Using an adsorption isotherm framework to classify removal efficiency of arsenic in full-scale groundwater treatment plants

Arsenic is mainly removed from groundwater by adsorption onto, or co-precipitation with iron flocs and -deposits. The efficiency of this process depends on various factors, amongst which the oxidation state of arsenic, adsorption competition or poor iron floc removal. The aim of this study was to assess the adsorption efficiency of arsenic in full-scale groundwater treatment plants in the Netherlands. Adsorption efficiency is dependent on the concentration of adsorbent (Fe) and adsorbate (As), both of which vary considerably at the various treatment plants. To allow comparing treatment plants at these different Fe and As concentrations, the framework of a linearized isotherm graph was used. As a reference, jar tests were executed to derive adsorption isotherms of As(III) and As(V) based on co-removal with Fe²⁺ precipitation. All treatment plants have a higher adsorption efficiency than the As(III) reference, and lower adsorption efficiency than the As(V) reference. Treatment plants with an efficiency close to the As(III) reference may suffer from incomplete arsenic oxidation, although other causes cannot be ruled out. Classifying removal efficiency can be used in an initial research phase to identify treatment plants with relatively poor performance as candidates for improvement. Furthermore, treatment plants can be grouped into categories with similar adsorption efficiency, aiding in identification of common factors responsible for As removal.