Skip to main content
NCBI home page
EPA Author Manuscripts logoLink to EPA Author Manuscripts
. Author manuscript; available in PMC: 2021 Sep 1.
Published in final edited form as: Environ Sci Pollut Res Int. 2020 Jun 10;27(26):32842–32855. doi: 10.1007/s11356-020-09482-w

Surface heterogeneity mediated transport of hydrochar nanoparticles in heterogeneous porous media

Jing Yang 1, Ming Chen 1, Han Yang 1, Nan Xu 1, Gang Feng 1, Zuling Li 1, Chunming Su 2, Dengjun Wang 3
PMCID: PMC7520070  NIHMSID: NIHMS1627965  PMID: 32519110

Abstract

The effects of clay particles (montmorillonite, M) and phosphate (P) on the transport of hydrochar nanoparticles (NPs) in water-saturated porous media (uncoated and aluminum (Al) oxide-coated sands) were explored in NaCl (1–50 mM) solutions. Our results showed that the deposition behaviors of hydrochar NPs affected by M and phosphate were significantly different between pH 6.0 and pH 9.0, especially in Al oxide-coated sand. This can be attributed to their distinct surface characteristics: hydrochar agglomerates with a larger pore size distribution, more carboxylate groups, and less negative charges on the surface at pH 9.0 than those at pH 6.0. In Al oxide-coated sand, block adsorption of hydrochar was alleviated appreciably with the presence of M due to the preferential preoccupies of M on these favorable retention sites. On the contrary, M substantially increased the hydrochar retention on uncoated sand due to the formation of nanoaggregates between hydrochar and M. Differently, phosphate substantially enhanced the transport of hydrochar, even in coated sand, due to the strong phosphate adsorption onto Al oxide on the surface of sand and hydrochar. Our findings will provide useful insights into designing effective strategies for land application of hydrochar while minimizing potential environmental risks.

Keywords: Hydrochar nanoparticles, Transport, Surface heterogeneity, Montmorillonite, Phosphate, Heterogeneous porous media

Introduction

Hydrochar as one of carbonaceous materials (i.e., fullerene, active carbon, carbon nanotubes, graphene oxide, and biochar) is a carbonized material with nano- and micro-size (Sun et al. 2014), which is converted by biomass through hydrothermal carbonization (HTC) (Fang et al. 2018; Stemann et al. 2013). Hydrochar has been widely used for land application such as a slow-release N fertilizer and a low-cost adsorbent due to its high chemical and thermal stability, large specific surface area and pore volumes, and rich surface oxygen functional groups (Berge et al. 2011; Fang et al. 2015; Fang et al. 2018; Flora et al. 2013; Takaya et al. 2016). However, during the land application, the nano-scale hydrochar will inevitably migrate down along the soil profile into groundwater via infiltration, posing potential environmental risks because of its intrinsical heavy metals and other toxic compounds, and associated contaminants (Chen et al. 2019; Kambo and Dutta 2015). Therefore, a full understanding is needed to investigate the transport and retention of hydrochar nanoparticles (NPs) in the subsurface environment.

Over the past few years, the environmental impact, compatibility with agricultural and horticultural systems, and subcritical water treatment of NPs have received considerable attention (Abel et al. 2013; Gajic and Koch 2012; George et al. 2012). The fate and transport of carbonaceous NPs in saturated porous media have also attracted attention (Kasel et al. 2013; Wang et al. 2010b, 2013a, b). For instance, Fang et al. (2017) reported that surface oxidation could reduce the effluent recovery rates (mobility) of multi-walled carbon nanotubes in soils. Wang et al. (2013a, b, 2019) found that biochar NPs with the aging process, various particle sizes prepared at different pyrolysis temperatures, and methods showed the distinct transport characteristics in saturated porous media. The variability in the characteristics of carbon black NPs is expected to contribute to the different transport and retention behaviors in soil. As obviously distinct from carbon black NPs in terms of their physical and chemical properties (Kambo and Dutta 2015), hydrochar NPs may present the different transport behavior from those of carbon black NPs. Hydrochar has been reported to facilitate the transport of TiO2 and SiO2 NPs in porous media by serving as a transport vehicle/carrier (Liu et al. 2017; Xu et al. 2018). However, the transport and deposition of hydrochar NPs in soils are largely unknown (Huangfu et al. 2019).

Due to the complexity of soil, the transport and retention of carbonaceous NPs in soils can be altered by physicochemical properties like pH, ionic strength and type, natural organic matter, surface roughness, and clay contents (Chen et al. 2017; He et al. 2015; Lohwacharin et al. 2015; Tong et al. 2010). For example, the transport of biochar NPs reduced with decreasing the pH and increasing the ionic strength (Zhang et al. 2010). Fang et al. (2013a) reported that the mobility of carbon nanotubes varied with soils, which was found to be correlated positively to the average soil particle diameters and soil sand contents, while correlated negatively to soil clay contents. Specifically, various clays (i.e., montmorillonite, diatomite) showed the different effects on the transport of hydrochar NPs in homogeneous sand media (Huangfu et al. 2019). In addition, the type and composition of collector grains also affect the transport and retention of engineered NPs (Wang et al. 2012a). As known, the positively charged iron oxide coating on sand surfaces immobilized the negatively charged graphene oxide NPs in the primary minimum, yielding hyperexponential retention profiles particularly in Ca2+ (Wang et al. 2017). Currently, there is limited information in the transport of hydrochar NPs with clays in heterogeneous collectors. Additionally, the inter-forces between clay platelets in slurries can be weakened by phosphate adsorption (Teo et al. 2009). Therefore, studying the influence of clay and phosphate on the transportability of hydrochar NPs in heterogeneous collectors under environmental solution chemistry is necessary.

In this study, the transport of hydrochar NPs was systematically investigated in water-saturated sand with surface charge heterogeneity (uncoated and aluminum (Al) oxide-coated sand) in single and binary systems of phosphate and/or montmorillonite (M). The two-site kinetic attachment model (TSKAM) was employed to simulate the transport of the hydrochar NPs, which can be used to decipher the underlying mechanism controlling the retention of hydrochar NPs in Al oxide-coated sand with the effect of interaction between montmorillonite and phosphate.

Materials and methods

Preparation of the hydrochar NPs and characterization

The hydrochar NPs used in this study were synthesized using a hydrothermal method (Hu et al. 2010; Wang et al. 2010a). The specific synthesis methods were given as follows. Before use, the rice hull was cleaned with distilled water, dried at 110 °C, crushed, and then passed through a 60-mesh sieve. A 72% sulfuric acid was then applied to treat the rice hull. The resultant mixture was heated at 50 °C in a water bath. After the reaction, the supernatant was collected and diluted with deionized (DI) water, followed by stirring at 95 °C for 6 h. Finally, the deposited solid was collected and washed 3 times with DI water. The solids were obtained after drying at 120 °C in an oven. The hydrochar NPs used in the transport experiments (described below) were finally collected by grinding the solids into powders with an agate mortar.

