Impact of Biochar Particle Shape and Size on Saturated Hydraulic Properties of Soil

Introduction

The saturated hydraulic conductivity (K_sat) of soil is a function of soil texture, soil particle arrangement, clay content, organic matter content, soil aggregation, bioturbation, shrink-swelling, and overall soil structure (Hillel, 1998; Moutier et al., 2000; Vervoort and Cattle, 2003; West et al., 2008). The K_sat is one of the main physical properties that aid in predicting complex water movement and retention pathways through the soil profile (Keller et al., 2012; Quin et al., 2014). It is also widely used as a metric of soil physical quality (Reynolds et al., 2000).

It appears that the impact of biochar on the soil hydraulic properties is a complex interaction of the physical properties of soils and biochars. Several studies have reported that the incorporation of biochar to soil increased the K_sat (Herath et al., 2013; Moutier et al., 2000; Oguntunde et al., 2008), but other studies have observed decreased K_sat following biochar additions (Brockhoff et al., 2010; Uzoma et al., 2011; Githinji et al., 2014). In contrast, Barnes et al. (2014) found that biochar addition decreased by 92% the K_sat in sand, but increased K_sat by 328% in clay-rich soil. However, it is unclear which physical characteristics of biochar have the greatest effect on the transport and the interaction of water within the soil profile.

The shape and size of external biochar pores is a function of particle size and particle morphology. The biochar particle size and particle morphology depend on the shape of raw materials, which can be processed into a range of shapes from platy to spherical (Abel et al., 2013; Yargicoglu et al., 2014). The particle size of biochar also impacts the hydraulic conductivity due to the increase of tortuosity when it added to soil as biochar consists of mostly different particle size distributions (Lim et al., 2016). Those features could affect the pore distribution of soil after addition of biochar to soil and, the important flow characteristics like hydraulic conductivity (Sperry and Peirce, 1995; Lim et al, 2016).

Biochar porosity has been divided into micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm) based on internal diameter (Rouquerol et al., 1994; Rouquerol et al., 1999). However, Gray et al. (2014) reported this classification system does not adequately account for micrometer size pores that dominates soil water retention and transport (typically >15,000 nm).

Pores between 1 and 100 μm in diameter are regarded to contain the majority of pore volume within biochar (Sun et al., 2012; Gray et al., 2014). Hydraulic conductivity in saturation condition is significantly influenced by the presence of large macropores (>100 nm) (Bouma et al., 1977; Bouma, 1982). An interconnected network of macropores will facilitate the rapid downward movement of water through soil profile (Mallants et al., 1997; Vervoort and Cattle, 2003). For soil applications, the presence of macropores in biochar could also affect the hydraulic conductivity of soil.

This research is an investigation of the influence of the shape and the size of biochar particles on hydraulic conductivity. The objectives of this research were (1) in order to determine the influence of biochar particle size on the K_sat when two different types of biochar were added to coarse sand and (2) to determine whether course sand amended with spherical and non-spherical beads can be used to predict K_sat values of soils amended with biochar.

Materials and Methods

Sand

Coarse sand was obtained from QUIKCRETE Company (Atlanta, GA USA). Particle size distribution of coarse sand was determined by manual dry sieving of a 500 g subsample of homogenized sand. Dry sieving was performed using seven different sized sieves arranged in decreasing sizes of 4.0, 2.0, 1.0,0.5, 0.25, 0.1, and 0.05 mm and agitating for 10 min. The sand was 4% >2mm, 9% 1-2 mm, 39% 0.5-1 mm, 43% 0.25-0.5 mm, and 5% <0.25mm (median 0.5 mm) by weight.

Biochars and beads

The biochars used in the experiments were derived from pine wood chips (bark and limbs, Pinus ponderosa and P. banksiana), and macadamia nutshell (Macadamia integrifolia). Dry sieving 100 g of the biochars (10 min agitation) resulted in size fractions from 8 to 4 mm, from 4 to 2 mm, and <2 mm (Fig. 1). The average diameter of macadamia nutshell remaining on the 8 mm sieve size after sieving was 18 mm. Spherical and non-spherical beads (15.0, 8.0, 3.0, 1.0, and 0.1 mm) were purchased from various commercial companies (Fig. 2).

Preparation of columns

The two biochars and spherical or non-spherical beads were each combined at 5% volume per weight with coarse sand and thoroughly mixed to provide a homogeneous mixture. It should be noted, that we normalized the additions to volume, such that the density difference of beads and biochar would be minimized.

To determine the hydraulic conductivity, the coarse sand and biochar or bead mixtures were gently packed into a soil column (6 cm diameter×20 cm high) to approximately a 5 cm height with light tamping and vibration of the PVC column to eliminate any gaps and voids during packing. Four independent replicates of each soil treatment were prepared.

