William has been working on the issue of water retention in biochar for the last month and has published his research so far in the following paper.

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Evaluating Soaking Times on the Hydrophobicity of Biochar Using the Water Droplet Penetration Time Method

by William Stevenson (University of Amsterdam)

supervision by David Friese-Greene (The Soil Fertility Project)


Biochar can be used as a soil amendment to increase water retention capacity and improve overall fertility. A key component to achieving these beneficial properties is the water-absorbing potential. It is believed that biochars pyrolysed in the lower temperature range and dry-cooled may initially have hydrophobic properties. This study aims to assess the water-repellence of freshly produced biochar and evaluate soaking times that will render the biochar more water-absorbent. The water droplet penetration time test was employed as an indicator of wettability. It was found that water-quenching biochar post-pyrolysis inhibits the condensation of hydrophobic aliphatic tars onto pore surfaces. Another important discovery was that dry-cooled biochar is initially water-repellent but that water soaking can remove these undesirable hydrophobic properties on short time-scale (<8 hours).


Biochar can be used as a soil amendment to improve both nutrient and water availability to plants. In arid landscapes it is primarily this second factor, plant-available water, that will make life possible to flora. It has been shown that the integration of char into sandy soils can effectively aid plant growth and survival (Mulcahy et al., 2013). The biochar-soil interactions are sometimes complex and often case-specific and therefore further investigations are still required. One key factor to these interactions is whether the biochar is in fact able to absorb and retain water, or not. Previous research has shown that freshly made biochar may be hydrophobic (Briggs et al., 2012; Kinney et al. 2012; Abel et al., 2013; Gray et al. 2015), particularly that produced at low pyrolysis temperatures (<400 °C) and may therefore be unsuitable for soil application for the purpose of increasing water storage. However, through the process of soaking (or flooding) the biochar in water, it is believed that the initial hydrophobic properties of the biochar can be removed (Das & Sarmah, 2015). This investigation is concerned with testing this theory and ascertaining soaking durations for the biochar to become water-absorbent rather than water-repellent. 

The water drop penetration time (WDPT) is a useful method for assessing the wettability of substrates. It involves placing a drop of water on the substrate surface and recording the time taken for complete penetration (Doerr, 1998; Letey et al., 2000). If the drop does not penetrate immediately, the substrate can be considered hydrophobic and the time of penetration will be a measure of this hydrophobicity.

It is thought that the initial hydrophobicity of biochar is caused by aliphatic functional groups created from the condensation of volatile gases into tars onto pore surfaces during pyrolysis (Das & Sarmah, 2015). This seems to be particularly prevalent in lower temperature pyrolysis conditions (≈≤ 500 °C) since they do not volatilise and escape the carbonising biomass as readily as they do in higher pyrolysis conditions (Gray et al., 2014).

Following the removal of aliphatic compounds from pore surfaces (through soaking), biochar is no longer considered hydrophobic. It is also, per se, not hydrophilic by definition since it does not acquire (through new chemical groups) the ability to make hydrogen bonds with water molecules (Das & Sarmah, 2015). Rather it can be considered to gain greater affinity towards water due to the removal of aliphatic tars and the unclogging of its pores, thus enabling these to become water-absorbent. One commonly used term to describe the absorptive capacity of biochar is wettability. 

The effects of integrating biochar into soils are diverse and long-lasting. Figure 1 displays the immediate and long-term impacts on soil hydrology (Lehmann & Joseph, 2015). Key to the successful implementation of any biochar scheme lies in obtaining rapid positive results, such as improvements in crop size, health and yield. In a water-stressed environment this relates to an increase in plant-available water in the soil. Biochar is also known to decrease the infiltration rate in coarse-grained, sandy soils. It is therefore of key importance to widen our knowledge on soil hydrophobicity immediately post biochar amendment which is itself directly related to biochar hydrophobicity prior to soil integration. Figure 2 outlines the physicochemical characteristics of the biochar and the soil medium that effect soil hydrology (Lehmann & Joseph, 2015).