Afterwards, the hydrochar NPs were suspended in NaCl electrolyte solutions, which were similar to the influents in the transport experiments (as described in the following section). A series of hydrochar samples were obtained for further analyses after drying under vacuum. The synthesized samples were then analyzed on a powder X-ray diffraction (XRD, D8-Focus, Bruker AXS Co., Ltd., Germany). The XRD patterns were obtained by a diffractometer equipped with a Cu Kα radiation source. The scanning range starts from 10 to 70° at a scanning speed of 2° min−1. The elemental composition of the hydrochar NPs was analyzed using an elemental microanalyzer (Vario Macro cube, Elementar Analysensysteme GmbH, Germany). The surface morphology of the samples was determined by using a scanning electron microscope (SEM; Quanta 400 FEG). The SEM images were taken for particles suspended in the electrolyte from the column influent after freeze-drying. The samples for transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-Twin) measurements were prepared with a thin carbon film. To determine the pore size distribution of hydrochar NPs, a mercury porosimeter (Auto Pore IV 9510, Micrometrics Instrument) was used for high-pressure mercury intrusion analysis (Liu et al. 2019). The injection pressure was increased from 0.5 up to 60,000 psi (413.68 MPa). The Fourier-transform infrared (FTIR) spectrometer (Nicolet iS10, Thermo Fisher Scientific, USA) was used to obtain the surface functional group properties of the hydrochar NPs.

Heterogeneous porous media transport experiments

Prior to use, quartz sands with sizes ranging from 580–620 μm were cleaned thoroughly as uncoated granular media for column studies. The Al oxide-coated sand was prepared using the modified method (Kuan et al. 1998). The detailed preparation procedures were provided in the Text S1 in Supplementary Information (SI). The hydrochar NP (0.2 g L−1) suspensions were prepared by adding the hydrochar NPs to electrolyte NaCl (1–50 mM) solutions, which contained single or binary 0.2 g L−1 M and/or 1.0 mM phosphate at pH values of 6.0 and 9.0. To avoid the precipitate of calcium phosphate, NaCl solution was chosen as an electrolyte in this study. The suspension pH was adjusted using a 0.1 M HCl or NaOH solution. Then, the hydrochar NP suspensions were homogenized by sonication for 30 min. The hydrochar NPs were found to be well suspended in 1 mM NaCl solution for 2 h without sedimentation.

Glass chromatography columns (2.5 cm inner diameter and 20 cm long) were evenly packed with the pretreated quartz sands (uncoated and Al oxide-coated sands) using a spatula and then covered with the 80-μm nylon net film at both ends. The pore volume and porosity of the packed sand columns were 52.0 mL and 0.530, respectively. Once packed, the column was flushed with DI water by using a peristaltic pump at a constant Darcy velocity (0.188 cm min−1) of 11.3 cm/h to saturate the sand for at least 24 h. The transport experiments were then conducted the following three steps. First, 5 pore volumes (PVs) of a NaCl solution at the desired concentrations were pumped into the sand columns in an upward mode. Second, another 5 PVs of the hydrochar NP suspension (shown above) were then injected into the column. Meanwhile, the outflow from the sand columns was collected using 10-mL glass tubes on a fraction collector at a specific time interval. Then, another 5 PVs of the NaCl background electrolyte were pumped into the sand column until no hydrochar was detected in the column effluent. The hydrochar NP suspension solutions in the presence or absence of M and/or phosphate were also injected into the sand columns at certain pH values. Finally, the hydrochar NP concentrations in the influent (C0) and effluent (C) were analyzed to obtain the breakthrough curves (BTCs) of C/C0 as a function of the PVs passing through the columns. A UV-vis spectrophotometer (UV-2450, Shimadzu Scientific Instrument, Japan) was used to measure the UV absorbance at 320 nm to determine the hydrochar NP concentration in solution based on pre-established calibration curves. All the transport experiments were performed in triplicate. After the completion of the column transport experiment, the retention profiles (RPs) of the hydrochar NPs retained in each column were obtained by determining the concentrations of hydrochar NPs in the retentates extracted from the dissection experiments (Wang et al. 2013a). Please note that Al oxide interferes the quantification of hydrochar NPs in the retentates by UV-vis measurement; only the spatial distribution of hydrochar NPs in the clean sand was performed. Additional descriptions of the physical conditions of the saturated sand columns are included in SI Table S1.

Hydrodynamic radius and zeta potential measurements

A series of hydrochar NP suspensions were prepared in NaCl solutions (1–50 mM) with M and/or phosphate at pH 6.0 and 9.0, which is similar to those prepared for the column experiments shown above. The samples were then sealed in a cup with plastic wrap and ultrasonicated for 30 min at room temperature (25 °C). The Zetasizer Nano ZS90 (Malvern Instruments Ltd., Malvern, UK) was employed to determine the hydrodynamic radius of the NPs. The zeta potentials of suspended NPs in solution were measured using dynamic light scattering (DLS) measurements. All measurements were run in triplicate, and the average values and standard deviations were reported.

Transport model

A one-dimensional form of the convection-dispersion equation based on the two-site kinetic attachment model (TSKAM) was applied to describe the transport of hydrochar NPs at the desired experimental conditions. The model parameters primarily including the first-order reversible attachment and detachment rate coefficients on site 1 (k1 and k1d), and the first-order irreversible retention coefficient on site 2 (k2), were numerically obtained, which were then used to explore the potential retention mechanisms of hydrochar NPs in water-saturated uncoated and Al oxide-coated sands under various experimental conditions. The mathematical model and the governing equations about the TSKAM were provided in SI Text S2 in detail.

Statistical analysis

The one-way ANOVA analysis was performed to identify statistically significant differences in measured parameters using Tukey’s honestly significant different (HSD) test. All statistical analyses were conducted using SPSS 17.0, and the differences of means were considered significant at p < 0.05.

Results and discussion

Surface characteristics of the synthesized hydrochar NPs

As shown in Fig. S1 in the SI, the XRD pattern of the synthesized material showed a broad diffraction peak from 10° ≤ 2θ ≤ 30°, indicating the amorphous structure (Wang et al. 2010a, b), which was consistent with the macroscopy of hydrochar NPs (Fig. 1a). The analyses obtained from the energy-dispersive X-ray (EDX) spectrum in Fig. S1 showed the C and O contents for the hydrochar NPs, consistent with the element content analysis results that showed that hydrochar NPs contains 59.35% carbon (C), 4.361% hydrogen (H), 35.39% oxygen (O), 0.254% nitrogen (N), and 0.547% sulfur (S). The TEM (Fig. S2a) showed the hydrochar NPs assembled to form roughly spherical particles with a size of approximately 80 nm × 80 nm. However, the SEM images present the obviously different pore sizes for the hydrochar agglomerates prepared by suspended in a NaCl electrolyte solution at pH values (6.0 and 9.0) and dried (Fig. 1 a vs. b), although similar morphologies were observed for these two samples. To further identify this observation, the macro-pore size distribution for hydrochar NP samples (suspended in the electrolyte at pH 6.0 and 9.0) was investigated by conducting the mercury intrusion porosimetry (MIP) analysis (Fig. 2). The differential pore size distribution (PSD) can be regarded as a function reflecting the main aspect of solid porosity. As shown in Fig. 2 a and b, the extrusion branches of capillary pressure curves were found to above the intrusion one and stick together, suggesting the adequate stiffness of the sample. Therefore, the pore size distribution data for the hydrochar sample (obtained at pH 6.0 and 9.0) as shown in Fig. 2 c and d were reliable. This function permits the comparison of samples with variable pore volume. As shown in Fig. 2c, more hydrochar aggregates obtained at pH 6.0 possess a pore size mainly around 21.4 μm. The pore size distribution of hydrochar aggregates for pH 9.0 mainly lied between 30.0 and 60.7 μm (Fig. 2d). These results indicated that the hydrochar agglomerated at pH 9.0 with a larger pore size than that at pH 6.0, which was consistent with SEM images (Fig. 1 a vs. b). The possible reason could be due to the different functional groups on the surface of hydrochar NPs at a different pH. This needs further analysis using FTIR.

