Water repellency (hydrophobicity) of granular materials such as soil is usually assessed on bulk samples or arrays of grains, with their wettability being influenced by the often-variable properties of individual particles. Numerous methods exist to assess the wetting behaviour of granular bulk materials, whereas methods for determining the wettability of individual grains are scarce. Here we introduce a new technique, based on the Wilhelmy plate method and termed "micro-Wilhelmy plate method" (mWPM) that allows quantification of the water repellency of an individual particle. We developed two complementary variants of the methods, which involve the rupture of a water lamella after a particle has been brought into contact with water and then withdrawn from it. They were applied to individual wettable or water repellent spherical glass of diameter 120 and 270 mu m and polymer particles (270 mu m), as well as those of both wettable and natural water repellent particles (of similar size i.e. 120 to 270 mu m) from sandy soils of the UK and the Netherlands. Spherical glass and polymer particles were examined in their native condition and following treatment with a hydrophobic (silicone-based water proofing) agent. In one method the break point of lamella was determined gravimetrically (g-mWPM, using an electronic 5-digit balance) and in the other it was determined optically (o-mWPM) from a video sequence obtained from a contact angle goniometer as the distance between the particle and the water surface at the point of lamella rupture. The latter required the use of image analysis and computation to estimate the potential energy of the water lamella. Both methods provided meaningful assessments of particle wettability. Man-made particles showed limited variability, whereas those drawn from naturally wettable or water repellent soils exhibited substantial variability, indicating that wettable soils contain water repellent particles and vice versa. Both methods introduced here offer a relatively quick examination of the wettability of individual particles. The o-mWPM is a particularly simple method only requiring a video camera, stepper-motor driven sample holder and a low magnification optical system. Additionally, it offers the possibility to investigate particle shape.

Charcoals have long been used to adsorb organics from water and other substrates; we hypothesise that biochar may act in a similar way when mixed with soil, removing hydrophobic organic compounds from the soil surfaces. To test this hypothesis, we developed quantitative methods for addition of two hydrophobic organic compounds (octadecane and octadecanoic acid, commonly found in naturally hydrophobic soils) to, and their subsequent extraction from, acid washed sand (as a model for sandy soil). We then measured the quantity of the organic compounds which remained on the sand after: deposition; subsequent addition of 0, 1, 5, 10, 25 or 40 % wettable biochar; and storage for 1, 10, and 30days in solutions of pH3, 6 or 9. We found that there were small reductions in hydrophobic compound on sand with 1 and 5 % biochar additions, but that 10 % biochar removed ~50 %, and ≥25 % biochar removed ~100 %. The significance of these results in understanding the potential of wettable biochar to remove hydrophobic compounds from sandy soils, and thus act as an ameliorant of soil water repellency, is discussed.

Soil water retention curves are usually obtained on the assumption that soils are wettable. While this is true at high water contents, where pores are filled by water or a film of water covers the particles, it is less likely for drier states, where the adhesion of water menisci to the surface of the particles is controlled by the material chemistry and surface roughness, the presence of organic matter and the soil water content. Here, we present suction, water content and wettability measurements for model samples following drying paths. A comparison is done for a model material (sandy clay) in an initially wettable and sub-critical water repellent state (<90 degrees). Wettability was manipulated by treatment with organic acids which mimic the chemistry of natural water repellent substances. Suction was measured with a high suction tensiometer and wettability via contact angles by the sessile droplet method. The results revealed lower water retention and greater contact angles for the water repellent samples.

Soil water retention curves (SWRCs) are usually obtained on the assumption small water menisci spread in the surface of particles yielding contact angles near zero degrees. While this is true at high water contents, where pores are filled by water or a film of water covers the particles, it is less likely for drier states, where the adhesion of water menisci to the surface of the particles is controlled by the nature of the particle surface (chemistry and surface roughness) and the presence of organic matter. Here, we investigate hysteretic effects in the relation between water content and soil particle wettability versus suction for model samples following drying and wetting paths. A comparison is done for a model material (mixture of sand and clay) with and without water repellent substances, being the samples with water repellent substances in a subcritical water repellent state (contact angles <90°). Wettability was manipulated by treatment with organic acids that mimic the chemistry of natural water repellent substances. Suction was measured directly with a high suction tensiometer and wettability via contact angles by the sessile drop method. The results revealed lower water retention and greater contact angles for the subcritical water repellent samples, following both drying and wetting paths. Hysteresis was present in the relation between the contact angles and suction for the subcritical water repellent samples.

