Mixed in place permeability reductive layer through Al and OM precipitation

Abstract

The control of infiltration and seepage of
water is one ofthe most challenging tasks in water management and civil­-engineering
and, inan attempt to control this, methods for forming a water­impermeable
layer inthe soil have been widely practised in soil engineering (Laumann et
al., 2018;Proto et al., 2016). The use of natural processes to modify the
engineeringproperties of the subsurface could help to develop cost-­effective,
robust andsustainable engineering technologies and is attracting increasing
attentionfrom the industry (Zhou, 2020). This research aims to reduce the
permeabilityby using aluminium (Al) and organic matter (OM) precipitates mixed
ex­-situwith porous media to create a horizontal barrier. The Al­-OM
precipitates wereexpected to clog the pore space with a reduction in
permeability and hydraulicconductivity as result. To find out if it is feasible
to use Al­-OMprecipitates mixed ex­-situ with porous media for a permeability
reductivelayer, the Al­-OM precipitates and the permeability of the medium
wereresearched.  To characterise the
flocculation reaction, experiments onthe flocculation were performed. The yield
of the reaction was obtained byadding certain amounts of Al and OM solutions to
form particular amounts of drymass of flocs. This experiment confirmed the
hypothesis that 85% of the addedmass of OM will result in dry mass of flocs.
The concept of a critical metal tocarbon ratio (M/C­-ratio), indicating
flocculation regardless of the inputconcentrations, was tested by measuring the
pH over an increasing M/C-­ratio.From this titration curve, the found critical
M/C ratio is between 0.023­0.031,and the pH stabilises at a level lower than pH
4. This result proves theconcept behind the numerical scenarios describing the
titration of OM solutionswith Al3+, with a critical molar M/C ratio independent
of the inputconcentrations (Veerkamp, 2018; Zhou, 2020). After determining the
yield of thereaction and concluding that the concentration of Al and OM was not
ofinfluence, the by-products of the Al­-OM reaction were quantified. The
ionicstrength of the supernatant of an increasing density of flocs in solution
wasdetermined by measuring the electrical conductivity (EC). The results
showedthat the ionic strength increased linearly with an increasing density of
flocs.The relationship between the ionic strength and the density of flocs was
coupledto the linear relationship between concentrations potassium chloride
(KCl) andits EC. From the results, the measured EC can be used as a tracer
since the K+and Cl­ are non­reactive.  The
hydraulic conductivity measurements were conducted by afalling head test to be
able to make an indication of the change inpermeability when adding the Al­-OM
precipitates to the sand. To find theoptimal method to mix the Al, OM and porous
media, the influence of differentmethods of producing, adding and mixing the
materials on the permeabilityreduction is explored. In the first mixing method,
the Al and OM were added insolution, the solution containing Al-­OM
precipitates was centrifuged until thereduced ionic strength was at an EC value
less than 700 µm/cm. The hydraulicconductivity measurements were used to obtain
the relationship between thehydraulic conductivity reduction over an increasing
concentration of flocsretained by one kilogram of sand. The hydraulic
conductivity was reducedexponentially over an increasing concentration of flocs
up to a magnitude of 3.The results imply a large variability in the achieved
reduction dominated bythe amount of retained flocs. The second method is using
Al and OM in powderformat and adding them to the dry sand and adding 500 ml of
water to thismixture. This mixing method resulted in a completely different
floc structure.The flocs produced by mixing in solution have a shear dependency
feature, whiledry mixing created particles that have a constant size. For this
method, theincrease of the reduction is linear over an increase of
concentration of flocsretained by the soil. The highest reduction for this
method was found to be ofa magnitude of two, measured at 50 grams of flocs
retained by one kilogram ofsand. Finally, this research gave proof of principle
of using Al­-OMprecipitates mixed directly with sand could reduce the
permeability up to amagnitude of 3. These results present a new road to research
on this Al-­OM­-sandmixture’s strength parameters and compaction over time over
an increasing flocdensity, since these parameters are critical for using the
layer in practice.