Fig. 1.

Fig. 1

Open in a new tab

SEM images of hydrochar alone (a, b), with montmorillonite (c, d), and with a combination of montmorillonite and phosphate (e, f) and the corresponding EDX spectra (g, h) at a pH of 6.0 (a, c, e, g) and 9.0 (b, d, f, h)

Fig. 2.

Fig. 2

Open in a new tab

Capillary pressure curves (a, b) and pore size distribution determined by means of mercury intrusion porosimetry (MIP) (c, d) for the hydrochar samples prepared at pH 6.0 (a, c) and 9.0 (b, d)

The FTIR spectra of the hydrochar NP samples suspended in solutions with pH values of 6.0 and 9.0 were collected after the samples were dried (under vacuum). Substantial differences were observed between the FTIR spectra of the samples at the two-pH values, as shown in Fig. 3. Generally, the IR adsorption bands at ~ 1700 and 1210 cm−1 were characterized as representative of the hydrochar (Sevilla and Fuertes 2009). And, the IR bands at 3400 cm −1 were assigned to the O–H (hydroxyl or carboxyl) stretching vibrations (Liu et al. 2010). Additionally, the characteristic absorption peaks at 800 cm−1 were attributed to aromatic C–H out-of-plane bending; the presence of these peaks indicated that the hydrochar NPs contained large amounts of aromatic structures and phenol compounds at both pH values of 6.0 and 9.0 (Gu et al. 2016; Ibarra et al. 1996). The vibration bands at 1600 cm−1 and 1210 cm−1 are associated with the stretching of the C=C group and the OH deformations of the carboxyl-C, respectively (Pradhan and Sandle 1999). Noted that more carboxylic and hydroxyl groups were found at 1700 and 1210 cm−1 at pH 6.0 compared with these at pH 9.0. Meanwhile, at pH 9.0, new peaks appeared at 1566 and 1386 cm−1, which indicates the presence of more carboxylate groups (Zhang et al. 2014). Thus, the repulsive forces among hydrochar NPs at pH 9.0 could be stronger than those at pH 6.0. This result contributed to the larger pore size distribution of hydrochar aggregates at pH 9.0 than that at pH 6.0. Overall, the functional groups on the surface of hydrochar were different at pH 6.0 from those at pH 9.0.

Fig. 3.

Fig. 3

Open in a new tab

FTIR spectra of the synthesized hydrochar in solutions with different pH values

Zeta potentials of hydrochar NPs and sand under various conditions

The electrokinetic potentials of NPs are documented to significantly affect the transport and retention behaviors of colloids and NPs including biochar NPs (Wang et al. 2013a, b). The hydrochar NP zeta potentials (ZPs) in 10 mM NaCl solutions in the presence of M and/or phosphate versus pH were investigated. Figiue 4 a showed that the hydrochar had a negative charge in NaCl solutions over a wide pH range from 3.0 to 9.0. Interestingly, the negative potential of the hydrochar NPs increased as the pH was increased to 7.0. The negative potential then decreased in the pH range from 7.0 to 9.0, which might be attributed to less hydroxyl groups on the surface of hydrochar NPs associated with IR band at 1210 cm−1 at alkaline pH than that at acidic pH (Fig. 3). In the presence of phosphate, the hydrochar NP ZP became more negative due to phosphate adsorption (Chen et al. 2018), whereas the surface charge was slightly less negative in the presence of M due to the heterogeneity of the surface charge (Xu et al. 2019). However, the hydrochar ZP in the co-presence of M and phosphate was more negative than that of hydrochar NPs alone, which suggested that phosphate plays a predominant role compared with the M. As shown in Fig. 4a, the hydrochar NP ZP exhibited a pronounced change with the addition of phosphate and/or M at pH < 7.0; by contrast, a slight difference occurred at pH > 7.0. Notably, the ZPs of hydrochar NPs with M and/or phosphate under NaCl concentrations were more negative than that of uncoated and Al oxide-coated sands at pH 6.0 (Fig. 4b), whereas the opposite results were observed at pH 9.0 (Fig. 4c). As expected, an increase in the IS decreased the overall negative charge of all the particles. In addition, the coated sand was less negative than uncoated sand irrespective of the solution pH and IS. This result is attributed to the Al oxide normally being positively charged under ambient pH conditions (pH of point of zero charge; pHPZC ≈ 9.0) (Parks 1965). Overall, hydrochar NP electrokinetic properties were clearly different at pH < 7.0 and pH > 7.0, which implies distinct mechanisms and different transport behaviors in (un)coated sand at pH values of 6.0 and 9.0.

Fig. 4.

Fig. 4

Open in a new tab

Zeta potentials of hydrochar with and without montmorillonite (M) and/or phosphate (P), quartz sand, and aluminum oxide-coated sand in a 10 mM NaCl solution as a function of the pH (a) and the concentrations of NaCl solution at pH 6.0 (b) and 9.0 (c)

Hydrodynamic radius of hydrochar NPs under various conditions

As shown in Fig. 5, an increase in the IS and a low pH induced the slightly increase in the hydrodynamic radius (HR) of individual hydrochar NPs. Phosphate has been reported to adsorb on hydrochar (Cui et al. 2011; Yao et al. 2013). Nevertheless, the HRs of the hydrochar NPs showed negligible change upon the addition of phosphate at pH values of 6.0 and 9.0, which was consistent with the TEM images of hydrochar NPs with and without phosphate (Fig. S2a vs. 2b). This result suggests that the hydrochar NPs were relatively stable in an electrolyte solution. Due to the patch-wise surface heterogeneity of M lamellar particles (Tombacz and Szekeres 2006), the positively charged edges of M could be partially adhered to the negatively charged hydrochar NPs. Accordingly, a less negative surface charge was observed when hydrochar and M were both present in solution (Fig. 4). Thus, weaker electrostatic force occurred between the hydrochar and M, which are prone to form larger hydrochar-M heteroaggregates, especially in a 50 mM NaCl solution (Fig. 5). The size distribution diagram (Fig. S3 in the SI) showed that the particle size of individual M greatly increased as IS increased from 1 to 50 mM at pH 6.0 and 9.0. The TEM and SEM images in Fig. S2c and Fig. 1c indicate that the addition of M induced heteroaggregate formation between hydrochar and M compared with that observed for individual hydrochar (Fig. 1a) or M (Fig. S4).