Saturated Hydraulic Conductivity

Saturated hydraulic conductivity (K_sat) was measured using a falling head method (Klute and Dirksen, 1986). A piece of filter paper was placed on the soil surface to minimize soil disturbance when filling with water. Tap water was gently poured into column until it was full (20 cm height of column) and hydraulic testing was performed after steady flow conditions were attained, usually after 3-4 repetitive flushing of the entire column. The average drop in hydraulic head over a known time period was used to calculate the K_sat value for each sample by the following equation (Klute and Dirksen, 1986):

where L is the length of the soil sample (3-6 cm), t is the time period (sec), ho is the initial height of water in the column referenced to the soil column outflow (cm), and hf is the final height of water also referenced to the soil outflow (cm). Since the diameters of the column and water column were equivalent these factors cancelled out from the equation.

Bulk density

The bulk density of each individual column was determined by dividing the known mass of the oven dried sample added to the columns by the measured sample volume. This soil volume measurement occurred immediately after the hydraulic conductivity assessments.

Statistical analysis

Averages and standard deviations of the quadruplicates were calculated. The one-way analysis of variance (ANOVA) is used to determine whether there were any significant differences between the means of the independent treatment groups (R Core Team, 1980).

Results and Discussion

Bulk density

Biochar had a statistically significant influence on the bulk density of coarse sand after amendment (p<0.05; Table 1). This decrease in bulk density following biochar incorporation has also been observed in other studies (e.g., Mukherjee et al., 2014; Pathan et al., 2003; Laird et al., 2010) and is expected due to the lower particle density of the biochar materials compared to soils (Laird et al., 2010; Brewer et al., 2014; Rogovska et al., 2014). The bulk density of the 6 to 13 mm non-spherical bead/sand mixture decreased similarly to the sand/biochar mixture; the 2.5 mm non-spherical bead/sand mixture is not different (Table 2). In contrast, the sand amended with spherical beads had a higher bulk density than that of the non-spherical bead/sand mixtures, likely linked to a possible improved packing and geometric packing arrangement.

Hydraulic Conductivity

The K_sat of coarse sand amended with spherical beads were significantly affected (p<0.05) as compared to the K_sat of coarse sand (Fig. 3). Increasing the size of spherical beads in the sand from 1.0 to 15 mm decreased the K_sat. For example, the K_sat values of coarse sand added with 1 mm and 15 mm spherical beads were decreased to 206, and 112 mm/h, respectively as compared to 229 mm/h for sand. Those reasons might be attributed to greater tortuosity and increased length of the water pathway following amendment of larger sized beads to sand as compared to the unamended sand, for which >85% of the particles were <1.0 mm, with a median size of 0.5 mm (McKeague et al., 1982; Kameyama et al., 2012). It should be noted for the case of 0.1 mm beads also reduced K_sat, possibly as the result of clogging existing pores. Keren et al. (1980) attributed the reduction in hydraulic conductivity of soils following gypsum additions was due to small gypsum particles mechanical plugging existing pores.

The K_sat was also significantly affected by nonspherical bead and its particle size representing what could be affected by particle shape. Those K_sat value showed similar patterns with the K_sat of biochars in the condition of 3 and 6 mm, corresponding intermediate sizes of from 2-4 mm and 4-8 mm, indicating do not following the K_sat curve of the spherical bead added into coarse sand. It might be attributed to similar non-spherical shapes as shown Fig. 1 and Fig. 2. It could be also inferred that it does not well match with those methods such as Campbell (1985), and Smettem and Bristow (1999), and Saxton et al. (1986) models in predicting of K_sat, presumably because application of biochar and the shape of biochar play an important role in predicting and modeling of K_sat.

In the soil treated with biochar, there are two possible theoretical pathways in water flow through soil profiles (Barnes et al., 2014). One is water migration through the pores within the biochar, the other is water migration through external space between sands or biochar-sand mixtures.

Firstly, there are potential water pathways through the pores within the biochar particle. The size distribution of pores of biochar appear a different range of from sub-nano (<1 nm) to macro (>50 nm), and their ability are well known for improving water holding capacity (Yu et al., 2006; Atkinson et al., 2010; Joseph et al., 2010). From soil capillary forces, a given height of water rise in a capillary column can be related to the pore radius by the following equation:

Where is the height of rise in the capillary column (pore, m), γ is the surface tension of water [@25℃=71.97 kg/sec²], θ_contact is the contact angle (assumed=0° rad), g is the acceleration due to gravity (9.8 m/sec²), p_water is the density of water (999.97 kg/m³), and r is the radius of the pore (m) (Hillel, 1998). Therefore, the largest pore that will be holding water at a soil moisture potential at the wilting point (-1500 kPa) is 0.2 μm. In other words, soil pores <200 nm are not of agronomic significance, since this soil moisture will not be plant or microbe available, as well as not contributing significantly to saturated water flow.

