50. The fitted RP, RCa, and RMg values (mmol kg-1 h-1; t > 24 h) were significant at P e 0.05 and are presented in SI Table S-2. reactions, followed by zero-order reactions at t > 24 h which persisted as long as the system was far from chemical equilibrium (Figure 3a, c, d). Initial (up to 24 h) P, Ca and Mg release was pH-dependent (Figure 3a, c, d), exhibiting an increase in released quantities as pH decreased from 8.9 (native pH) to 4.5. In accordance with eq 1, the fitted reaction rates (t > 24 h) of P, Ca, and Mg (RP, RCa, and RMg, respectively) were pH-dependent as well, and increased with decreasing pH (Figure 3a, c, d). Potassium released to the solution exhibited a different pattern, whereby initial rapid K release to the solution was not pH-dependent, nor was it followed by a zero-order reaction at any pH (Figure 3b, data of pH 5.7 and 6.9 were omitted as they overlapped with results for pH 4.5 and 8.9). Charge Characteristics and Solution Ionic Composition. The zero point of titration (ZPT; pH of suspension prior to the addition of protons or hydroxyls) of the CS suspensions decreased as the ionic strength increased from 0.001 to 0.01 and 0.1 M (pH 8.7, 8.6, and 8.4, respectively) (Figure 4a). This general behavior may indicate the presence of pH-dependent charge moieties above the point of zero salt effect (PZSE). The pH titration lines obtained with the three different NaCl concentrations overlapped and it was not possible to detect any intersection point (Figure 4a). Unlike the intact (CS) biochar, the effect of solution ionic strength on the ZPT values in CSAc suspensions was negligible (4.0 ( 0.65) and the pH titration curves were distinct (Figure 4b). The main difference between the intact and the acidified materials derived from higher quantities of K, Ca, Mg, and P released from the solid phase of the intact material. Both pH and ionic strength had specific effects on cation releases from the intact CS biochar and consequently, solution Ca, Mg, and K concentrations (CCa, CMg, and CK, respectively) exhibited distinct patterns (Figure 5). The concentration of released Ca increased significantly as pH declined, while the effect of ionic strength was minor (Figure 5; 0.01 M data omitted because of similarity with 0.001 M data). Unlike Ca, the effect of pH on CMg was 9320

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FIGURE 4. Potentiometric titration curves of intact and acidified cornstraw biochars (a, and b, respectively). Solid symbols: Net acid/base addition (0.1N HCl and NaOH, respectively) as functions of pH and ionic strength (0.001, 0.01, and 0.1 M NaCl); biochar:solution ratio of 1:50; equilibrium period of 48 h. Empty symbols represents the net amounts (cations-anion) of ions released (∆ion, eq 2) into the solution phase.

FIGURE 5. The effects of pH and ionic strength (0.001, 0.01, and 0.1 M NaCl) on solution-Ca, -Mg and -K concentrations in intact cornstraw biochar. Biochar:solution ratio of 1:50; equilibrium period of 48 h. Symbols represent experimental data ( SE (SE not shown when smaller than the symbol). minor and although CMg and CCa prior to acid addition were almost similar (0.3-0.9 mmol L-1 at pH > 8.5 for both Ca and Mg), CMg increased merely to ∼2.8 mmol L-1 at pH 2.9, compared with ∼8.6 mmol L-1 for CCa (Figure 5). The concentrations of CK prior to acid addition were the highest and increased as ionic strength rose (5.9, 6.6, and 8.1 mmol L-1 for 0.001, 0.01, and 0.1 M, respectively). However, CK was not affected by pH (in accordance with the kinetics experiment), and the effect of ionic strength diminished as pH dropped (Figure 5). The effect of ionic strength on solution-P concentrations (CP) was not important, however, that of pH was significant and CP concentrations systematically increased from 0.4 to 1.4 mmol L-1 as pH decreased from the ZPT to 2.9 (SI Figure S-4).