Fig. 5.

Fig. 5

Open in a new tab

Hydrodynamic radius of hydrochar with and without montmorillonite (M) in the absence or presence of phosphate (P) under different NaCl concentrations at pH 6.0 (a) and 9.0 (b)

Hydrochar NP transport in uncoated and in Al oxide-coated sands

Hydrochar NP transport and retention in uncoated and in Al oxide-coated sands were investigated via column experiments. Breakthrough curves (BTCs) in uncoated sand were obtained, as shown in Fig. 6a and b. Correspondingly, the retention profiles (RPs) of individual hydrochar were included in Fig. S5 in the SI. In general, the C/C0 value in the hydrochar BTCs substantially decreased with increasing NaCl concentration at pH 6.0 as shown in Fig. 6a. This behavior was attributed to the increase in repulsion as the hydrochar NPs approached the sand surfaces, which was a result of the less negative charge of hydrochar NPs at higher ISs (Fig. 4a). This result agreed with the charge shielding effect and the compression of the surface double layer (Chen and Elimelech 2006). Furthermore, hydrochar NPs showed much higher transportability than biochar in quartz sand under a weak acid pH condition (Wang et al. 2013a). Note that the BTCs showed a typical blocking pattern in a high-IS (50 mM) solution at a pH of 6.0. However, the IS (< 50 mM) had a negligible effect on the transport, and a blocking phenomenon was not observed at pH > 7.0 (9.0), as shown in Fig. 6b. This result indicates the different transport behaviors at pH 6.0 and pH 9.0, which was mainly attributed to the distinct surface properties of hydrochar NPs at pH < 7.0 and pH > 7.0 (as discussed above), except that the negligible change HRs of individual hydrochar NPs in various ISs at pH 9.0 (Fig. 5b). The pore size distribution of agglomerated hydrochar NPs was relatively smaller at pH 6.0 from that at pH 9.0 (Fig. 1 a vs. b). This additional reason might also contribute to this difference in the transport of hydrochar NP suspension at acidic and alkaline pHs.

Fig. 6.

Fig. 6

Open in a new tab

Observed (dots) and fitted (lines) breakthrough curves (BTCs) of 0.2 g L−1 hydrochar under different NaCl concentrations in uncoated sand (a, b) and aluminum oxide-coated sand (c, d) at pH 6.0 (a, c) and 9.0 (b, d), respectively. The two-site kinetic attachment model (TSKAM) was employed to simulate the BTCs of hydrochar

The BTCs of hydrochar NPs in Al oxide-coated sand were present in Fig. 6 c and d. Notably, a typical blocking adsorption pattern occurred in BTCs of hydrochar NPs at both pH 6.0 and 9.0, especially at high pH. This observation indicated that a large number of hydrochar NPs initially deposited onto the Al oxide-coated surface at the early stage, subsequently reducing the available deposition for particles, and thus inducing the hydrochar NP transport at the later stage. At both pHs of 6.0 and 9.0, the influence of IS (< 50 mM) on the mass recovery of hydrochar NPs could be neglected. As shown in Table S2, when the IS increased from 1 to 50 mM, the C/C0 of hydrochar NPs was basically maintained at approximately 19% at pH 6.0 (exp. 1 to 3) and 45–46% at pH 9.0 (exp. 4 to 6). The hydrochar NPs showed high stability due to a slight change of HRs in 1–50 mM IS (Fig. 5), despite their ZPs reduced with the increase of IS (Fig. 4 b and c). Thus, the IS (< 50 mM) showed the subtle effect on the transport of hydrochar NPs in coated sand at pH 6.0 and 9.0 (Fig. 6 c and d). Particularly, Al oxide-coated sand was more positively charged than uncoated sand as shown in Fig. 4 b and c. Therefore, the negatively charged hydrochar NPs would be prone to adsorption onto coated sand. Wang et al. (2013b) have reported that the deposition of biochar NPs increased with increasing fractional surface coverage of an iron oxyhydroxide coating on sand grains. This is due to the increased electrostatic attraction between negatively charged NPs and positively charged iron oxyhydroxides. Nevertheless, hydrochar NPs showed a much stronger attachment in coated sand, which were slightly dependent on IS (< 50 mM) within the wide pH range, than biochar NPs in iron oxyhydroxide-coated sand at pH 6.9 (Wang et al. 2013b). This could be attributed to the distinct physiochemical properties and the surface characteristics of hydrochar NPs.

A TSKAM was applied to fit the hydrochar NP BTCs under all the experimental conditions. The fitted model parameters for uncoated and coated sands are provided in Tables S1 and S2, respectively. The TSKAM provided a good description of the BTCs (R2 > 0.912). First, the reversible coefficients (k1 and k1d) corresponding to site 1 were substantially greater than the irreversible coefficient (k2) on site 2 irrespective of the sand heterogeneity (uncoated or coated by Al oxide). Nonetheless, the hydrochar k1d/k1 values for (un)coated sands were generally less than 1. For example, the k1 for hydrochar alone in uncoated sand decreased from 1.625 to 0.841 min−1 and k1d decreased from 1.342 to 0.597 min−1 at pH 6.0 as the IS was increased from 1 to 50 mM (exp. 1 to 3, Table S1). These results suggest that the attachment forces between the hydrochar NPs and (un)coated sands were greater than the detachment forces and that the blocking on site 1 was strong. In addition, site 2 can be related to the surface roughness and surface charge heterogeneity of sand due to the irreversible attachment observed at these retention sites (Shellenberger and Logan 2002). The value of k2 at site 2 in uncoated sand increased with increasing IS (Table S1). The k2 value is proportional to the fraction of the sand surface area that favors attachment (Cai et al. 2014) and the concentration of electrolyte, in accordance with the greater transport of particles at lower ISs (Fang et al. 2013b; Wang et al. 2012b). Thus, a high IS strengthened the process governing the hydrochar NP retention on site 2. This result agrees with a similar linear relationship about NP transport as a function of IS (Chen et al. 2011; Chowdhury et al. 2011, 2012; Sun et al. 2015; Xu et al. 2017). However, the value of k2 in Al oxide-coated sand changed slightly with increasing IS (Table S2), consistent with the BTCs of hydrochar NPs in coated sand, as shown in Fig. 6 c and d. Notably, the k2 for hydrochar alone in coated sand was substantially greater than that in uncoated sand (Table S1 vs. S2). This result implies that the irreversible deposition of synthesized hydrochar NPs was markedly higher in coated sand than in uncoated sand.