According to pore classification in relation to pore function, it is mentioned that pore sizes of between 0.005 and 0.5 μm are recognized as retention and diffusion ions in solutions as residual pores, and pore sizes of between 0.5 and 50 μm are capable of holding of water against gravity and release as storage pores, and pore sizes above 50 μm play a drainage in excess water as transmission pores (Lal and Shukla, 2004). In other words, only pore sizes above 50 μm play a critical role for water transport.

Gray et al. (2014) found that the sizes of biochar’s macropore were centered in the low micrometer range and Shaaban et al. (2013) reported average pore diameter for rubber wood sawdust biochar treated at 300, 500, and 700℃ was 7, 13, and 7 nm, respectively. All these median pore sizes are significantly below those pores theoretically available at the soil wilting point. Besides, Barnes et al. (2014) mentioned water pathway within biochar has greater tortuosity and the restriction of water flow due to the size of the smallest pore as well as the lack of interconnectivity. These facts justify the lack of significant impacts of biochar’s intra-particle pores in water transport.

Secondly, there is also water pathway through external spaces between sands or biochar-sand mixtures. The size of external pores counts on particle size, particle morphology, and compaction (Juang and Holtz, 1985; Gray et al., 2014). Bigelow et al. (2004) found that the K_sat values in coarse sand increased 6 times due to the higher presence of macro-porosity in the coarse sand (0.347 cm³/cm³) compared to fine sand (0.182 cm³/cm³), although the total porosity in fine sand, was higher in the fine sand (0.45 cm³/cm³ compared to 0.38 cm³/cm³). This result highlights the importance of pore size in regards to controlling K_sat. For example, according to Jong et al. (2007), the required time for water to move 30 mm through a channel with a 5 cm water head water was 200 sec for a 200 μm channel compared to 1,400 sec for a channel of 50 μm of diameter. Thereby, K_sat depends on the presence and proportion of macropores (50 μm) existing within sands or biochar-sand and meanwhile the water pathway through internal pores of biochar was largely restricted or had a little impact on the K_sat values.

Particle shape was also important parameter in ground-water flow such as hydraulic conductivity (Coelho et al., 1997). Sperry and Peirce (1995) reported porous media consisted of irregular particles showed lower hydraulic conductivity for the larger (700 to 840 μm) particles, though particle shape had no observable influence for the smaller (150 to 180 μm) particles on hydraulic conductivity. Those results could be extrapolated on the basis of two things. The first thing is that non-spherical bead might put in denser configurations than spherical bead, creating smaller pore passage sizes and greater tortuosity. The other thing is when beads pour into a column, the shape the bead affects the angle of repose. For example, Friedman and Robinson (2002) found while the minimum and maximum angle of glass beads were 22.1 and 23.1 degrees respectively, those of soil grains were between 34 and 37 degrees. In other word, the angle of repose is influenced by the shape and roughness of the particlesand is greater for non-spherical particles, which makes the K_sat increase (Sperry and Peirce 1995; Yun et al., 2005).

Conclusions

Saturated hydraulic conductivity (K_sat) is influenced by the particle size distribution and the shape of bead and biochar treated with coarse sand. The application of larger particles sizes compared to the median sand (0.5 mm) decreased K_sat, while non-spherical particles had a more significant impact on decreasing K_sat than the spherical counterparts. The application of smaller particles size materials also decreased the K_sat value, most likely due to the mechanical clogging of original water pathways. The downward migration of water through inner holes within biochar in the welldrained coarse sand had little impact on the K_sat value. This data also demonstrated that an important consideration for biochar additions in altering hydraulic properties is the overall particle morphology also must be considered. However, further research is needed to understand the duration of these effects.

Notes

The authors declare no conflict of interest.

Tables & Figures

Fig. 1. Photos by different particle sizes of Macadamia nutshell (a-d) and Pine chip (e-g) biochar used in this experiment: a) 18 mm, b) 8.0-4.0 mm, c) 4.0-2.0 mm, d) <2.0 mm, e) 8.0-4.0 mm, f) 4.0-2.0 mm, g) <2.0 mm.

Fig. 2. Photos by different particle sizes of spherical beads (a-e) and non-spherical beads (f-i): a) 15.0 mm, b) 8.0 mm, c) 3.0 mm, d) 1.0 mm, e) 0.1 mm, f) 13.0 mm (13.0×18.0×2.0 mm), g) 10.0 mm\(10.0×10.0×4.0 mm), h) 7.0 mm (7.0×7.0×2.0 mm), i) 6.0 mm (6.0×6.0×4.0 mm), j) 2.5 mm\(2.5×2.5×2.0 mm).

Table 1. The change of coarse sand bulk density after different size of biochar and bead were added to coarse sand

The values in parentheses are the standard deviation for the 4 assessments of the corresponding bulk densities. Values for the soil mixtures with the same letter are not statistically different from one another.

Fig. 3. The comparison of saturated hydraulic conductivity between biochar (pine chip and macadamia nutshell) and beads (spherical and non-spherical bead) treated with different particle sizes on the coarse sand.

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