FIGURE 6. Effect of pH on net surface charge of CS (open symbols) and CSAc (closed symbols) biochars. Net surface charge was calculated according to eq 3. Triangles, circles and squares symbols denote 0.001, 0.01 and 0.1M, respectively. Model lines (SE of the fitting parameters given in parentheses) were plotted according to linear equations: 23.0 (3.62) × pH -68.1(20.94); 33.6(5.42) × pH 93.9(31.01); 51.2(4.83) × pH 150.1(27.47) for 0.001, 0.01, and 0.1 M, respectively. On the assumption that H+/OH- consumption occurs during cation/anion exchange, the net charge at each pH value can be evaluated by subtracting the net quantity of cations/anions accumulated in the solution of the substrate (∆ion releases) from the quantity of acid/base added at the pertinent pH. Hence, ∆ion releases (∆ion; cmol kg-1) is defined as ∆ion ) (Σcat - Σan)pH - (Σcat - Σan)ZPT

Ri ) Ki[H+]n

(4)

where Ki is the mineral specific rate constant for element i, [H+] is the proton activity in solution and n is the order of the reaction with respect to [H+]. The consumption of one mole of H+ by the surface must be balanced by the release/ retention of one mole of net positive/negative charge into the solution; therefore the rate of net proton consumption (NPC) can serve as an estimate of the constant dissolution rate, R (30). Analogous with eq 1, elements released (Qit) vs time were calculated as follows: Qit ) Qi24 + Ri × t (t > 24h)

(5)

(2)

where ∑cat and ∑an include all the cationic and anionic species in the solution, respectively, according to speciation calculations. As reflected by ∆ion in Figure 4b for the acidified biochar, CCa, CMg, and CK and CP were very low, and diminished at pH ∼3 to less than 0.01, 0.1, 0.3, and 0.02 mmol L-1, respectively. The amount of H+/OH- added to the CS suspensions for reaching a desired pH was significantly higher compared with CSAc suspensions (compare Figure 4a to Figure 4b). This difference predominantly resulted from H+ consumption through dissolution or/and exchange reactions which resulted in a higher quantity of ∆ion releases (eq 2) into solution (Figure 4a, b), rather than a discrepancy in charge characteristics. Net surface charge (NSC) at a given pH calculated according to eq 3 is shown in Figure 6. NSC ) H+biochar - H+blank - ∆ion

(17). This is demonstrated by only minor differences in FTIR spectra (SI Figure S-3) and the lack of significant change in atomic ratio (SI Table S-1). Agronomic Implications. Nutrient release from minerals associated with biochar will have direct benefits for crop production. It is commonly accepted that dissolution of minerals comprises an initial rapid element detachment reaction, followed by a zero-order reaction which persists as long as the system is far from chemical equilibrium (26). The rapid reaction is attributed to ion exchange, preferential dissolution at crystal imperfections, and dissolution of submicrometer particles (27). The constant (zero-order) reaction rate of element i (Ri) is controlled by the rate of detachment of structural ions into solution (28) and is pH dependent, as described by eq 4 (29):

(3)

where H+biochar designates amount of protons added to the biochar suspension, and H+blank designates amount of protons added to the blank. Note that the ZPT values of the CS biochar were high (pHs 8.4-8.7) and the titration ended at pH ∼ 3, whereas the ZPT values of the CSAc were acidic (pH ∼4.0) and the titration ended at pH ∼ 8 (Figure 4a and b, respectively). To facilitate straightforward comparison between the two biochars, NSC curves for CS were drawn by taking the absolute NSC values, thus simulating an initially acidic titration point (∼3). As presented in Figure 6, at each ionic strength, a single fitted line connected the calculated values of CS and CSAc materials, indicating that both had similar surface charge characteristics. Based on this, it can be assumed that the pH-dependent CEC of the CS biochar would be similar to that measured for the CSAc biochar, given that potentiometric titration and pHdependent CEC analyses are actually alternative methods for assessing the surface charge properties of a medium. This suggests that while the mild acidification procedure simulates the effect of mineral leaching from biochar, it does not capture essential changes to surface structure that occur over time

Here Qi24 stands for element i releases during the first 24 h. Comparable to Ri (eq 4), the pH- dependency of Qi24 is expressed as + n ] Qi24 ) K*[H i

(6)