SummaryThe potential of biochar to ameliorate soil water repellency has not been widely studied. Previous studies have focused on the potential for biochar to induce or exacerbate existing water repellency rather than alleviate it. This study investigates the effect of adding wettable biochar to water‐repellent soil by comparing the water drop penetration times (WDPTs) of a control and biochar‐amended soil. The potential of wettable biochar to act as a physical amendment to water‐repellent soil was evaluated by mixing coarsely‐ground biochar (CGB, particle size range 250–2000 µm) or finely‐ground biochar (FGB, particle size range < 250 µm) with one strongly and one severely naturally water‐repellent soil in various quantities, and then measuring the WDPT for each mixture. When biochar particles did not fall within the size range of existing soil particles, an initial increase in both mean WDPT (WDPTM) and variation in WDPT was observed with small additions of biochar. These effects possibly result from increased surface roughness and inhibition of infiltration by the suspension of drops above the average soil–air interface at a few hydrophobic points. Both CGB and FGB reduced soil water repellency, FGB more effectively than CGB. The addition of 10% w/w FGB reduced soil WDPT by 50%, and 25% FGB eliminated repellency. Direct absorption of water by biochar and an increase in soil surface area in contact with water are the predominant physical mechanisms involved. This exploratory study suggests biochar has the potential to amend water‐repellent soil.

Pyrogenic carbon (PyC) produced during vegetation fires represents one of the most degradation resistant organic carbon pools and has important implications for the global carbon cycle. Its long-term fate in the environment and the processes leading to its degradation are the subject of much debate. Its consumption in subsequent fires is considered a potential major mechanism of abiotic PyC degradation; however, no quantitative data supporting this removal pathway have been published to date. To address this gap, we quantified consumption of residual PyC at the forest floor during an experimental fire, representative of a typical boreal wildfire, complemented by exploratory laboratory heating experiments. Labelled PyC (pinewood charcoal from a slash pile burn), in granular form contained in stainless steel mesh bags and as individual pieces, were placed at 2-cm depth within the forest floor. The median mass loss of granular charcoal was 6.6%, with 75% of the samples losing <15%, and of individual pieces 15.1% with 75% of the samples losing <25%. The mass losses under laboratory conditions, although somewhat higher than in the field, confirm an overall low consumption of PyC. The limited losses of PyC found here do not support the widely held notion that wildfire is a major cause of loss for residual PyC.

The water drop penetration time (WDPT) test is commonly used to evaluate the persistence of soil water repellency by placing water drops on the soil surface and recording the time to complete infiltration. Currently no standard protocol exists regarding the number or volume of drops applied, and often neither detail is provided when WDPT is reported. This study evaluated how mean WDPT (WDPTM) varies with drop volume and the number of drops required to obtain WDPTM within a given precision. Two naturally water-repellent soils were tested with 416 drops each of 15, 20, 80, and 200 μL. Wettable analogs of each soil were prepared and combined with the repellent soils to create various soil mixtures, to which 20- and 200-μL drops were applied. The WDPTM was found to vary significantly (α = 0.05) with drop number and volume, and the relationship likely depends largely on soil heterogeneity from variations in particle size, organic matter content, and the distribution of hydrophobic compounds within the soil. The results of this study support the following recommendations: (i) a reliable WDPTM (±10%) can be obtained with 95% confidence from 30 drops of 80- to 200-μL size (smaller drops can be used but will reflect microtopographical variability as heterogeneity increases); and (ii) water repellency class can be determined with 95% confidence from six drops and with 90% confidence from one drop, regardless of volume. Standard deviations should always be included to give some indication of the heterogeneity of water repellency within the soil.

