Transport of Biochar Particles in Saturated Granular Media: Effects

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Transport of Biochar Particles in Saturated Granular Media: Effects of Pyrolysis Temperature and Particle Size Dengjun Wang,†,§ Wei Zhang,‡ Xiuzhen Hao,† and Dongmei Zhou*,† †

Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, No. 71 East Beijing Road, Nanjing 210008, China ‡ Department of Plant, Soil and Microbial Sciences; Environmental Science and Policy Program, Michigan State University, East Lansing, Michigan 48824, United States § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Land application of biochar is increasingly being considered for potential agronomic and environmental benefits, e.g., enhancing carbon sequestration, nutrient retention, water holding capacity, and crop productivity; and reducing greenhouse gas emissions and bioavailability of environmental contaminants. However, little is known about the transport of biochar particles in the aqueous environment, which represents a critical knowledge gap because biochar particles can facilitate the transport of adsorbed contaminants. In this study, column experiments were conducted to investigate biochar particle transport and retention in water-saturated quartz sand. Specific factors considered included biochar feedstocks (wheat straw and pine needle), pyrolysis temperature (350 and 550 °C), and particle size (micrometer-particle (MP) and nanoparticle (NP)). Greater mobility was observed for the biochars of lower pyrolysis temperatures and smaller particle sizes. Extended Derjaguin−Landau−Verwey−Overbeek (XDLVO) calculations that considered measured zeta potentials and Lewis acid−base interactions were used to better understand the influence of pyrolysis temperature on biochars particle transport. Most biochars exhibited attractive acid−base interactions that impeded their transport, whereas the biochar with the greatest mobility had repulsive acid−base interaction. Nonetheless, greater retention of the MPs than that of the NPs was in contrast with the XDLVO predictions. Straining and biochar surface charge heterogeneity were found to enhance the retention of biochar MPs, but played an insignificant role in the biochar NP retention. Experimental breakthrough curves and retention profiles were well-described using a two-site kinetic retention model that accounted for depth-dependent retention at one site. Modeled first-order retention coefficients on both sites 1 and 2 increased with increasing pyrolysis temperature and particle size.

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INTRODUCTION Biochar is a charred form of organic matter, oftentimes derived from plant biomass or biowaste and has carbon content at least 2-fold higher than ordinary biomass.1−4 Decomposing carbon is locked up into a recalcitrant form that may persist for centennial to millennial time scales.1,3,5 Consequently, biochar is increasingly being considered as a means to sequester carbon and to mitigate climate change.1−4,6 Land application of biochar may also offer other agronomic and environmental benefits. For example, biochar may improve soil structure and fertility,2,7 increase the pH of acidic soils,8−11 enhance retention and efficiency of fertilizers,12 and increase biomass production.2 In addition, biochar has been reported to decrease the mobility and toxicity of inorganic, organic, and pathogenic microorganism contaminants.8,13−18 Conversely, dissolved or colloidal biochar particles may also act as a carrier to facilitate the transport of these same contaminants.17,19 Much is known about the stability of biochar in soils and its critical role in the biochar’s ability to act as a carbon sink in terrestrial ecosystems.3 The stability and recalcitrance of biochar against decomposition depend upon the properties © XXXX American Chemical Society

and origin of the biochar itself as well as the conditions of environmental exposure (e.g., soil mineral, microbial activity, nutrient, oxygen, and moisture).3,12,20−24 More importantly, the stability of biochar significantly depends on the pyrolysis temperature that determines the physicochemical properties of the biochar such as pH, specific surface area, and surface charge (e.g., zeta potential).10,18,19,25,26 Kawamoto et al.27 found that charcoal biochar produced at 400 °C exhibited greater stability against abiotic oxidation by ozone than the one produced at 1000 °C. Apart from this abiotic oxidation, microbial decomposition of biochar is also a critical process in soils that is influenced by pyrolysis temperature.24 Thus, pyrolysis temperature in biochar production is a crucial consideration for carbon sequestration by biochar land application.2,3 On the other hand, very little is known about the transport of biochar particles away from the site of application.3,28 Zhang et Received: September 20, 2012 Revised: December 13, 2012 Accepted: December 18, 2012