Fast (24 h) P release (QP24) increased from 20 to 87 mmol kg-1 as pH decreased from 8.9 to 4.5 (Figure 3). These amounts are significant (16.4 and 71.6% of the total P in the biochar, respectively) and may replace conventional P fertilizers. Considering an application rate of 10 ton biochar ha-1 and assuming conditions similar to those prevailing in the experiment, between 6 and 27 kg of P would be released within 24 h and be available for plant uptake. Reported total P uptakes by common low-input crops (sugar beet, potato, soybean, bean, and rice) are 4-34 kg ha-1 (31), and hence, a reasonable biochar application rate may successfully fulfill almost all the plant demand for P throughout the growing season. The t > 24 h rate of P release (RP) varied between 0.015 to 0.02 mmol kg-1 h-1 (Figure 3a and SI Table S-2). Transforming these rates to 10 ton biochar ha-1 gives 0.11-0.14 kg P ha-1 d-1. Reported values of total P uptake by high-input crops (lettuce, bell pepper, tomato, egg plant) were 20-65 kg ha-1 and the daily P consumption rates were 0.1-0.6 kg ha-1 d-1 (32). Thus, even for high input crops, P releases from the biochar may substitute significant P fertilization. Total K amount in the cornstraw biochar is 48.7 g kg-1, and almost 1/3 of that was released during the first hour. The unexpectedly low total K release relative to P release may be connected to the temperature of char formation (500 °C) as discussed in (15). As with P, the amount of K released from the biochar is significant and may replace conventional K fertilizers. However, in contrast to P, K release from the biochar cannot supply long-term plant demand. At pH 4.5, almost 90 and 95% of the total Ca and Mg content of the CS biochar (444 and 300 mmol kg-1, respectively) were released during the 21 day-long kinetics experiment. As such, cornstraw biochar may be a good initial source of Ca and Mg for VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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plant nutrition in acidic soils. Under neutral and alkaline conditions, release of Ca and Mg decreased (Figure 3), reducing the potential value of the biochar as a source of Ca and Mg. Application of biochars in agricultural soils is a suggested management tool for ameliorating crop productivity and soil fertility also as a result of enhanced cation retention (17). Yet, taking into account the very high CEC and pH dependency of native soil organic constituents (2800 mmolc (kg C)-1 at pH 7, 296 mmolc (kg C)-1 per each pH unit, ref 33), and suggested rates of biochar amendment (usually between 5 and 20 tons/ha, ref 34), it is apparent that improved soil fertility as a result of amendment with biochars having CEC values similar to those of the biochar studied herein (750 mmolc (kg C)-1 at pH 7, 187 mmolc (kg C)-1 per pH unit), will be favorable probably only in sandy and low organic matter-bearing soils. In such soils, the proportion of biochar relative to the native soil organic matter and clay components may be sufficiently high to have a substantial impact. If, however, there is a substantial increase in CEC during aging of the biochar in the soil environment (17), or alternatively, if biochars are specially produced to have high CEC values, they may improve soil fertility also of organic matter and clay rich soils as a result of enhanced cation retention.

Acknowledgments We are grateful to the reviewers for their constructive comments. This research was supported by the Chief Scientist of the Ministry of Agriculture and Rural Development of Israel, project number 301-0693-10. Biochar was kindly provided by Prof. Xueyuan Bai, Shandong University of Technology, and his generosity is gratefully acknowledged. Amir Sandler of the Israel Geological Survey made the XRD analyses. Ludmilla Tsechansky assisted in preparing the biochar. This paper is contribution No. 608/10 of the Agricultural Research Organization, the Volcani Center, Israel.

Supporting Information Available Experimental details for determination of pH dependent CEC and AEC, kinetics of pH-dependent element release, potentiometric titrations and effects of ionic strength and pH on element solubilization, and determination of pH-dependent surface properties. It also includes Table S-1with chemical analyses of the biochars, Table S-2 with best fitted equations for eq 1, SEM photomicrographs of the biochars (Figure S-1), XRD diffractogram (Figure S-2), FTIR spectra of the biochars (Figure S-3), and the effect of pH and ionic strength on solution-P concentrations (Figure S-4). This material is available free of charge via the Internet at http:// pubs.acs.org.

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