Determine the hydrophobicity of freshly made biochar (dry-cooled vs. quenched) and soaking durations to render the biochar less hydrophobic.

Research Questions

1 - How does the hydrophobicity of biochar that has been dry-cooled compare to that which has been water-quenched?

2 - What is the water-soaking duration necessary for biochar to lose its initial hydrophobic properties and become water-absorbent?


Freshly-made biochar that has been water-quenched following pyrolysis will not have hydrophobic properties whilst that which has been left to dry-cool under anoxic conditions will be water-repellent.

Dry-cooled biochar will lose these hydrophobic characteristics through soaking in water. It is expected that the longer the biochar is left to soak the more ‘water-absorbent’ it will become.


The experimental procedure can be broadly separated into two stages: a preparatory stage and a WDPT test (figure 3).


Pyrolysis was carried-out using a TLUD (Top-Lit-Upper-Draft) kiln with retort. The was made using a 12 L paint can as kiln and 6 L biscuit tin as retort (figure 4). Maximum Heat Treatment Temperature (MHTT) is unknown (to be measured on future burn) but assumed to be equal for each pyrolysis procedure.

Feedstock – Untreated, virgin willow tree (Salix alba) wood, dried and chopped into 5-8 cm long pieces (figure 5). The same wood was used as fuel.

Preparatory phase

Quenching is carried out by pouring water over the still hot freshly carbonised char until the retort is full and the char has been fully submerged. The biochar was left to soak for 10 minutes before draining and drying. Quenching is performed when volatile flammable gases leaving the retort are insufficient to maintain combustion.

Dry-cooling involves leaving the kiln for approximately 8 hours following the end of the pyrolysis/combustion process; the single hole on the underside of the retort is sufficiently small to prevent the entrance of oxygen for combustion.

Biochar is sometimes integrated into soils in pellet-form, more often however it is applied in powder-form. The biochar was therefore milled and sieved to < 2 mm, before being placed into small glass containers.

Soaking was performed by pouring relatively large volume of distilled water to the biochar containers. The contents were stirred thoroughly to ensure no biochar was left floating above water-level. A small portion (tea-spoon full) of biochar was removed before water-addition and following the times stated below. The soaked samples were oven-dried for 4 hours at 105 °C before initiation of the WDPT test.

WDPT tests were carried out at soaking times of 0, 0.5, 1, 2, 4, 8, 16, 24, 48 and 72 hours. 

Water droplet penetration time test

Figure 6 – Water droplet penetration time test with un-soaked biochar.

Figure 6 – Water droplet penetration time test with un-soaked biochar.

A simple yet effective hydrophobicity test is the water droplet penetration time (WDPT) test (figure 6). This involves measuring the penetration time of a distilled water droplet into the biochar. A 1 mm syringe is used to release a droplet from a height of < 10 mm, but not touching the surface, to reduce splash whilst ensuring a constant drop volume. Timing begins at the moment of water-biochar first contact and ends when fully submerged. (Doerr, 1998; Dekker et al., 2009; Herath et al., 2013; Hallin et al, 2015)





  • Pyrolysis: 3 ‘runs’ with dry-cooling and 1 ‘run’ with water-quenching
  • WDPT test: 2 water droplets per sample


In answer to the first research question, it was found that water-quenching fresh biochar immediately after pyrolysis disables (or inhibits) its hydrophobic properties; the WDPT test showed that the biochar (dried and milled) was immediately able to absorb water. On the other hand, the dry-cooled biochar was hydrophobic for the WDPT test before soaking.

The second research question is concerned with assessing soaking times necessary to remove the original water-repellence of dry-cooled biochar. The recorded data (figure 7) shows that this effect is very fast, if not quasi-immediate. Within 30 minutes, water droplet penetration times had dropped from above 35 minutes un-soaked, to below 5 minutes (figure 8). Although a penetration time of 12 minutes (S.E. 3) at 1 hour soaking time was recorded for sample ‘dry cooled 3’, this may have been caused by over-compaction or another unknown factor. What’s more, all samples had soaking times of less than 1 minute after only 4 hours.