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Table 1. Properties of Biochar Background Influents, Electrokinetic Potentials of Biochar Particles and Sand Grains, and Extended DLVO Parameters for the Particle−Particle and Particle−Sand Interactions biochar W350_MPa,b W550_MP P350_MP P550_MP W350_NPa,b W550_NP P350_NP P550_NP

pH 6.8 6.7 6.7 6.8 6.8 6.7 6.7 6.8

ac a a a a a a a

ECd μS cm−1 17 bc 18 c 14 abc 16 abc 15 abc 16 abc 12 a 13 ab

DLS sizee nm 2015 1706 1600 1420 126 106 105 98

± ± ± ± ± ± ± ±

13 d 23 c 12 c 7.7 b 3.5 a 3.3 a 3.5 a 3.1 a

(dp)/(dc)f 4.0 3.4 3.2 2.8 2.5 2.1 2.1 2.0

× × × × × × × ×

10−3 10−3 10−3 10−3 10−4 10−4 10−4 10−4

b b b b a a a a

ζpg,h mV

ζgg,h mV

Φmax‑ppi,j kT

Φmax‑psi,j kT

Φmin2k kT

−30.1 −20.6 −28.0 −25.6 −36.6 −32.9 −34.0 −28.9

−68.1 −66.4 −66.4 −68.1 −68.1 −66.4 −66.4 −68.1

518 c 193 ab 2201 d 228 b 44.0 ab 26.2 ab 180 ab 18.9 a

1037 d 386 b 4402 e 457 c 88.0 a 52.4 a 360 b 37.7 a

−0.5 b −0.4 b −0.4 b −0.3 b −0.03 a −0.03 a −0.03 a −0.02 a

c a c b e d d c

a a a a a a a a

a,b MP and NP refer to micrometer-particle and nanoparticle, respectively. cMean values in each column followed by the same lowercase letters are not significantly different using Tukey’s HSD test at p < 0.05. dElectrical conductivity. eMean size ± standard deviation (triplicate experiments). fdp and dc are diameters of suspended particle and collector grain, respectively. g,hζ-potentials of biochar particle and sand grain, respectively. i,jEnergy barriers to primary minimum for particle−particle and particle−sand interactions, respectively, as calculated by XDLVO theory. kThe secondary minimum for particle−sand interactions.

al.29 examined the effects of pH, ionic strength, and particle size on the transport of biochar particles (size 0.999) was constructed by diluting the 600 mg L−1 biochar suspension (i.e., adding 0.060 g of dry biochar MP into 100 mL of DI water at pH 6.8) over the range of 0−600 mg L−1 (Figure S2). It is noted that the calibration curves of determining the concentrations of the biochar MP and NP suspensions were identical. Size and morphology of biochar MPs and NPs were determined using a transmission electron microscope (TEM JEM-2100, Japan). Average hydrodynamic sizes and intrinsic size distributions of the biochar MP and NP suspensions were determined by dynamic light scattering (DLS) (S2). Electrokinetic Properties of Biochars. The zeta (ζ) potentials of the biochar MP and NP suspensions, as well as colloidal quartz particles (size less than 2 μm and pulverized from the sand used in the column experiments),32 were measured for a minimum of 10 times using a ZetaPlus analyzer (Brookhaven Instruments Corporation). The measured ζpotential values were used in place of surface potentials for extended Derjaguin−Landau−Verwey−Overbeek (XDLVO) calculations as described in S3.

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MATERIALS AND METHODS Biochar Preparation. A crop residue (wheat straw) and a soft woody biomass (pine needle) were utilized to prepare the biochars in this study. The biochars were produced using a stepwise procedure under anaerobic conditions in a patented facility (ZL 2009202321919). Prior to biochar production, the wheat straw and pine needle were oven-dried for 12 h at 80 °C, and then transferred into the biochar reactor. The starting temperature was fixed to 200 °C, and gradually elevated to 250, 300 (each temperature interval was maintained for 1.5 h), then to the target temperature of 350 or 550 °C, and remained at the target temperature until no further smoke emission from the gas exhaust pipe. The resultant biochar sample was allowed to cool to room temperature, ground into powder, and then passed through a 150-μm sieve. All the biochars were stored in a desiccator. The biochars were designated as W350 and W550 for wheat straw biochars, and P350 and P550 for pine needle biochars. Biochar Characterization. A number of methods were used to investigate the physicochemical characteristics (e.g., pH, specific surface area, particle size, and contact angle) of the biochars that may be related to the biochar transport in B

