(545ah) Analysis and Control of Al Concentration in Groundwater Based on Mathematical Modeling and Laboratory Tests

Aluminium (Al) is present in all types of natural water sources and additionally comes to drinking and industrial waters from alum used as a coagulant in water treatment technologies (Sposito, 1996). The recommended upper boundary limit of Al concentration in drinking water is 0.20 mg∙L^−1 in accordance with the U.S. Environmental Protection Agency (USEPA, 2012). The problem of elevated Al level has to be considered as a growing number of epidemiological studies linked the high Al content of drinking water as being proportional to the degree of neurological disease incidence (Wang et al., 2016). Current investigation has an industrial dimension: the existing problem of elevated Al concentrations in the groundwater of the Khibiny alkaline massif (Kola Peninsula) — area with a large-scale industrial production of apatite (Konukhin et al., 2012). A high Al level in water of this aquifer is described as a serious health concern when used for household water supply. According to the available data set of groundwater quality monitoring, during the past 10 – 12 years there is a general increase of Al concentrations in groundwater of Khibiny water aquifer. The current concentrations exceed the guideline up to nine times. The main objectives of current investigation are the quantitative analysis of the long-term results of field data monitoring program by using chemometric methods and a study of reliable adsorbents to remove Al effectively under the water aquifer-specific natural conditions.

The time series (1999 – 2012) of 12 physico-chemical variables monitored from ten groundwater wells were examined in both time and frequency domains by using multiple and multivariate regression methods as well as spectral analysis based on fast Fourier transform (FFT) algorithm. Computed pairwise correlation coefficients matrix revealed that pH values, concentrations of sulfate SO₄^2−, nitrate NO₃^−, chloride Cl^−, and total dissolved solids TDS correlate with Al concentrations at a statistically significant level (α-level=0.05). According to the coefficient of determination R² the multiple regression model considering the result of multicollinearity check explained up to 54% of Al temporal variation linked to pH values, concentrations of Cl^− and NO₃^− in groundwater. The sequential chain of contribution to the R² percentage is as follows: pH > NO₃^− > Cl^−. The cluster analysis (CA) identified three different groups of similarity based on correlation between variables within the dataset. The factor analysis/principal component analysis (FA/PCA) revealed three factors defined as linear combination of the original variables, capturing 58.7% of the total variance of the data set and allowed to group the selected variables according to common features. The first factor accounting for the most of total variance (34%) of the data set includes Al, pH, TDS, NO₃^−, Cl^−, SO₄^3−. Considering the results of multiple, multivariate regression analyses and multicollinearity check the Fourier coefficients, power spectral density (PSD) and cumulative spectral power (CSP) were calculated for Al, pH, NO₃^−, and Cl^− time series to identify and analyze the strength of their variations as a function of frequency. Calculations revealed the spectrums of these compounds include three frequency bands corresponding approximately to 5 – 7, 13 – 17 and 20 – 34 months periods. The fluctuations within these bands contribute mostly to the total temporal variation of Al, Cl^−, NO₃^− concentrations and pH values.

The Khibiny alkaline massif groundwater most of the time does not meet established drinking water standards at least for two variables: the level of pH and concentration of Al. These variables are highly correlated, have similar variability patterns and belong to one cluster as the results of mathematical modeling revealed. The laboratory study has been undertaken to analyze adsorbents that can be reliable and cost-effective when applying to the water treatment process in the investigated area for Al removal under alkaline conditions – that is one of the major features of Khibiny massif groundwater. The initial Al concentration for all experiments was controlled at approximately 2 mg∙L^−1– the concentration is close to the maximum levels of Al observed in the groundwater based on the monitoring data set. Titanium dioxide Degussa P25 (TiO₂), a crushed vermiculite concrete modified by iron oxyhydroxide (VC-FeOOH) (Martemianov et al., 2017), montmorillonite K10 and silica gel (SiO₂) were four adsorbents tested in a concentration range 0.5 – 10 g∙L^−1.

The results of laboratory tests revealed that K10 and SiO₂ are favourable for the adsorption of Al in the acidic pH range, however no significant adsorption was found at pH > 8. An equilibration time of 2 hours for K10 and 10 hours for SiO₂ resulted in the removal of 99% and 57% of Al at pH 4 respectively. The alkaline conditions were further investigated for VC-FeOOH and TiO₂. The maximum adsorption of Al up to 90 – 91% onto VC-FeOOH took place within 4 hours under the constant alkaline conditions (pH 9). Laboratory testing showed that VC-FeOOH can act as a coagulant that accelerates the Al removal rate.VC-FeOOH loading ≤ 2 g∙L^-1 resulted in up to 95% removal due to Al precipitation within 30 min at final pH 8.1. Laboratory experiments with TiO₂ showed Al removal up to 75 – 78% at pH 9. The Al concentration decreases in three to five times within initial 7.5 min of experiments where TiO₂ loading was ≥ 1 g∙L^-1 and reached an equilibrium in 30 min. The mathematical description of equilibrium data was fitted most satisfactorily with Langmuir isotherm model.

The current investigation is an initial and necessary step to develop and implement a reliable method to reach a target level of Al removal in the groundwater of Khibiny alkaline massif. The application of chemometric methods allowed better understanding the major features of Al concentration variability as well as associations between Al and other compounds examined at both time and frequency domains. The evidence of direct anthropogenic influence on the elevated level of Al in groundwater was not releveled. The results obtained rather support that high Al concentration is caused by aluminum leaching from minerals. At big depths (more than 100 m) there are reduction conditions formed promoting the leaching in accordance with the recent geochemical analysis conducted in the area (Mazukhina et al., 2012). The implementation of adsorption process to the current technology at the Khibiny local water treatment facility can be an effective way to decrease Al concentrations below the guideline. Moreover, it is well known that TiO₂ can act as catalyst (Chen & Ray, 2001), therefore, the main direction of future works is to study how photocatalysis can improve Al removal. Although the current investigation has been done for the particular metal and groundwater source, similar analysis can be successfully applied to any other groundwater sources and could be extended to other toxic metals found in the groundwater.