Effect of montmorillonite on hydrochar NPs transport

The effect of M alone (single system) on the transport of hydrochar NPs was investigated. As previously discussed, nanoaggregates of hydrochar-M were formed in solution. The BTCs (Fig. 7 a and b) showed that the deposition of hydrochar-M nanoaggregates in uncoated sand depends on the change in IS. Moreover, in uncoated sand, the transport of hydrochar-M (Fig. 7a) was relatively lower than that of individual hydrochar in high IS (50 mM) at pH 6.0 and 9.0 (Fig. 6 a and b vs. 7 a and b). We attributed this result to hydrochar-M with less negatively charged in comparison with individual hydrochar (Fig. 4), which was easily entrapped in the uncoated sand and could further narrow and plug the pore throat (grain-to-grain contact) to trap more hydrochar-M nanoaggregates near the entry surfaces of the porous media (Chen et al. 2011; Phenrat et al. 2009). Similarly, M inhibited the transport of nTiO2 in the sand and destabilized nTiO2 at low pH (Guo et al. 2018; Zhou et al. 2012). However, the opposite trend, where the transport of hydrochar-M (Fig. 7 c and d) was greater than that of hydrochar alone (Fig. 6 c and d), occurred in Al oxide-coated sand. This result means that M facilitated hydrochar transport in coated sand. Furthermore, the value of C/C0 exhibited a greater change with various ISs, especially at acidic pH (Fig. 7c). At pH 6.0, 78.3% of hydrochar-M passed through the coated sand column in 1 mM NaCl, whereas only 6.1% in 50 mM (Fig. 7 c and d, exp. 13 vs. 15, Table S2). This behavior likely occurred because of competitive adsorption between M and hydrochar on the Al oxide-coating surface. Due to the surface charge heterogeneity, the negatively charged basal plane of M may preferentially adhere to the positively charged Al oxide and subsequently reduce the number of available deposition sites for hydrochar NPs. Thus, the retention of hydrochar NPs in the coated sand was decreased by the presence of M. Nevertheless, a high pH favored the transport of hydrochar-M in coated sand, consistent with the ZP results in Fig. 4c.

Fig. 7.

Fig. 7

Open in a new tab

Observed (dots) and fitted (lines) breakthrough curves (BTCs) of 0.2 g L−1 hydrochar with montmorillonite under different NaCl concentrations in uncoated sand (a, b) and aluminum oxide-coated sand (c, d) at pH 6.0 (a, c) and 9.0 (b, d), respectively. The two-site kinetic attachment model (TSKAM) was employed to simulate the BTCs of hydrochar

As expected, the k2 for hydrochar upon the addition of M increased for uncoated sand compared with that for hydrochar alone (exp. 13 vs. exp. 1, Table S1). The presence of M appeared to induce further deposition of hydrochar NPs due to the attachment forces being stronger than the detachment forces with uncoated sand. Nevertheless, the opposite trend for k2 change was observed in coated sand (exp. 13 vs. exp. 1, Table S2). This result indicates that the existence of M obviously changed the deposition force of hydrochar NPs in coated sand, consistent with the preferential adhesion of M to the Al oxide-coating surface. Moreover, the k2 decreased with increasing pH, thus inhibiting the retention of hydrochar NPs in coated sand at high pH.

Combined effects of montmorillonite and phosphate on hydrochar NPs transport

The combined influence of M and phosphate (binary system) on the transport of hydrochar NPs in uncoated and Al oxide-coated sands was examined in the NaCl solution. As previously discussed, nanoaggregates of hydrochar-M formed when they coexisted in solution. In the presence of phosphate, the transport of hydrochar-M was sensitive to the IS in uncoated and coated sands at both pH 6.0 and 9.0 (Fig. 8). For example, when the IS increased from 1 to 50 mM, the transport percentage in coated sand was reduced from 93.9 to 45.6% at pH 9.0 (Fig. 8d, exp. 22 to 24 in Table S2 in SI). Compared with the presence of M alone (Fig. 7), the hydrochar transport in the presence of both M and phosphate was slightly higher in various ISs over a wide pH range (Fig. 8), regardless of uncoated or coated sand. This result implies that phosphate might facilitate the transport of hydrochar-M. In addition, the k1d/k1 value for hydrochar-M in uncoated sand was 0.583, which was associated with a k2 value of 0.053 min−1 in a 50 mM solution at pH 6 (exp. 15, Table S1). In the presence of phosphate, the responding k1d/k1 and k2 of hydrochar-M in uncoated sand were 0.572 and 0.023 min−1 under the same conditions (exp. 21, Table S1), respectively. Apparently, the value of k1d/k1 increased and that of k2 decreased for hydrochar-M with phosphate. These results agree with the facilitated transport of hydrochar-M by phosphate (Fig. 8). This result might be due to the stronger repulsive forces between hydrochar-M and (un)coated sands upon the addition of phosphate, which are caused by more negative charges on the hydrochar-M (Fig. 4). Additionally, the values of k2 decreased slightly with the lower IS and higher pH, which indicates that the increase in IS or decrease in pH favors the restraint of the hydrochar NPs (even in the simultaneous M and phosphate system) near the column inlet. Notably, the deposition of hydrochar-M in the presence of phosphate onto coated sand in 50 mM NaCl solution at pH 9.0 was better than that at pH 6.0 whereas hydrochar-M exhibited better transportability at a lower IS (≤ 10 mM) at pH 9.0 than at pH 6.0.

Fig. 8.

Fig. 8

Open in a new tab

Observed (dots) and fitted (lines) breakthrough curves (BTCs) of 0.2 g L−1 hydrochar with montmorillonite under different NaCl concentrations in uncoated sand (a, b) and aluminum oxide-coated sand (c, d) in the presence of phosphate at pH 6.0 (a, c) and 9.0 (b, d), respectively. The two-site kinetic attachment model (TSKAM) was employed to simulate the BTCs of hydrochar

To better clarify the interactions between phosphate and hydrochar, the BTCs of hydrochar NPs with 1 mM phosphate alone in coated and uncoated sands at pH 6.0 and 9.0 are displayed in Fig. S6 in the SI. A careful comparison between Fig. 6 and Fig. S6 reveals that the presence of phosphate alone substantially enhanced hydrochar transport in coated and uncoated sands. This result is attributed to phosphate adsorption onto hydrochar NPs, which increased the negative charge on the surface (Fig. 4) and decrease the particle size (Fig. 5). This result agrees with that of a previous study that phosphate adsorption favors the transport of nTiO2 due to an increase in the repulsive force between NPs and sand (Chen et al. 2015; Liu et al. 2017). The k2 value of hydrochar NPs in 1 mM IS decreased from 0.012 to 0.010 after phosphate was added at pH 6.0 (cf. exp. 1 and exp. 7, Table S1), which indicates the weak and irreversible retention on site 2 of uncoated sand. Thus, the lower k2 can explain the facilitated transport of hydrochar with phosphate. In addition, a high pH favors the transport of hydrochar with phosphate due to the increase in the k1d/k1 value. For example, the k1d/k1 value for hydrochar with phosphate increased from 0.703 to 0.603 at 50 mM IS as the pH increased from 6.0 to 9.0 (cf. exp. 9 and exp. 12, Table S1). Similar behavior was observed for hydrochar with phosphate alone in coated sand. Particularly, the block adsorption of BTCs was more pronounced in coated sand than in uncoated sand due to the strong phosphate adsorption onto Al oxide on the surface of the sand.