Extreme soil water repellency can have substantial implications for soil hydrology, plant growth and erosion, including enhanced splash erosion caused by raindrop impact. Previous studies of water droplet impact behaviour on man‐made super‐hydrophobic surfaces, with which water‐repellent soil shares similar characteristics, revealed three distinct modes of splash behaviour (rebound, pinning and fragmentation) distinguished by two transition velocities: rebound‐to‐pinning (vmin) and pinning‐to‐fragmentation (v*). By using high‐speed videography of single water droplet impacts we show that splash behaviour is influenced by the hydrophobicity of immobile particles, with hydrophobic glass spheres exhibiting all three modes of splash behaviour in the hydrophobic state but hydrophilic spheres exhibiting solely pinning behaviour. We found that increasing the particle size of fixed glass spheres increases vmin. A study of droplet impact on hydrophobic sand shows that the increased roughness of the immobile particles makes impacting droplets more likely to fragment at slower impact velocities. The mobility of the particles influenced droplet impact behaviour, with loose, hydrophobic particles displaying significantly greater vmin values than their fixed analogues. The surface tension of the water droplet also lifted loose, hydrophobic particles from the surface, forming highly mobile ‘liquid marbles'. Water‐repellent soil was also shown to form ‘liquid marbles' at both the slow (approximately 0.3–2.1 m s−1) and fast (about 7 m s−1) droplet impact velocities studied. The observation of very mobile liquid marbles upon water droplet impact on water‐repellent soil is significant as this provided a mechanism that may enhance erosion rates of water‐repellent soil.

Environmental contextHeating of soils under wildfires can substantially reduce their ability to absorb rainfall, causing reduced vegetation recovery and increased erosion and flooding. This study examines, for the first time, the chemical changes in soil organic matter associated with heating in the oxygen-limited conditions typical under many wildfires. There was a noticeable tendency for production of non-polar compounds, which may ultimately contribute to a more persistent form of soil water repellency with important implications for managing fire affected terrain. AbstractSoil heating, as experienced during wildfires or management burns, can lead to extreme soil water repellency (WR). Previous work has focussed on the chemical composition of soil organic matter (SOM) that may be associated with WR in natural soil samples or samples heated in air. Under wildfires, however, oxygen supply is typically reduced and previous work has shown that the extreme WR induced under such conditions resists eventual destruction at temperatures ~200 °C higher than that of the same soil heated in air. This study examines, for the first time, the chemical changes in SOM associated with extreme WR following heating under oxygen limited conditions. Extracts obtained by accelerated solvent extraction (ASE), using mixtures of isopropyl alcohol/aqueous ammonia (IPA/NH3) and dichloromethane/methanol (DCM/MeOH), were analysed using gas chromatography–mass spectrometry (GC/MS). The data were compared with the SOM composition of the same soil unheated and following heating in air. In the absence of oxygen during soil heating, phthalic acid esters, substituted benzaldehydes, unsaturated amides and organophosphate esters were produced. In comparison with extracts of the same soil heated in air, there was a decreased methoxyphenol/phenol ratio, suggesting progressive demethoxylation and synthesis of new aromatic structures likely to promote extreme WR in soil.

Soil water repellency (SWR) is widely thought to be influenced by soil pH, however, few studies have systematically investigated the relationship between these variables. Specifically the hypothesis that the pH may control repellency via changes in the variable surface charge of soil material has not yet been tested. In previous work, methods for changing soil pH have also involved changes in soil moisture, but it has been argued that the potential influence of soil moisture changes needs to be eliminated before the actual relationship between pH and SWR can be isolated.The paper addresses this research gap using a new method, which enables adjustment of the pH of soils with low moisture content via the gas phase and thus involves minimal change in moisture content, allowing the response of SWR to pH changes to be evaluated. The method was applied to 14 soil samples from Germany, The Netherlands, the UK, and Australia, using the water drop penetration time (WDPT) as the indicator of SWR. Additionally, sessile drop contact angles (theta(ess)) were measured on the four samples from Germany and the titratable surface charge of these four soils was measured at selected pH values using a particle charge detector (PCD).Changes in SWR with soil pH were found to be influenced by the density and type of sites able to interact with protons at the available surfaces of organic and mineral materials in soil. The maximum SWR occurred for soil at natural pH and where the charge density was minimal. As pH increased, negative surface charge increased due to deprotonation of sites and WDPT decreased. Two types of behaviour were observed: i) a decrease in repellency with decreasing pH, probably because of a sufficient number of proton accepting surface sites with a significant amount of positive surface charge, ii) no decrease in repellency with decreasing pH in soils with insufficient proton accepting surface sites to develop significant positive surface charge. The data suggest that the availability and relative abundance of proton active sites at mineral surfaces, and those at organic functional groups influence the response of the soil samples to changes in pH. The variety of geographic origins and histories of the soils examined provides a distinction between site specific and more widely applicable soil characteristics of pH dependent SWR. (C) 2010 Elsevier B.V. All rights reserved.

