The role of organic compounds in soil wettability

1
The role of organic compounds in soil
wettability
University of Wales Swansea UWS

• E. Urbanek, S.H. Doerr, R.A. Shakesby

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UWS key responsibilities

• Collection of soil physical parameters

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UWS key responsibilities

• Collection of soil physical parameters
• Chemical characterisation of soil samples
• (where appropriate)

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Characterisation of organic compounds

• Reason
• organic compounds can affect wettability
• (e.g. Wallach et al. 2005)

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Characterisation of organic compounds

• Reason
• a) by inducing water repellency
• (e.g. Wallach et al. 2005)

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Characterisation of organic compounds

• Reason
• a) by inducing water repellency
• (e.g. Wallach et al. 2005)

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Characterisation of organic compounds

• Previous EU project Water Repellent Soils
• Analysis of 19 (non-irrigated) samples from 5
  countries.
• Question which organic compounds cause
  repellency?

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Characterisation of organic compounds

• Published as Feature Paper in Austr. J. Soil
  Res.

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Characterisation of organic compounds

• Our questions in Water Reuse
• a) which compounds in waste water may cause
  repellency?
• (e.g. Wallach et al. 2005)

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Characterisation of organic compounds

• Our questions in Water Reuse
• a) which compounds in waste water may cause
  repellency?
• (e.g. Wallach et al. 2005)
• b) could organic compounds in soils increase
  wettability?
• (e.g. Graber et al. submitted)

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Characterisation of organic compounds

• Our questions in Water Reuse
• a) no samples yet available
• b) ? address this issue first at UWS

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b) How surface active compounds affect
infiltration
Infiltration increase into repellent soil by
decreasing surface tension of water
Surface tension of water 72.8 mN/m

• Surface tension decrease
• addition of alcohol, surfactants
• humic acid?
• soil organic matter?

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b) How surface active compounds affect
infiltration
Infiltration increase into repellent soil by
decreasing surface tension of water
Surface tension of water 72.8 mN/m

• Surface tension decrease
• addition of alcohol, surfactants
• humic acid?
• soil organic matter?

Waste water may cause repellency, but also
provides surface active substances that may
enhance wetting
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b) Role of surface active compounds in
infiltration
Questions for normal and irrigated soils

• Does soil wetting cause dilution of surface
  active substances?
• Does it change the surface tension?
• Does it change the infiltration?

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Preliminary soil sample analyses

• 33 soil samples of various origin
• Only one so far from waste water irrigated plots
  (UMH)

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Sample wettability Repellency Severity (CST) and
Persistence (WDPT)
1 10 100 1000 10,000
in seconds)
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Methods preparation of aqueous soil solution
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Methods Contact angle and surface tension
measurements
EasyDrop System (Krüss) DSA1 v 1.9 Drop Shape
Analysis Software
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Pendant drop surface tension measurements
Sessile drop contact angle measurements

• CA gt 90 water repellent
• CA lt 90 wettable
• (subcritical repellent)

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Results test solution with humic acid (HA)
Humic acid (key organic soil component) contains
surface active substances used here as natural
model material to observe the change in surface
tension
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Results surface tension of soil extracts

• no relationship of TOC content with surface
  tension
• even organic soils or soils after waste-water
  irrigation did not show much change in surface
  tension

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Results initial soil wettability and solution ST

• surface tension of soil solution independent of
  sample repellency

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Does only surface tension influence the
infiltration into the soil?
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Infiltration into the soil role of solution
chemistry
Solutions of different chemical structure alcohol
at 60 mN/m surface tension
strongly repellent

• no effect of chain length or change of linear
  alcohol to a keton on infiltration
• significant increase of infiltration with
  addition of aromatic alcohol

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Conclusions

• Surface tension of model solution water/humic
  acid lineally decreases with increased
  concentration of HA because of increased
  concentration of surface active materials

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Conclusions

• Surface tension of model solution water/humic
  acid lineally decreases with increased
  concentration of HA because of increased
  concentration of surface active materials
• Surface tension of soil extracts slightly
  decreases in some samples. (Does not depend on
  the TOC OR soil wettability) ? May depend on
  waste water type. ? May help to alleviate
  repellency.

?
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Conclusions

• Surface tension of model solution water/humic
  acid lineally decreases with increased
  concentration of HA because of increased
  concentration of surface active materials
• Surface tension of soil extracts slightly
  decreases in some samples. (Does not depend on
  the TOC OR soil wettability) ? May depend on
  waste water type. ? May help to alleviate
  repellency.
• No conclusion yet about WW-irrigated sample
  possible ? More samples needed.

?
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Conclusions

• The infiltration of liquid into the soil does not
  depend only on the surface tension chemical
  structure of added surface active material plays
  also very important role
• ? can also be tested by chemical analysis
  (GC-MS)

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Conclusions

• The infiltration of liquid into the soil does not
  depend only on the surface tension chemical
  structure of added surface active material plays
  also very important role
• ? can also be tested by chemical analysis
  (GC-MS)
• Samples for chemical analysis required to
  address
• cause of water repellency in waste water
  irrigated soils
• role of surface active substances in the wetting
  process

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• Thank you