Notably, the percentage of the injected hydrochar with phosphate alone through the coated sand columns was 85.9% in 50 mM NaCl solution at pH 9.0 (Fig. S6d) but only 45.4% of hydrochar with M alone was detected in effluent imder the same conditions (Fig. 7d, exp. 12 vs. exp. 18 in Table S2). This result indicates that phosphate played a predominant role in the influence on hydrochar transport in coated and uncoated sands compared with M. This result is attributed to the stronger repulsive forces between hydrochar and coated sand in the presence of phosphate than those between hydrochar and coated sand in the presence of M, which was a consequence of the more negative charge of hydrochar NPs with phosphate alone than with M alone at pH 9.0 (Fig. 4). Therefore, at the same IS level at alkaline pH, M induces more hydrochar retention than phosphate on both uncoated and coated sand. However, an abnormal phenomenon was observed in that, in coated sand at pH 6.0, hydrochar with phosphate exhibited stronger deposition than hydrochar with M. This result is attributed to the adsorption affinity of phosphate onto the Al oxide coating being stronger than that of M at pH 6.0 and the adsorption onto Al oxide at alkaline pH being negligible.

Furthermore, after the addition of phosphate in uncoated sand, the BTCs of hydrochar-M (Fig. 8 a and b) were slightly lower than that of hydrochar alone (Fig. S6a and b). This result is attributed to the less negative charge (Fig. 4) and slightly larger size (Fig. 5) of hydrochar with both M and phosphate compared with those of phosphate alone due to phosphate adsorption onto M. However, for coated sand in the presence of phosphate, the transport of hydrochar-M was greater than that of hydrochar alone at pH 6.0 (cf. Fig. 8c and Fig. S6c), whereas the opposite trend occurred at pH 9.0 (cf. Fig. 8d and Fig. 8S5d). For example, only 33.3% of hydrochar NPs broke through the coated sand columns in 50 mM NaCl solution at pH 6.0 in the single system of phosphate (Fig. S6c, exp. 9 in Table S2); by contrast, 16.4% of hydrochar was detected in the effluent in the binary system of both phosphate and M (Fig. S6c, exp. 21 in Table S2). In contrast, the transport percentage of hydrochar with combined P and M in coated sand in 50 mM NaCl solutions at pH 9.0 was 45.6%, whereas that with P alone was 85.9% (Figs. 8d and S5d, exp. 24 vs. exp. 12 in Table S2). The preferential attachment of M onto the Al oxide coating and phosphate adsorption onto the hydrochar may occiu simultaneously under acidic conditions (e.g., pH 6.0 in this study), thereby facilitating the transport of hydrochar in coated sand. The SEM images in Fig. 1 e and f show that the hydrochar in the presence of both M and phosphate exhibited a similar morphology as the hydrochar alone (Fig. 1 a and b) and the hydrochar with phosphate alone (Fig. 1 c and d), except that the surface became rougher and more irregular. Moreover, the corresponding EDX spectra in Fig. 1 g and h contain the peaks of Si, Ca, Al, and C, further supporting the coexistence of M and hydrochar at pH 6.0 and 9.0. However, the mass weight ratio of Si vs. C for hydrochar-M with phosphate was obviously higher at pH 9.0 than that at pH 6.0, which suggests that hydrochar-M aggregates contained a higher content of M than hydrochar at pH 9.0 and that the opposite result was observed at pH 6.0. These observations further prove that, in the presence of phosphate, hydrochar-M aggregates were prone to attach to coated sand at pH 6.0, whereas these aggregates exhibited higher transportability at pH 9.0 (cf. Fig. 8 c and d) because they contained a large amount of M at higher pH. Again, this phenomenon indicates that the addition of M enhanced the repulsive force between hydrochar-M and Al oxide-coated grains, especially at high pH.

Overall, the transportability of hydrochar in uncoated sand exhibits the order hydrochar with phosphate > hydrochar-M with phosphate > hydrochar alone > hydrochar-M. In coated sand, the transportability follows the order hydrochar-M with phosphate > hydrochar-M > hydrochar with phosphate > hydrochar alone at pH 6.0 and hydrochar with phosphate > hydrochar-M with phosphate > hydrochar-M > hydrochar alone at pH 9.0. The different transport behaviors are attributed to the combined mechanisms of phosphate adsorption on hydrochar or nanoaggregates between hydrochar and M and phosphate or M adsorption onto Al oxide coating due to heterogeneous porous media.

Conclusions

This study provided new insights into understanding the effects of M or/and phosphate on the transport of hydrochar NPs in heterogeneous porous media. Our findings demonstrate that hydrochar showed distinct mobility from other carbonaceous NPs in the sand. Particularly, hydrochar NP transport behavior differs on uncoated and aluminum oxide-coated sands at pH values of 6.0 and 9.0. This difference may be due to the distinct functional groups and characteristics of hydrochar surfaces at pH < 7.0 and pH > 7.0 and the heterogeneity of acceptors. In Al-coated sand, a stronger blocking adsorption pattern occurred for the hydrochar transport, especially at pH 9.0 compared with pH 6.0. The hydrochar deposition was decreased by the presence of phosphate and M. However, in uncoated sand, the hydrochar transport was compressed by the presence of M and facilitated by phosphate. Future work on the transport of hydrochar NPs in natural soil is needed to better understand the transport behaviors of hydrochar and to evaluate potential environmental risks. Our results herein provide in-depth insights into assessing the environmental exposure, potential risks, and ecological implications of hydrochar and for eventually developing regulations for such nanomaterials.

Supplementary Material

Supplementary Materia

Acknowledgments

Funding information The authors appreciate the research funding provided by the National Natural Science Foundation of China (21777110 and 21377090) and the Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment.

Footnotes

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11356-020-09482-w) contains supplementary material, which is available to authorized users.