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Column Transport Experiments. The column transport experiments were conducted in triplicate using glass chromatography columns measuring 2.6-cm in inner diameter and 20-cm long. The vertically oriented columns were wetpacked with angular quartz sands (0.50−0.60 mm, 0.55-mm median grain size, Sinopharm Chemical Reagent Co., Ltd., China). The sand was thoroughly cleaned using a procedure described in our previous study.32 Porosity of the packed columns was determined gravimetrically and varied between 0.38 and 0.40. The column experiments were run in an upward mode using a peristaltic pump. The packed column was equilibrated by sequentially pumping 10 pore volumes (PVs) of DI water followed by 10 PVs of the biochar-free background electrolyte solution (0.1 mM NaHCO3 at pH 6.8) through the column at a constant pore-water velocity of 8.0 × 10−3 cm s−1. A stable biochar MP or NP suspension at a concentration of 200 mg L−1 was then introduced into the column for 3.0 PVs, followed by several PVs of biochar-free background electrolyte solution (0.1 mM NaHCO3) to flush any unattached particles remaining in the column. Column effluent was collected using a fraction collector. The concentrations of biochar particles in the effluent were determined spectrophotometrically at the wavelength of 221 nm. Particle size and ζ-potential of biochar in the influent and effluent were measured for selected samples to better understand the potential influence of biochar particle aggregation and charge heterogeneity on retention processes. Following completion of the transport experiments, the spatial distribution of biochar particles in each column was determined. The end fitting was removed and the quartz sand in 2-cm segment was carefully excavated into a 50-mL centrifuge tube. Ten centrifuge tubes were used to recover the quartz sand from a particular 20-cm column. Excess (25 mL) DI water was added to the centrifuge tubes. These tubes were slowly shaken at 30 rpm for 2 h using a shaker to liberate the retained colloids. The concentration of the biochar particles in the excess aqueous suspension (decanted from the tubes) was measured spectrophotometrically at the wavelength of 221 nm. A mass balance was calculated by comparing the quantities of biochar in the effluent to those retained in the sands and to those injected into the column. Transport Model. A one-dimensional form of the convection−dispersion equation that considers kinetic retention at two types of sites was used to simulate biochar particle transport and retention.34,35 Detailed description of the mathematical model, including governing equations, is given in S4. Statistical Analysis. One-way analysis of variance (ANOVA) was performed to identify statistically significant differences in measured parameters. Mean separations were performed using Tukey’s honestly significant difference (HSD) test. All statistical analyses were conducted using SPSS 17.0 and the differences were considered significant at p < 0.05.

cations (e.g., P, K, Ca, Mg, Fe, Zn, and Mn) increased significantly with increasing pyrolysis temperature (p < 0.05) due to the condensation effect at higher pyrolysis temperatures. This was consistent with the results reported elsewhere.4,10,25,26,36 The bulk density of the biochars increased significantly (p < 0.05) with increasing pyrolysis temperature and its value varied between 0.30 and 0.41, consistent with the results reported in the literature.29,37 The biochar BET surface area also increased significantly with the increase in pyrolysis temperature (p < 0.05), indicating that higher pyrolysis temperatures led to biochars of higher specific surface areas. Similar results were observed elsewhere.4,11,19,25,26 More importantly, the pH of the biochars increased significantly (p < 0.05) from 7.4 to 9.7 and from 7.4 to 9.9, respectively, for the wheat straw and pine needle biochars when the pyrolysis temperature increased from 350 to 550 °C. This is due to increased alkalinity of biochars at a higher pyrolysis temperature (Figure S3), consistent with previous studies.10,11,36 Electrokinetic Properties and Sizes of Biochar MPs and NPs. The ζ-potentials of the biochar MPs and NPs, and sand grains, under different experimental conditions are shown in Table 1. The results indicated that both the biochars and quartz sand were negatively charged under tested experimental conditions, consistent with the existing literature.10,29,36,38,39 For quartz sand, the magnitude of the ζ-potential increased slightly with suspension pH due to dissociation of proton groups on the sand grain.32,40 For both biochar MPs and NPs, the magnitude of the ζ-potential decreased significantly with increased pyrolysis temperature (p < 0.05). This trend is probably due to reduced density of carboxylic and phenolic functional groups with increasing pyrolysis temperature (p < 0.05, Table S2). Similar results were observed elsewhere.10,26 Also, the NPs of both biochars were more negatively charged than the MPs, consistent with the ζ-potential results on fullerene (C60) particles at different particle sizes.41 For both ground biochar MPs and NPs, the DLS data shown in Table 1 indicate that their sizes reduced with increased pyrolysis temperature and that the particle size of the wheat straw biochar was larger than that of the pine needle biochar (W350 > W550 > P350 > P550). The TEM micrographs of biochar NPs (Figure 1) showed that most biochar colloidal particles were roughly spherical (except for the sheet and polyhedral W550 NPs, Figure 1b). These spherical biochar NPs were very similar to other black carbon NPs such as activated

