Review on the Determination of Oxidant Demand for in-situ Chemical Oxidation Application

In-situ chemical oxidation (ISCO) refers to the use of appropriate conveyance technology to transport oxidants into soil or aquifer, where the contaminants are converted to low- or non-toxic substances by chemical oxidation. ISCO has been widely used for the remediation of organic contaminants in soil and groundwater due to its high efficiency and cost effectiveness.

Oxidant demand is an important parameter in the application of ISCO. Once injected into the contaminated soil or aquifer, the oxidant reacts not only with the target contaminants, but also with natural organic matter (NOM) and reductive minerals (RM) such as Fe²⁺, Mn²⁺, S^- and S^2- in the medium. In addition, the oxidant may decompose naturally due to its own properties. If the amount of oxidant used is insufficient, the remediation of the contaminated site may not meet the project objectives and residual contaminants or reaction intermediates may be formed, resulting in a rebound of the contaminant concentration at the site. If too much oxidant is used, it would not only increase the cost of remediation, but also seriously damage the physical and chemical properties of soil, reducing the diversity of microbial communities. Therefore, accurate measurement of oxidant requirements is important to achieve good remediation performance.

In previous studies, the specific meaning of terms related to oxidant demand is not consistent and they are often mixed in different studies, which would lead to ambiguity.In this review the composition and definition of oxidant demand is clarified and unified. The total oxidant demand (TOD) in ISCO is composed of pollutant oxidant demand (POD, the amount of oxidant consumed by oxidative degradation of pollutants), natural oxidant demand (NOD, the amount of oxidant consumed by natural organic matter and reducing minerals in soil or aquifer media), and decomposed oxidant (DEO, the amount of oxidant naturally decomposed due to its nature and the influence of environmental conditions).

The methods for determining oxidant demand can be divided into two categories: experimental method and stoichiometric model method. The basic principle of the experimental method is to mix the treated medium sample with the oxidant in a system for a period of time, and then calculate the oxidant consumption before and after reaction to obtain the oxidant demand per unit of medium. Specifically, the experimental method can be divided into batch test, column test, push-pull test and reaction kinetics model according to the scale and dimension of the experiment. Among them, the batch test has been relatively widely used due to its excellent characteristics of simplicity and efficiency. The column test is more capable of reflecting the actual consumption of oxidant in the field medium, taking into account the transport process and interaction of oxidant in porous media. Although the push-pull test is carried out at the field scale, which can effectively overcome the error caused by the difference between laboratory conditions and field conditions, it is relatively complex to operate. Finally, the reaction kinetics model can be used to analyze the oxidant consumption in the reaction process based on experiments. This method can characterize the oxidant consumption on a longer time scale, which is of great importance for potassium permanganate or sodium persulfate oxidants with longer reaction times.

The determination of oxidant demand by experimental method is affected by factors such as the initial concentration of oxidant, reaction time, solid-to-liquid ratio and mixing conditions of the reaction. To facilitate comparison of data and costing of the project, it is necessary to incorporate experimental conditions into the expression of oxidant demand. For example, the difference of the oxidant demand obtained at different times for the same sample can be as much as 37%. Therefore, the time required for the pollutant concentration to be reduced to the target limit by oxidation should be used as the oxidant demand determination time.

Stoichiometric model method is the determination of the theoretical oxidant demand value by the establishment of the stoichiometric relationship model between the oxidant demand and the components in the medium. The stoichiometric model method was originally designed to explore the relationship between the total organic carbon in the medium and the oxidant demand. It was then developed to consider the oxidant demand of natural organic matter, other mineral ions and organic pollutants as a whole. Calculation results of the stoichiometric modelling method depend on the accuracy of the model and the precise determination of the content of relevant components in the medium.

Both experimental method and stoichiometric model method mostly take potassium permanganate oxidant as the research object, but there is a lack of research on the demand of Fenton reagent, ozone and sodium persulfate. In particular, the DEO of more decomposable oxidants such as Fenton reagent and ozone may reach more than 50% of TOD. The existing methods still face difficulties in accurately measuring DEO, which deserves special attention. As a component of TOD that is easily overlooked, the importance of DEO needs to be further clarified and more accurate measurement methods need to be explored. In the actual project, the number of injection wells, well spacing, oxidant injection rate and other design parameters will have an important impact on the total oxidant demand, which still needs to be further investigated. The remediation of contaminated sites by ISCO is a complex project and a more scientific workflow or guideline is necessary for engineers to determine oxidant demand.