References

  1. Abel S, Peters A, Trinks S, Schonsky H, Facklam M, Wessolek G (2013) Impact of biochar and hydrochar addition on water retention and water repellency of sandy soil. Geoderma 202-203(2): 183–191 [Google Scholar]
  2. Berge ND, Ro KS, Mao J, Flora JR, Chappell MA, Bae S (2011) Hydrothermal carbonization of municipal waste streams. Environ Sci Technol 45(13):5696–5703 [DOI] [PubMed] [Google Scholar]
  3. Cai L, Tong M, Wang X, Kim H (2014) Influence of clay particles on the transport and retention of titanium dioxide nanoparticles in quartz sand. Environ Sci Technol 48(13):7323–7332 [DOI] [PubMed] [Google Scholar]
  4. Chen KL, Elimelech M (2006) Aggregation and deposition kinetics of fullerene (C60) nanoparticles. Langmuir 22(26):10994–11001 [DOI] [PubMed] [Google Scholar]
  5. Chen G, Liu X, Su C (2011) Transport and retention of TiO2 rutile nanoparticles in saturated porous media under low-ionic-strength conditions: measurements and mechanisms. Langmuir 27(9):5393–5402 [DOI] [PubMed] [Google Scholar]
  6. Chen M, Xu N, Cao X, Zhou K, Chen Z, Wang Y, Liu C (2015) Facilitated transport of anatase titanium dioxides nanoparticles in the presence of phosphate in saturated sands. J Colloid Interface Sci 451:134–143 [DOI] [PubMed] [Google Scholar]
  7. Chen M, Wang D, Yang F, Xu X, Xu N, Cao X (2017) Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions. Environ Pollut 230:540–549 [DOI] [PubMed] [Google Scholar]
  8. Chen M, Alim N, Zhang Y, Xu N, Cao X (2018) Contrasting effects of biochar nanoparticles on the retention and transport of phosphorus in acidic and alkaline soils. Environ Pollut 239:562–570 [DOI] [PubMed] [Google Scholar]
  9. Chen M, Tao X, Wang D, Xu Z, Xu X, Hu X, Xu N, Cao X (2019) Facilitated transport of cadmium by biochar-Fe3O4 nanocomposites in water-saturated natural soils. Sci Total Environ 684:265–275 [DOI] [PubMed] [Google Scholar]
  10. Chowdhury I, Hong Y, Honda RJ, Walker SL (2011) Mechanisms of TiO2 nanoparticle transport in porous media: role of solution chemistry, nanoparticle concentration, and flowrate. J Colloid Interface Sci 360(2):548–555 [DOI] [PubMed] [Google Scholar]
  11. Chowdhury I, Cwiertny DM, Walker SL (2012) Combined factors influencing the aggregation and deposition of nano-TiO2 in the presence of humic acid and bacteria. Environ Sci Technol 46(13):6968–6976 [DOI] [PubMed] [Google Scholar]
  12. Cui HJ, Wang MK Fu ML, Ci E (2011) Enhancing phosphorus availability in phosphorus-fertilized zones by reducing phosphate adsorbed on ferrihydrite using rice straw-derived biochar. J Soils Sediments 11(7): 1135–1141 [Google Scholar]
  13. Fang J, Shan X, Wen B, Huang R (2013a) Mobility of TX100 suspended multiwalled carbon nanotubes (MWCNTs) and the facilitated transport of phenanthrene in real soil columns. Geodemra 207:1–7 [Google Scholar]
  14. Fang J, Xu MJ, Wang DJ, Wen B, Han JY (2013b) Modeling the transport of TiO2 nanoparticle aggregates in saturated and unsaturated granular media: effects of ionic strength and pH. Water Res 47(3):1399–1408 [DOI] [PubMed] [Google Scholar]
  15. Fang J, Gao B, Chen J, Zimmerman AR (2015) Hydrochars derived from plant biomass under various conditions: characterization and potential applications and impacts. Chern Eng J 267:253–259 [Google Scholar]
  16. Fang J, Wang M, Shen B, Zhang L, Lin D (2017) Distinguishable cotransport mechanisms of phenanthrene and oxytetracycline with oxidized-multiwalled carbon nanotubes through saturated soil and sediment columns: vehicle and competition effects. Water Res 108:271–279 [DOI] [PubMed] [Google Scholar]
  17. Fang J, Zhan L, Ok YS, Gao B (2018) Minireview of potential applications of hydrochar derived from hydrothermal carbonization of biomass. J Ind Eng Chem 57:15–21 [Google Scholar]
  18. Flora JFR, Lu X, Li L, Flora JRV, Berge ND (2013) The effects of alkalinity and acidity of process water and hydrochar washing on the adsorption of atrazine on hydrothermally produced hydrochar. Chemosphere 93(9):1989–1996 [DOI] [PubMed] [Google Scholar]
  19. Gajic A, Koch HJ (2012) Sugar beet (l.) growth reduction caused by hydrochar is related to nitrogen supply. J Environ Qual 41(4): 1067–1075 [DOI] [PubMed] [Google Scholar]
  20. George C, Wagner M, Kucke M, Rillig MC (2012) Divergent consequences of hydrochar in the plant-soil system: arbuscular mycorrhiza, nodulation, plant growth and soil aggregation effects. Appl Soil Ecol:68–72 [Google Scholar]
  21. Gu X, Guo Y, Zhang C, Ding L, Han X, Liu Q, Zou B (2016) The effect of phenolic compounds on the preparation of hydrochars from saccharides. Environ Prog Sustain Energy 35(1): 189–194 [Google Scholar]
  22. Guo P, Xu N, Li D, Huangfu X, Li Z (2018) Aggregation and transport of rutile titanium dioxide nanoparticles with montmorillonite and diatomite in the presence of phosphate in porous sand. Chemosphere 204:327–334 [DOI] [PubMed] [Google Scholar]
  23. He J, Li C, Wang D, Zhou D (2015) Biofilms and extracellular polymeric substances mediate the transport of graphene oxide nanoparticles in saturated porous media. J Hazard Mater 300:467–174 [DOI] [PubMed] [Google Scholar]
  24. Hu B, Wang K, Wu L, Yu S, Antonietti M, Titirici MM (2010) Engineering carbon materials from the hydrothermal carbonization process of biomass. Adv Mater 22(7):813–828 [DOI] [PubMed] [Google Scholar]
  25. Huangfu X, Xu N, Yang J, Yang H, Zhang M, Ye Z, Wang S, Chen J (2019) Transport and retention of hydrochar-diatomite nanoaggregates in water-saturated porous sand: effect of montmorillonite and phosphate at different ionic strengths and solution pH. Sci Total Environ 703:134487–134487 [DOI] [PubMed] [Google Scholar]
  26. Ibarra J, Munoz E, Moliner R (1996) FTIR study of the evolution of coal structure during the coalification process. Org Geochem 24(6):725–735 [Google Scholar]
  27. Kambo HS, Dutta A (2015) A comparative review of biochar and hydrochar in temrs of production, physico-chemical properties and applications. Renew Sust Energ Rev 45:359–378 [Google Scholar]
  28. Kasel D, Bradford SA, Simunek J, Heggen M, Vereecken H, Klumpp E (2013) Transport and retention of multi-walled carbon nanotubes in saturated porous media: effects of input concentration and grain size. Water Res 47(2):933–944 [DOI] [PubMed] [Google Scholar]
  29. Kuan WH, Lo SL, Wang MK, Lin CF (1998) Removal of Se (IV) and Se (VI) from water by aluminum-oxide-coated sand. Water Res 32(3):915–923 [Google Scholar]
  30. Liu Z, Zhang F, Wu J (2010) Characterization and application of chars produced from pinewood pyrolysis and hydrothermal treatment. Fuel 89(2):510–514 [Google Scholar]
