Mobility of Rare Earth Elements in Coastal Aquifer Materials under Fresh and Brackish Water Conditions

The indispensable role of rare earth elements (REEs) in manufacturing high-tech products and developing various technologies has resulted in a surge in REE extraction and processing. The latter, in turn, intensifies the release of anthropogenic REEs into the environment, particularly in the groundwater system. REE contamination in coastal aquifer systems, which serve as drinking and domestic water sources for large populations, demands a thorough understanding of the mechanisms that govern REE transport and retention in these environments. In this study, we conducted batch and column experiments using five representative coastal aquifer materials and an acid-wash sand sample as a benchmark. These experiments were conducted by adding humic acid (HA) to the REE solution under fresh and brackish water conditions using NaCl, representing different groundwater compositions in coastal aquifers. The REEs were shown to be most mobile in the acid-wash sand and natural sand samples, followed by two types of low-carbonate calcareous sandstone and one type of high-calcareous sandstone and the least mobile in red loamy sand. The mobility of REEs, found in solution primarily as REE–HA complexes, was controlled mainly by the retention of HA, which increases with increasing ionic strength and surface area of the aquifer material. Furthermore, it was found that the presence of carbonate and clay minerals reduces the REE mobility due to enhanced surface interactions. The higher recoveries of middle-REE (MREE) in the column experiment effluents observed for the acid-wash sand and natural sand samples were due to the higher stabilization of MREE–HA complexes compared to light-REE (LREE) and heavy-REE (HREE) HA complexes. Higher HREE recoveries were observed for the calcareous sandstones due to the preferred complexation of HREE with carbonate ions and for the red loamy sand due to the preferred retention of LREE and MREE by clay, iron, and manganese minerals.

The Supporting Information contains figures and tables showing the sampling locations of the different coastal aquifer materials (Table S1), a list of constants used for REE speciation using Stockholm Humic Model (Table S2), a list of logKMb and ΔLK2 values used for REEs speciation calculations using Stockholm Humic Model (Table S3),.Retained REE mass per 1 g of coastal aquifer materials in fresh and brackish water (Tables S5-S16).The Concentrations of REEs in the different coastal aquifer materials (Figure S1); XRD measurements of all six examined samples are presented in Figures S2-7, batch adsorption experiment results (Figure S8), and column transport experiment results (Figure S9), Humic acid retention on the different coastal aquifer materials in fresh and brackish water (figure S10), Concentrations of Mn and Fe in the different coastal aquifer materials (Figure S11).Table S1.Coastal aquifer porous media sampling locations.

Comparing REE retention between batch and column experiments
The retained REEs were calculated for column experiments (after 20, 50, 100 and 150 PV) and for batch experiments (after 8 days).

Method of calculation:
Batch experiments: The concentrations of retained REEs (µg REE/ gr soil) were calculated by first measuring the REE concentrations in solution after 8 days and subtracting the REEs in the solution from the initial REE concentrations.The retained REE concentrations were multiplied by the solution volume to determine the retained REE mass.The retained REE mass was divided by the soil mass in the experimental bottles.An average (and standard deviation), "representative" REE concentration, based on all REE concentrations, was then determined.

Data:
• Initial REE concentrations: 70 µg/L for each REE • Volume of solution: 500 mL • Mass of soil in experimental bottles: 10 g

Column experiments:
The concentration of retained REEs (µg REE/ gr soil) in the column after injecting 20, 50, 100, and 150 PV was calculated by measuring the mass of soil in each column and calculating the PV of each column by subtracting the unsaturated weight of the column from its saturated weight.The volume of solution that flows through the column after 20, 50, 100, and 150 PV was calculated by multiplying the number of PV by the volume (mL) of one PV of the column.The mass of REEs that entered the column was calculated by multiplying the initial REE concentrations by the volume of solution that flowed through the column.The recoveries of REEs (mass) after injecting a given number of PV were determined by integrating their BTCs.The retained REE mass was calculated by subtracting the recovered REEs from the total REEs that entered the column.The retained REE for a g of soil was determined by dividing the retained REE mass by the soil mass.

Data:
• Initial REE concentrations: 70 µg/L for each REE • PV and soil mass for the different aquifer materials are presented in Table S4 Table S4 Figure S2.XRD of acid-wash sand bulk sample.Q=quartz

Figure S8 .
Figure S8.Adsorption curves of REE in different Coastal Aquifer materials and salinities. A. acid-wash sand, B. natural sand.C. low-carbonate calcareous sandstone 1. D. low-carbonate calcareous sandstone 2. E. high-carbonate calcareous sandstone F. red sandy soil.Red line: REE average in fresh water conditions.Red background: REE distribution in fresh water conditions.Blue line: REE average in brackish water conditions.Blue background: REE distribution in brackish water conditions.

Figure S9 .
Figure S9.Breakthrough curve measurements of REEs (average concentration) and Br tracer in different Coastal Aquifer materials and salinities.Note: different y-axis values.A. acid-wash sand, B. natural sand.C. low-carbonate calcareous sandstone 1. D. low-carbonate calcareous sandstone 2. E. high-carbonate calcareous sandstone F. red sandy soil.Red line: REE average in fresh water conditions (IS=2.5x10-3).Red background: REE distribution in fresh water conditions.Blue line: REE average in brackish water conditions (IS=2.5x10-2).Blue background: REE distribution in brackish water conditions.Black circles: Br tracer.

Figure S11 .
Figure S11.Concentrations of Mn and Fe in the different coastal aquifer materials.

Table S2 .
List of constants used for REE speciation using Stockholm Humic Model.

Table S3 .
logKMb and ΔLK2 values for REEs speciation calculations using Stockholm Humic Model.

Table S6 .
Retained REE mass per 1 g of acid-wash sand for batch and column experiments with brackish water.

Table S7 .
Retained REE mass per 1 g of natural sand for batch and column experiments with fresh water.

Table S8 .
Retained REE mass per 1 g of natural sand for batch and column experiments with brackish water.

Table S9 .
Retained REE mass per 1 g of low-carbonate calcareous sandstone 1 for batch and column experiments with fresh water.

Table S10 .
Retained REE mass per 1 g of low-carbonate calcareous sandstone 1 for batch and column experiments with brackish water.

Table S12 .
Retained REE mass per 1 g of low-carbonate calcareous sandstone 2 for batch and column experiments with brackish water.

Table S13 .
Retained REE mass per 1 g of high-carbonate calcareous sandstone for batch and column experiments with fresh water.

Table S14 .
Retained REE mass per 1 g of high-carbonate calcareous sandstone for batch and column experiments with brackish water.

Table S15 .
Retained REE mass per 1 g of red loamy sand for batch and column experiments with fresh water.

Table S16 .
Retained REE mass per 1 g of red loamy sand for batch and column experiments with brackish water.
Figure S1.Concentrations of REEs in the different coastal aquifer materials.