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RESULTS AND DISCUSSION Properties of Biochars. Chemical compositions of the biochars at two different pyrolysis temperatures are shown in Table S1. With increasing pyrolysis temperature, the C content increased significantly for both wheat straw biochars and pine needle biochars (p < 0.05). However, there was no consistent trend regarding the change of N and H content with pyrolysis temperature. Except for the K content of wheat straw biochar and the Zn content of pine needle biochar, all other extractable

Figure 1. Representative TEM micrographs of W350 (a), W550 (b), P350 (c), and P550 (d) biochar NPs. C

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Figure 2. Measured (symbols) and fitted (lines) breakthrough curves (a, c) and retention profiles (b, d) for biochar MPs (a, b) and NPs (c, d), respectively, at a particle concentration of 200 mg L−1 in saturated sand-packed columns (pH = 6.8). Fitted curves were obtained using the two-site kinetic retention model. In b and d the normalized solid-phase concentration, S/C0, was plotted as a function of distance from the column inlet. Error bars represented the standard deviations in triplicate transport experiments.

Table 2. Mass Balance Percentages and Fitted Parameters of the Two-Site Kinetic Retention Model as Estimated from the Transport Data in Saturated Column Experiments biochar W350_MP W550_MP P350_MP P550_MP W350_NP W550_NP P350_NP P550_NP

Meffa,b,

and d

51.2 12.6 52.4 19.8 81.4 76.5 88.3 56.5

i

c a c b f e g d

%

Mreta,b,

and d

54.2 f 89.1 h 47.6 e 83.1 g 18.9 b 23.3 c 10.8 a 40.6d

%

Mret‑inletc % 62.2 80.3 60.0 73.8 46.6 53.3 41.3 57.4

e g de f b c a d

Mtota,b,

and d

105 102 100 103 100 99.8 99.1 97.1

%

k1e,f min−1 2.88 4.57 2.78 4.15 1.51 1.64 1.25 2.28

× × × × × × × ×

−1

10 10−1 10−1 10−1 10−1 10−1 10−1 10−1

k1de,f min−1 e g e f b c a d

5.23 9.25 5.97 8.82 3.93 5.16 4.64 5.73

× × × × × × × ×

−1

10 10−1 10−1 10−1 10−1 10−1 10−1 10−1

k2g min−1 c g e f a c b d

2.19 6.18 1.79 4.91 6.72 6.92 4.26 1.74

× × × × × × × ×

−1

10 10−1 10−1 10−1 10−2 10−2 10−2 10−1

e g d f b c a d

β

R2h

0.432 0.432 0.432 0.432 0.432 0.432 0.432 0.432

0.957 0.996 0.974 0.992 0.984 0.978 0.969 0.989

a,b, and d

Effluent, retained, and total percentages of biochar particles recovered from column tests, respectively. cBiochar particles retained near the column inlet (0−6 cm). e,fFirst-order retention and detachment coefficients on site 1, respectively. gFirst-order retention coefficient on site 2. h Squared Pearson’s correlation coefficient. iMean values in each column followed by the same lowercase letters are not significantly different using Tukey’s HSD test at p < 0.05.