SummaryThe structure and surface chemistry of soil particles has extensive impact on many bulk scale properties and processes of soil systems and consequently the environments that they support. There are a number of physiochemical mechanisms that operate at the nanoscale which affect the soil's capability to maintain native vegetation and crops; this includes soil hydrophobicity and the soil's capacity to hold water and nutrients. The present study used atomic force microscopy in a novel approach to provide unique insight into the nanoscale properties of natural soil particles that control the physiochemical interaction of material within the soil column. There have been few atomic force microscopy studies of soil, perhaps a reflection of the heterogeneous nature of the system. The present study adopted an imaging and force measurement research strategy that accounted for the heterogeneity and used model systems to aid interpretation. The surface roughness of natural soil particles increased with depth in the soil column a consequence of the attachment of organic material within the crevices of the soil particles. The roughness root mean square calculated from ten 25 μm2 images for five different soil particles from a Netherlands soil was 53.0 nm, 68.0 nm, 92.2 nm and 106.4 nm for the respective soil depths of 0–10 cm, 10–20 cm, 20–30 cm and 30–40 cm. A novel analysis method of atomic force microscopy phase images based on phase angle distribution across a surface was used to interpret the nanoscale distribution of organic material attached to natural and model soil particles. Phase angle distributions obtained from phase images of model surfaces were found to be bimodal, indicating multiple layers of material, which changed with the concentration of adsorbed humic acid. Phase angle distributions obtained from phase images of natural soil particles indicated a trend of decreasing surface coverage with increasing depth in the soil column. This was consistent with previous macroscopic determination of the proportions of organic material chemically extracted from bulk samples of the soils from which specimen particles were drawn. Interaction forces were measured between atomic force microscopy cantilever tips (Si3N4) and natural soil and model surfaces. Adhesion forces at humic acid free specimen surfaces (Av. 20.0 nN), which are primarily hydrophilic and whose interactions are subject to a significant contribution from the capillary forces, were found to be larger than those of specimen surfaces with adsorbed humic acid (Av. 6.5 nN). This suggests that adsorbed humic acid increased surface hydrophobicity. The magnitude and distribution of adhesion forces between atomic force microscopy tips and the natural particle surfaces was affected by both local surface roughness and the presence of adsorbed organic material. The present study has correlated nanoscale measurements with established macroscale methods of soil study. Thus, the research demonstrates that atomic force microscopy is an important addition to soil science that permits a multiscale analysis of the multifactorial phenomena of soil hydrophobicity and wetting.

The effects of variation in heating temperature T (50–300°C), heating duration (20–60 min), and post-heating equilibration times (24–168 h at 20°C and 50% relative humidity) on the wettability, as measured by the Critical Surface Tension (CST) method, of 4 initially water repellent soils from Canada, Portugal, and the UK are reported. All soils show an increase in water repellency following heating at temperatures in the range of 50 to 150°C, followed by a considerable decline after heating to 200–250°C, and, except for one soil, the eradication of repellency after heating to 300°C. For two soils with a comparatively high organic carbon content and fine texture, water repellency levels were also affected by the length of the post-heating equilibration period. The results demonstrate that (i) the common practice of heating samples to 105°C does not provide a viable standard procedure for the measurement of water repellency as it may alter repellency to different degrees, and (ii) where heat treatment is required, a post-heating equilibration time of 24 h is not necessarily sufficient for sample repellency levels to adjust to atmospheric laboratory conditions; therefore it is advisable to prolong equilibration to at least one week prior to measurement.

Adverse effects of soil water repellency (hydrophobicity) are of concern during or following rainfall or irrigation, and are often preceded by conditions of high atmospheric relative humidity (RH). Assessments of repellency are, however, commonly conducted on air‐dried samples at ambient laboratory conditions. This study explores the effects of differing antecedent RHs (32–98%) on the water repellency of air‐dried soils of wide ranging characteristics under laboratory conditions using water drop penetration time (WDPT) and ethanol‐percentage tests. Most samples exhibited considerably higher water repellency after exposure (<1 d) to 98% RH compared with lower RHs, typical of ambient laboratory conditions. This work suggests that previous studies may have incorrectly classified some soils, likely to exhibit water repellency in the field, as wettable, and that tests carried out following exposure of samples to high RH provide assessments that best reflect critical field conditions.