Numbers in parentheses indicate the standard error of the mean. These all show a relatively low value (≤ 3.5) and thus indicate good precision of the data. However the study could have benefitted from a greater number of replicates.

One potential problem with the WDPT method in which long durations are involved (ie. before soaking) is that evaporation may decrease the drop size, thus reducing/falsifying penetration time (reference). However, due to the relatively short penetration times recorded in this investigation (except pre-soaking), this effect can be considered negligible.

Another commonly used method for measuring soil hydrophobicity is the molarity of an ethanol droplet (MED) technique. Known concentrations of standardized solutions of ethanol in water are dropped onto a substrate surface, following which their infiltration behaviour gives an indication of surface tension and hydrophobicity (Doerr, 1998). Although Kinney et al. (2012) deemed this method more suitable for pure biochar samples than the WDPT (due to the sometimes long penetration times involved), it was not employed in this investigation due to inherent difficulties in an out-of-lab setting.

Although the initial aliphatic functional groups are hydrophobic and detrimental to water retention capacity, they also act as nutrient exchange sites and may have positive effects on soil fertility (Das & Sarmah, 2015; Sun et al., 2011). Therefore biochar applied to water-rich, nutrient-poor soils could benefit from these aliphatic compounds. It is therefore important to determine which characteristics the amended soil would most benefit from. 

Biochars affinity towards water is determined by surface area, pore radius and aliphatic functional groups (Das & Sarmah, 2015; Gray, 2014), which in turn are functions of feedstock (type, particle size) and pyrolysis conditions (temperature).

It should be noted that very few studies exist on the wettability properties of biochar, and those which have tended to evaluate the hydrophobicity of biochar-soil mixtures (Herath et al 2013; Hallin et al, 2015). One recent study by Das & Sarmah (2015) however, examined the effect of water-flooding on biochar wettability. Here, a piece of grapevine biochar was subject to a constant stream of deionised water for 12 hours and analysed droplet contact angles using the WDPT test. They found that this treatment significantly decreased the initial hydrophobic behaviour (initial contact angle and penetration time).

Gray et al. (2014) employed a biochar post-production treatment involving exposure to saturated water vapour and found that its initial hydrophobic behaviour was lost after 7 days. They attributed this to water-filling of pore spaces and oxidation of pore surfaces.

Wettable biochars are also able to increase the water retention capacity of hydrophobic soils. Hallin et al. (2015) found that finely-ground biochar was significantly more effective at increasing the wettability of water-repellent coarse- and medium-textured soils than coarsely-ground biochar. 

This study can be extended and improved upon. It would be of interest for example to carry out the same hydrophobicity tests on other types of feedstocks (ie. other hardwoods, crop residues etc…). It would also be relevant to carry out an evaluation on the effect of pyrolysis temperature on hydrophobicity. Before biochar amendment can be implemented on large-scale, it is also necessary to carry out field trials in different textured soils and with different application rates


In a post-pyrolysis dry-cooling situation, the wettability potential of biochar is inhibited by hydrophobic tars on its pore surfaces. This study has shown that these aliphatic compounds can be quickly removed through milling and soaking the biochar in water. In contrast, it was observed that if the biochar is water-quenched immediately post-pyrolysis, dried and milled then it is able to absorb water without soaking. Rendering a biochar water-absorbent is clearly a desirable trait, particularly when used as a soil amendment in arid conditions in which soils are poorly able to retain humidity. The WDPT test showed that a soaking time of only 30 minutes was sufficient for a tenfold decrease in penetration times of two of the three samples, whilst the third experienced a relatively more gradual decrease. After 8 hours soaking time, all samples had penetration times of less than 1 minute. The results from this study are in accordance with those from the few other studies available in literature evaluating the wettability of biochar. It should be noted however that the hydrophobic behaviour of biochar alone does not give an indication of its water retention capacity (which is related to physical structure), nor can it predict the plant-available water in a soil-biochar mixture. It is however, a key factor to unleashing the full spectrum of the now well-established beneficial hydraulic properties of biochar amendment in water-stressed soils.


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