carbon and soot42 due to the homology between the biochar and other black carbon (e.g., activated carbon, soot, and charcoal).43 The mean sizes of the biochar NPs (determined by TEM measurements) were in the following order: W350 > W550 > P350 > P550, consistent with the particle size measurement of bulk biochars determined by the particle size analyzer (Table S1). Effect of Pyrolysis Temperature on the Transport of Biochar MPs and NPs. Figure 2a and b present breakthrough curves (BTCs) and retention profiles (RPs) for biochar MPs, respectively, in quartz sand at a particle concentration of 200 mg L−1 and pH 6.8. Figure 2c and d present similar information for the biochar NPs. The BTCs are plotted as dimensionless concentrations (Ci/C0) of biochar particles as a function of PVs. The RPs are plotted as the normalized solid-phase concentration, S/C0, as a function of distance from the column inlet. The corresponding mass recoveries of biochar particles in the effluent and from the sand are shown in Table 2. Because virtually all (97.1−105%) of the biochar particles were

recovered, there is a high degree of confidence in the experimental measurements. Peak breakthrough of biochar particles was somewhat retarded, arriving at ∼1.35 PV. The BTCs were symmetrical and exhibited low degrees of tailing. In contrast with classical filtration theory44 that predicts exponential profile with depth, the RPs for biochar particles typically exhibited a hyperexponential shape with greater retention in the section adjacent to the column inlet (0−6 cm) and rapidly decreasing retention with depth (Figure 2b and d). The amount of biochar transport and retention was strongly dependent on the pyrolysis temperature and the particle size, which will be discussed below. The transport capacities of biochar MPs and NPs were both reduced significantly (p < 0.05) with an increase in pyrolysis temperature (Figure 2a and c). For instance, the total effluent mass recovery of wheat- and pine-derived biochars decreased significantly (p < 0.05) from 51.2% (W350_MP) to 12.6% (W550_MP) and from 52.4% (P350_MP) to 19.8% (P550_MP), respectively, as the pyrolysis temperature D

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increased from 350 to 550 °C (Table 2). A similar trend of decreasing effluent mass recovery with increasing pyrolysis temperature was observed for the biochar NPs (Table 2). This is due, in part, to the less negative ζ-potentials of biochars and increased acid−base attractions at a higher pyrolysis temperature (Table 1, Table S3, and Figure S4), which consequently decreased total repulsive interaction energies between biochar particles and sand grains (Table 1). The transport potential of biochar particles increased significantly (p < 0.05, Table 2) with decreasing particle size (i.e., from MPs to NPs). The total effluent mass recovery ranged from 56.5 to 88.3% for the NPs, whereas it varied from 12.6 to 52.4% for the MPs at an input concentration of 200 mg L−1. Similar transport behavior of enhanced transport with reducing particle size was also reported for other carbonaceous nanomaterials such as fullerene (C60).41 This is partly due to the decreased (more negative) ζ-potentials for the biochar NPs than those of MPs (Table 1). However, the total repulsive interaction energies for the MP−MP and MP−sand systems were nearly 1−2 orders of magnitude larger than those for the NP−NP and NP−sand systems (Table 1). Furthermore, mass transfer correlations for colloidal filtration theory45 predict a higher mass transfer coefficient for the biochar NPs than the biochar MPs. The biochar MP and NP transport results are therefore in contradiction with the DLVO predictions and the filtration theory. Hyperexponential RPs were strongly correlated with the amount of retention (i.e., inversely related to the recovered amount in the effluent). Consequently, hyperexponential RPs became more substantial at higher pyrolysis temperatures and for MPs. For example, approximately 62.2% of the retained particles occurred near the column inlet for W350_MP, whereas this value increased to 80.3% for W550_MP (p < 0.05, Table 2). A similar RP trend was observed for biochar MPs derived from pine needles. Conversely, the NPs of the pine needle biochar at a pyrolysis temperature of 350 °C exhibited a RP shape that was nearly exponential. The two-site kinetic retention model provided a good description for both BTCs and RPs of biochar MPs and NPs at the two different pyrolysis temperatures, judging from the high Pearson’s correlation coefficients (Table 2). Values of the firstorder retention coefficient on site 1 (k1) and especially the depth-dependent retention coefficient on site 2 (k2) increased significantly (p < 0.05) with increasing pyrolysis temperature and decreased with decreasing particle size (i.e., from MPs to NPs). These observations suggest that mechanisms controlling biochar particle retention near the column inlet were sensitive to pyrolysis temperature and particle size. Values of the firstorder detachment coefficient on site 1 (k1d) exhibited an inconsistent trend with pyrolysis temperature but were always greater than k1, suggesting that attachment was approaching linear equilibrium.46 XDLVO Interaction Energies. Temporal changes of biochar sizes in fresh MP and NP suspensions (200 mg L−1) were measured every 30 min for 2 days to evaluate their stability using DLS. The DLS results (Figure S5) showed that the sizes of biochar MPs and NPs did not vary in the tested conditions over 2 days. Consequently, the DLS sizes of biochar particles presented in Table 1 were used for the calculation of XDLVO interaction energies. Table S3 presents measured contact angles on biochars layers in the presence of air−water, air−glycerol, and air−n-decane. This contact angle information was subsequently used to