Adverse effects of soil water repellency (hydrophobicity) are of concern during or following rainfall or irrigation, and are often preceded by conditions of high atmospheric relative humidity (RH). Assessments of repellency are, however, commonly conducted on air-dried samples at ambient laboratory conditions. This study explores the effects of differing antecedent RHs (32-98%) on the water repellency of air-dried soils of wide ranging characteristics under laboratory conditions using water drop penetration time (WDPT) and ethanol-percentage tests. Most samples exhibited considerably higher water repellency after exposure (<1 d) to 98% RH compared with lower RHs, typical of ambient laboratory conditions. This work suggests that previous studies may have incorrectly classified some soils, likely, to exhibit water repellency in the field, as wettable, and that tests carried out following exposure of samples to high RH provide assessments that best reflect critical field conditions.

A scheme for the separation, purification and preparation of sub-micron, homoionic, Na-attapulgite from a natural mineral deposit is presented together with represen-tative analysis of the particle size distribution. Transmission electron microscopic examination indicated that particles were predominantly <1μm long and “lath-like” with aspect ratios (length:width:thickness) 100:3:1 which provide for a variety of modes of particle/particle interaction.

Analytical surface charge (σa), electrophoretic, and physicochemical properties of an homoionic form of Na–attapulgite show some similarity with those exhibited by homoionic sodium forms of the platy clay minerals kaolinite and illite. Both electrophoretic mobility (ue) and analytic or titratable surface charge show dependency on pH and electrolyte concentration (c). Transmission electron microscopic examination indicated that particles were 1000 nm long, lath-like, and of aspect ratios (length:width:thickness) 100:3:1. Estimates of (hydrodynamic) equivalent spherical particle diameter (dse), made with photon correlation spectroscopy (PCS), were found to be ∼400 nm and reflect extreme nonisometric particle shape. Failure of PCS at pH < 6.4 suggests the onset of colloidal instability in this region. The correlation betweenueand σawas found to be approximately linear in the range 3 < pH < 6.5 with more complex behavior at pH > 6.5. Apportionment of titratable charge to particle edges suggests a charge site density that is reasonable for the structure of this clay mineral.

Electrochemical analyses of two illitic clay minerals (Fithian and Muloorina illite) are presented and show these minerals to have similar surface chemistries. Aqueous acid/base potentiometric titration of suspensions of these clays, both prior to and after conversion to fully homoionic forms, indicates that analytic or titratable surface charge density is particularly sensitive to surface preparation. Although hydrogen and hydroxyl ion adsorption/desorption are mechanisms responsible for the development of some surface charge on the particles, the electrolyte cation also exerts a significant influence. If surface sites are assumed to be evenly distributed over the particles then the changes in surface charge densities, over the pH range 3–10, are small (<0.2 C m−2; especially so for the Muloorina mineral) compared with those found for homogeneous oxides such as silica or alumina (>0.5 C m−2), over a similar range in pH. Even if this titratable charge were localized on the edges of these clay particles, estimates of changes in the local surface charge density for the Muloorina illite are still comparatively small. Titration of suspensions at various ionic strengths produced families of titration curves which show points of inflection within the range 3 < pH < 10. Curves for incompletely prepared homoionic sodium Fithian illite possessed a common intersection point which disappeared after further preparation. No common intersection points were observed in the titration curves of Muloorina illite. No point of zero charge, independent of electrolyte concentration, can therefore be unequivocally identified for the particle edges. Interference from cation exchange reactions involving exposed interlayer sites located at or near the particle edges is probably responsible for these effects.

The electrophoretic properties of colloidal particles sampled from suspended and deposited sediments of an ice-contact proglacial lake were examined as a function of solution composition and particle size. The finest size fraction of the deposited sediment, which is heavily dominated by mica, provided enough material for surface charge density determination.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTInfluence of sorption processes on aluminum determinations in acidic watersXabier. Goenaga, Robert. Bryant, and David J. A. WilliamsCite this: Anal. Chem. 1987, 59, 22, 2673–2678Publication Date (Print):November 15, 1987Publication History Published online1 May 2002Published inissue 15 November 1987https://pubs.acs.org/doi/10.1021/ac00149a010https://doi.org/10.1021/ac00149a010research-articleACS PublicationsRequest reuse permissionsArticle Views50Altmetric-Citations24LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts