Controlling subsurface flow is crucial to prevent: the spreading of contaminants, the failure of dikes or any undesirable water flow. Subsurface flow can be controlled by changing the soil permeability. In order to change soil permeability rigid structures are often installed (for example the installation of sheet pile walls into dike bodies), however, these solutions are costly and disrupt the environment. Alternatively, the SoSEAL project aims to provide an in-situ solution to reduce the permeability of highly permeable soil layers in an ecological friendly and cost-effective way. The SoSEAL project is inspired by podzolization, which is a soil formation process. It reduces soil permeability via the precipitation of metal-organic matter complexes in the pore space. During earlier practices a need has arisen for a cheap, environmentally friendly and naturally available aluminum source. Accordingly, the goal of this research is to investigate if gibbsite can be used as the aluminum source to form metal-organic matter precipitates.

A literature study was conducted to investigate the interplay between gibbsite and dissolved organic matter (DOM). Four processes are primarily studied in this research, they are: the release of free Al via dissolution of gibbsite; the complexation between free Al and DOM; the protonation and subsequently precipitation of DOM and the adsorption of H⁺/DOM onto the gibbsite surface. Thereafter the following hypotheses were formulated:

•The extent of gibbsite dissolution is larger at low pH. However, due to its low solubility and slow dissolution kinetics, a low concentration of free Al is expected.
•The organic matter (OM) source used in this study is a humic acid, therefore it is expected to become completely insoluble when the pH is below two.
•The contradicting pH dependency of gibbsite dissolution and OM solubility leads a narrow transitional pH range, that is favourable for the occurrence of complexation between free Al and DOM.

These hypotheses are tested by performing experiments in the laboratory, where the pH varies from 2 to 8 and the aluminum/carbon (mol/mol) ratio is 0; 0.05; 0.1; 0.3 and 1. A synthesized crystalline gibbsite powder and a potassium humate are used.
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The results are subdivided into three pH ranges in which similar behaviour was found. In the low pH range (pH = 2 - 2.5), the protonation and subsequently precipitation of DOM is the dominant process in short term. However, when considering the long term behaviour, gibbsite dissolution is the dominant process as the DOM has precipitated. Due to the difference in time scale, there is very little complexation between free Al and DOM. In the transitional pH range (pH = 3 - 3.5), the competition for H⁺ between DOM and gibbsite is profound. This increases the solubility of DOM and slows the dissolution of gibbsite down. As a consequence, this favours the gradual complexation between free Al and DOM. Nevertheless, due to the ambiguity of free Al measurement method used in this research, it is difficult to quantify the complexation. In neutral to high pH range (pH = 4 - 8) DOM undergoes protonation over time but stays soluble. The release of free Al via the dissolution of gibbsite is negligible in this pH range. Therefore complexation between free Al and DOM should not be expected.

The results presented in this research indicate that gibbsite may not be suitable as the aluminum source to form precipitates with organic matter in engineering solutions that require fast soil permeability reduction. However, the slow release of free Al from gibbsite in combination with the gradual formation of precipitates with organic matter might be interesting for robust problems that require a self-healing ability. For efficiency concern, another OM source that is rich in fulvic acids (higher solubility at low pH than humic acid) and a natural gibbsite source (higher solubility than synthesized gibbsite) should be investigated.

Leakage of contaminants to the subsurface is a potential threat to groundwater and is a major problem with storage tanks. When the contaminant reaches the groundwater, it can be subjected to subsurface flow due to which a contaminant plume may form and the contaminant can spread. In this research, it was investigated if a horizontal layer of reduced permeability can be engineered by using the SoSEAL (Soil Sealing by Enhanced Aluminium and DOM Leaching) concept to prevent leakage. Within SoSEAL, Aluminium-Dissolved Organic Matter complexation is used to clog the pore space with a reduction in permeability and hydraulic conductivity as a result. The concept is based on the natural process of podsolization in which a layer of reduced permeability (a hardpan) is formed by decades of organic matter leaching to the subsurface while complexing with metal cations. In this research, the complexation reaction using Aluminium-Chloride Hexahydrate and HUMIN-P 775 (Organic Matter) was investigated. The study roughly consisted of two parts, one concerning a situation of ideal mixing and one where the chemicals mixed inside a porous medium. When considering ideal mixing, eleven samples were created with 40 mL of Organic Matter solution and Aluminium solution in the range of 0 to 4 ml was added. These samples were created with an initial OM pH of 5, 7 and 9 to investigate the pH effect. The precipitation was quantified on the basis of measured pH, EC, UV254 and weight. With UV254 measurements it was found that 85% of the organic matter is able to form precipitates by binding aluminium ions. Depending on the pH, the amount of aluminium which needed to be added to reach this 85% varies. At a lower pH, less aluminium salt is needed to be added to form complexes with the available organic matter.
The precipitation as a result of the complexation was also formed in the three-dimensional experimental set up. The chemicals were injected via horizontal drains and a flow was created towards an overflow drain. A narrow precipitation band of a few millimetres thick was formed at the contact interface of the aluminium and organic matter solutions. The Al-DOM precipitation was visualized with a 3D interpretation of photographs of horizontal cross-sections in Matlab. The results of this interpretation showed that it was difficult to form an area-covering layer of precipitates. Most precipitation was shown near the injection- and overflow drains, this assumes that due to separation of flow no Al-DOM precipitation took place in the middle of the set-up. Due to the lack of an area-covering layer, a significant reduction of hydraulic conductivity could not be measured. An average reduction of about 25-30% was measured. In COMSOL it was assumed that in ideal situations the precipitation band could cause a reduction in permeability of about two order of magnitude. The results of this research show that the SoSEAL concept, using HUMIN-P 775 and aluminium hexahydrate, can be used to form a horizontal layer of reduced permeability, but that it is challenging to create an area-covering layer due to separation of flow and preferential flow paths.