calculate the Lewis acid−base interaction energy per unit area, ΔGh0AB, as described in S3. The value of ΔGh0AB equaled −8.95, −18.7, and −25.7 mJ m−2 for W350, W550, and P550, respectively, suggesting that these three biochars were hydrophobic and the Lewis acid−base interactions between W350−, W550−, and P550−sand systems were attractive.47 For biochar P350, however, the value of ΔGh0AB was significantly (p < 0.05, Table S3) larger than the other biochars and equivalent to +3.34 mJ m−2, indicating a higher hydrophilicity than other biochars. The acid−base interaction for the P350-sand system was repulsive in nature (Figure S4). The existence of substantive energy barriers between colloids and sand grains for all conditions tested (Table 1) indicated unfavorable attachment environments. However, the energy barriers for colloid−colloid interactions were substantively less than those for colloid−sand interactions at the same tested condition. In general, the energy barrier decreased with increasing pyrolysis temperature due, in part, to increased (less negative) ζ-potentials of biochars and greater acid−base attractions between biochars and sand grains at a higher pyrolysis temperature. Repulsive acid−base interaction occurred for P350−sand system, whereas attractive acid−base interactions occurred in W350−, W550−, and P550−sand systems. Hence, the energy barrier for P350−sand was significantly larger than that for the other three biochars (p < 0.05, Table 1) and P350 possessed the highest mobility in this study (Figure 2). The energy barriers for biochar MPs were in the following order: P350_MP > W350_MP > P550_MP > W550_MP, which was identical to the transport capacity order (Figure 2). However, the XDLVO theory failed to explain why the biochar NPs were more mobile than MPs in this study. The energy barriers for MP−sand systems were 1−2 orders of magnitudes larger than those for NP−sand systems (Table 1) due to their larger particle size (eqs 5−9 in S3). The intrinsic mechanisms governing biochar MP and NP transport and retention will be discussed in greater detail below. Mechanisms of Biochars Retention. Hyperexponential RPs occurred during all the considered experimental conditions. A number of potential explanations for hyperexponential RPs have been proposed in the literature including secondary minimum,48−51 colloid aggregation,52−54 straining,34,55−58 and surface charge heterogeneities.50,59−61 Additional experiments were therefore conducted to better understand and quantify the mechanisms of biochar particle retention. The depths of the secondary minima (Φmin2) were quite shallow, i.e., only −0.02 to −0.5 kT (where k is the Boltzmann constant and T is the absolute temperature, Table 1). These minima were 3- to 75-fold smaller than the average thermal energy of the Brownian particles themselves (∼1.5 kT).48 Consequently, deposition in secondary minima was negligible. Additionally, the depths of the secondary minima were close among the biochar MPs and the biochar NPs, respectively, suggesting that the secondary minimum retention cannot explain the observed different mobility of the biochars (Figure 2). Recently, NP aggregation has increasingly been invoked to explain the hyperexponential RPs.52,53 However, in this study, aggregation of biochar MPs and NPs did not occur under tested conditions (Figure S5). Straining has been demonstrated to cause the hyperexponential RPs when the lowest threshold value of (dp)/(dc) (diameters of suspended particle to collector grain) is E

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Figure 3. ζ-potentials of W550_MPs (a) and NPs (b) in the influent, effluent, and retained at the column inlet at the concentration of 200 mg L−1. Error bars represent the standard deviations. Mean values in each column with the same lowercase letters above are not significantly different using Tukey’s HSD test at p < 0.05.