  31. Liu C, Xu N, Feng G, Zhou D, Cheng X, Li Z (2017) Hydrochars and phosphate enhancing the transport of nanoparticle silica in saturated sands. Chemosphere 189:213–223 [DOI] [PubMed] [Google Scholar]
  32. Liu K, Ostadhassan M, Sun L, Zouc J, Yuan Y, Gentzis T, Zhang Y, Carvajal-Ortiz H, Rezaee R (2019) A comprehensive pore structure study of the Bakken Shale with SANS, N2 adsorption and mercury intrusion. Fuel 245:274–285 [Google Scholar]
  33. Lohwacharin J, Takizawa S, Punyapalakul R (2015) Carbon black retention in saturated natural soils: Effects of flow conditions, soil surface roughness and soil organic matter. Environ Pollut 205:131–138 [DOI] [PubMed] [Google Scholar]
  34. Parks GA (1965) The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems. Chern Rev 65(2): 177–198 [Google Scholar]
  35. Phenrat T, Kim HJ, Fagerlund F, Illangasekare T, Lowry GV (2009) Particle size distribution, concentration, and magnetic attraction affect transport of polymer-modified Fe(°) nanoparticles in sand columns. Environ Sci Technol 43(13):5079–5085 [DOI] [PubMed] [Google Scholar]
  36. Pradhan BK, Sandle NK (1999) Effect of different oxidizing agent treatments on the surface properties of activated carbons. Carbon 37(8): 1323–1332 [Google Scholar]
  37. Sevilla M, Fuertes AB (2009) The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 47(9):2281–2289 [Google Scholar]
  38. Shellenberger K, Logan BE (2002) Effect of molecular scale roughness of glass beads on colloidal and bacterial deposition. Environ Sci Technol 36(2):184–189 [DOI] [PubMed] [Google Scholar]
  39. Stemann J, Putschew A, Ziegler F (2013) Hydrothermal carbonization: process water characterization and effects of water recirculation. Bioresour Technol 143:139–146 [DOI] [PubMed] [Google Scholar]
  40. Sun Y, Yang G, Wen C, Zhang L, Wang YS (2014) Preparation of carbon sphere from lactose by hydrothermal reaction and its performance in gas separation. Environ Prog Sustain Energy 33(2):581–587 [Google Scholar]
  41. Sun P, Shijirbaatar A, Fang J, Owens G, Lin D, Zhang K (2015) Distinguishable transport behavior of zinc oxide nanoparticles in silica sand and soil columns. Sci Total Environ 505(505): 189–198 [DOI] [PubMed] [Google Scholar]
  42. Takaya CA, Fletcher LA, Singh S, Anyikude KU, Ross AB (2016) Phosphate and ammonium sorption capacity of biochar and hydrochar from different wastes. Chemosphere 145:518–527 [DOI] [PubMed] [Google Scholar]
  43. Teo J, Teo L, Liew WK, Leong YK (2009) Clay, phosphate adsorption, dispersion, and rheology. Water Air Soil Pollut Focus 9:403–407 [Google Scholar]
  44. Tombacz E, Szekeres M (2006) Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite. Appl Clay Sci 34:105–124 [Google Scholar]
  45. Tong M, Ding J, Shen Y, Zhu P (2010) Influence of biofilm on the transport of fullerene (C60) nanoparticles in porous media. Water Res 44(4): 1094–1103 [DOI] [PubMed] [Google Scholar]
  46. Wang L, Guo Y, Zhu Y, Li Y, Qu Y, Rong C, Wang Z (2010a) A new route for preparation of hydrochars from rice husk. Bioresour Technol 101 (24):9807–9810 [DOI] [PubMed] [Google Scholar]
  47. Wang Y, Li Y, Kim H, Walker SL, Abriola LM, Pennell KD (2010b) Transport and retention of fullerene nanoparticles in natural soils. J Environ Qual 39(6): 1925–1933 [DOI] [PubMed] [Google Scholar]
  48. Wang D, Bradford SA, Harvey RW, Gao B, Cang L, Zhou D (2012a) Humic acid facilitates the transport of ARS-labeled hydroxyapatite nanoparticles in iron oxyhydroxide-coated sand. Environ Sci Technol 46(5):2738–2745 [DOI] [PubMed] [Google Scholar]
  49. Wang D, Bradford SA, Harvey RW, Hao X, Zhou D (2012b) Transport of ARS-labeled hydroxyapatite nanoparticles in saturated granular media is influenced by surface charge variability even in the presence of humic acid. J Hazard Mater:170–176 [DOI] [PubMed] [Google Scholar]
  50. Wang D, Zhang W, Hao X, Zhou D (2013a) Transport of biochar particles in saturated granular media: effects of pyrolysis temperature and particle size. Environ Sci Technol 47(2):821–828 [DOI] [PubMed] [Google Scholar]
  51. Wang D, Zhang W, Zhou D (2013b) Antagonistic effects of humic acid and iron oxyhydroxide grain-coating on biochar nanoparticle transport in saturated sand. Environ Sci Technol 47:5154–5161 [DOI] [PubMed] [Google Scholar]
  52. Wang D, Shen C, Jin Y, Su C, Zhou D (2017) Role of solution chemistry in the retention and release of graphene oxide nanomaterials in uncoated and iron oxide-coated sand. Sci Total Environ 579:776–785 [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Wang Y, Zhang W, Shang J, Shen C, Joseph SD (2019) Chemical aging changed aggregation kinetics and transport of biochar colloids. Environ Sci Technol 53:8136–8146 [DOI] [PubMed] [Google Scholar]
  54. Xu X, Xu N, Cheng X, Guo P, Chen Z, Wang D (2017) Transport and aggregation of rutile titanium dioxide nanoparticles in saturated porous media in the presence of ammonium. Chemosphere 169:9–17 [DOI] [PubMed] [Google Scholar]
  55. Xu N, Cheng X, Zhou K, Xu X, Li Z, Chen J, Li D (2018) Facilitated transport of titanium dioxide nanoparticles via hydrochars in the presence of ammonium in saturated sands: Effects of pH, ionic strength, and ionic composition. Sci Total Environ 612:1348–1357 [DOI] [PubMed] [Google Scholar]
  56. Xu N, Huangfu X, Li Z, Wu Z, Li D, Zhang M (2019) Nanoaggregates of silica with kaolinite and montmorillonite: sedimentation and transport. Sci Total Environ 669:893–902 [DOI] [PubMed] [Google Scholar]
  57. Yao Y, Gao B, Chen J, Yang L (2013) Engineered biochar reclaiming phosphate from aqueous solutions: mechanisms and potential application as a slow-release fertilizer. Environ Sci Technol 47(15):8700–8708 [DOI] [PubMed] [Google Scholar]
  58. Zhang W, Niu J, Morales VL, Chen X, Hay AG, Lehmann J, Steenhuis TS (2010) Transport and retention of biochar particles in porous media: effect of pH, ionic strength, and particle size. Ecohydrology 3(4):497–508 [Google Scholar]
  59. Zhang J, Lin Q, Zhao X (2014) The hydrochar characters of municipal sewage sludge under different hydrothermal temperatures and durations. J Integr Agric 13(3):471–482 [Google Scholar]
  60. Zhou D, Abdel-Fattah AI, Keller AA (2012) Clay particles destabilize engineered nanoparticles in aqueous environments. Environ Sci Technol 46(14):7520–7526 [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary Materia
NIHMS1627965-supplement-Supplementary_Materia.doc (1.3MB, doc)

RESOURCES