≥0.0020.55,58 For biochar NPs, the highest value of (dp)/(dc) was only 0.00025 (Table 1), indicating that straining was unlikely to have contributed to the biochar NP retention. For biochar MPs, however, the values of (dp)/(dc) ranged from 0.0028 to 0.0040, suggesting that straining of colloids may have played a role in the biochar MP retention. Considering the wide size distribution (e.g., size-heterogeneity)60 of biochar particles, especially the biochar MPs (Table 1), it is reasonable to investigate whether the surface charge heterogeneity within colloid population was a contributing cause for the hyperexponential RPs.50,59,61 Figure 3 presents measured ζ-potentials (i.e., surface charge heterogeneity) for W350 MPs and NPs in the influent, effluent, and retained at the column inlet, respectively, when C0 = 200 mg L−1. No significant difference (p < 0.05) existed for W350_NP ζ-potentials in the influent, effluent, and the retained portion (Figure 3b). Similar results were observed for W550, P350, and P550 NPs (data not shown). These results suggest that surface charge heterogeneity within the biochar NP population was insignificant in this study. For MPs, however, significant difference (p < 0.05) occurred for W350_MP ζ-potentials between the effluent and the retained portion. The ζ-potential of W350_MP was generally in the following order: retained > influent > effluent (Figure 3a). That is, the fraction of the MP population with less negative ζpotentials was preferentially retained at the column inlet, whereas the remaining MPs experienced a smaller retention and correspondingly greater transport potential, resulting in the observed hyperexponential RPs. The above information indicates that straining and surface charge heterogeneity within MP population contributed to the hyperexponential RPs as well as the lower mobility of biochar MPs. Additional research is needed to fully resolve the amounts of retention from these two retention mechanisms, but is beyond the scope of this work. Environmental Implications. The results of this study have significant implications for the land application of biochars. Biochars produced at higher pyrolysis temperature had much greater C content and lower mobility. Thus, it could be more beneficial to land-apply high temperature biochars because this will allow for greater C sequestration potential while minimizing the biochar transport away from the point of placement and into underlying groundwater. From environmental and agronomic points of view, the lower mobility of the biochar means that the biochar will remain in place at the soil profile and offer lasting benefits in improving soil quality and concurrently adsorbing environmental contaminants. What is more appealing is that the higher temperature biochars

generally have higher sorption capacity for a variety of contaminants (e.g., heavy metals, herbicides, PCBs, and PAHs) due to greater specific surface area, surface hydrophobicity, and microporosity, and they would be better used to neutralize acidic soils due to higher pH and alkalinity. Nonetheless, caution is warranted that a significant fraction of biochar particles are transported in saturated sand, especially for the biochar particles in nanometer scales. These biochar NPs could behave as vehicle to facilitate the transport of adsorbed contaminants, which requires particular attention because these NPs normally have greater contaminant sorption capacity. Similar caution is also warranted for other black carbon such as activated carbon, charcoal, and especially soot due to its nanoscale size comparable to the biochar NP.42 While this study is relevant to the transport of biochar in sandy soil or groundwater, its applicability in other types of soils is limited. Given the complexities and heterogeneities of natural soils, it is important to understand how variations in environmentally relevant concentrations of dissolved salts and organic matter can affect the fate and transport of biochar particles. These issues will need to be considered and are a topic worthy of future studies.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the procedures involved in the biochar characterizations, extended DLVO interaction, transport model, and mass balance of column experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-25-86881180; fax: +86-25-86881000; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly acknowledge Ronald W. Harvey (USGS, Boulder, CO), Scott A. Bradford (US Salinity Laboratory, Riverside, CA), and three anonymous reviewers for their valuable suggestions and review of this manuscript. This research was supported by the National Science Foundation of China (41125007) and the Knowledge Innovative Program of Chinese Academy of Sciences (Y112000016). F

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