Ground water samples were collected from the entry point into the distribution system (prior to disinfection) of a groundwater treatment facility in Arizona. Samples were stored at 4 degrees Celsius (˚C) and brought to room temperature (20-25˚C) prior to testing.

ActiGuard® Fluorograde Activated Alumina, with a mesh size of 14 × 28, was purchased from Delta Adsorbents. The activated alumina was rinsed with 18-ohm water (generated from Millipore Sigma IQ 7005 water purification system) fifteen times to remove small particles (fines). The cleaned media was dried at 105˚C overnight before the testing.

The groundwater and activated alumina treated water samples were filtered through 0.45-micron filter paper using vacuum filtration prior to analysis. Filtered samples were analyzed for fluoride, chloride, nitrate, and sulfate using ion chromatography (IC) [AG18-Fast-4 micron RFIC 4 * 30 mm and Separation column Dionex IonPac AS18-fast-4 micron RFIC 4 * 150 mm] on a Thermo Fisher Dionex Integrion HPICTM System. Duplicate samples were analyzed with quality control samples with each batch of ten samples. Quality control samples included a laboratory blank, laboratory control standard, and laboratory spike sample. Thermo Scientific’s Dionex Six Anion-II standard (P/N 057590) was used to prepare laboratory control standards and to develop a five-point calibration curve. Analysis of total organic carbon (TOC) and dissolved organic carbon (DOC) was conducted using a Sievers M9 Laboratory TOC Analyzer. Unfiltered samples were analyzed for TOC and samples filtered through 0.45-micron syringe filters were analyzed for DOC. Hach TNTplus 843 test kits (range 0.05 - 1.50 mg P/L) were used for orthophosphate analysis with Method 10209/10210 (equivalent to USEPA Method 365.3). Hach Silica Powder Pillows (range 1 - 100 mg/L as SiO₂) were used for silica measurement using Method 8185. TDS was measured according to Standard Methods 2540 C.

Arsenic was analyzed using inductively coupled plasma-mass spectrometry following USEPA Method 6020, to capture the low level (µg/L) concentrations in samples. The samples were filtered through 0.45-micron filter paper using a vacuum filtration apparatus and acid-preserved prior to the analysis.

Alkalinity was analyzed using Hach TNTplus 870 test kits, with a range of 25 - 400 mg/L as calcium carbonate (CaCO₃), using Method 10239. Bicarbonate concentration was calculated from total alkalinity using the equation provided in the Standard Method 4500-CO₂.

Each batch adsorption experiment was conducted using 500 mL of pH-adjusted source water placed in high density polyethylene (HDPE) bottles at room temperature (20 - 25˚C). The optimum amount of activated alumina required, and the mixing time were evaluated by preliminary testing.

After measuring initial pH, temperature, and alkalinity of the groundwater, samples were aliquoted into HDPE bottles and pH was adjusted to 5.5 to 7.5 ( Table 1 ), except the control sample. A 1 normal (N) hydrochloric acid solution, 1 N sulfuric acid solution, and carbonated water were used for adjusting sample pH. All experiments were conducted in duplicate. Both mineral acids, hydrochloric and sulfuric, were obtained from Fischer Chemicals, and carbonated water was supplied by Burnett-Inc, which contained approximately 2,000 mg/L we carbon dioxide. Alkalinity was also measured in the pH adjusted samples.

Bottles were leak-tested and placed on an Eberbach E6145 orbital shaker at 150 rotations per minute (rpm) for 180 minutes (50, 51). After mixing the treated water was filtered through 0.45-micron filters and analyzed for parameters described in Section 2.3. After treatment with activated alumina adsorption, the pH of the water was adjusted with 0.1 N sodium hydroxide to match the initial pH of the groundwater. The alkalinity was measured once more, after sodium hydroxide addition.

To determine the optimum activated alumina dose required, a preliminary evaluation was conducted at a constant mixing time of 180 minutes, constant mixing speed of 150 rpm, and an adjusted initial pH of 6 (with hydrochloric acid). Activated alumina concentrations evaluated were 0.1, 0.5, 1, 2, 3, 5, 7.5 and 10 g/L and experiments were conducted in duplicate. Results showed a diminishing efficiency of treatment after 5 g/L activated alumina, where fluoride removal plateaued above 7 g/L ( Figure 1 ). Therefore, 5 g/L of activated alumina was chosen for further testing (considered optimum) for simultaneous removal of fluoride and arsenic.

Following optimization of the activated alumina dose, pH optimization was conducted with initial pH values of 5.5, 6.0, 6.5, 7.0 and 7.5 with 5 g/L of activated alumina, using a mixing time of 180 minutes, and mixing speed of 150 rpm. Experiments were conducted in duplicate. Results confirmed the pH dependance of fluoride removal, however, arsenic removal was independent of pH within the range evaluated; notably, in this case study, the observed initial concentration of arsenic in the groundwater was comparatively lower than other studies ( Figure 2 ). With the maximum fluoride removal observed at pH 5.5, its removal efficiency declined with increasing pH and plateaued after pH 7.0.

Mixing time ranges were evaluated after the activated alumina dose (5 g/L) and initial pH of 6.0 were optimized. The mixing/contact times evaluated were 7.5, 15, 30, 60, 90, 120, 180 and 240 minutes. Both arsenic and fluoride removal increased with the mixing time ( Figure 3 ). Table 1 summarizes all the preliminary testing tests carried out.

Based on these preliminary evaluations, further experiments were conducted with 5 g/L of activated alumina, at a mixing time of 180 minutes and mixing speed of 150 rpm.

The initial groundwater quality characterization is summarized in Table 2 .

Fluoride removal rate was marginally lower when sulfuric acid was used for pH adjustment versus hydrochloric acid and carbon dioxide, in the pH range of 5.5 and 6.5 ( Figure 4 ). As pH approaches a neutral range, fluoride removal is similar among the three acids tested for pH adjustment. This is specifically attributed to the fluoride removal chemistry, which is predominantly higher at lower pH ranges ( Figure 4 ), and not the type of acid used. Rathore et al. observed similar trends reporting that below pH of 3.5, the predominant species present is hydrogen fluoride, which is non-ionic; and, as pH increases the concentration of fluoride ion (F-)—the ionic species—increases and becomes predominant above pH of 5 [42] . While arsenic removal was marginally lower with both carbon dioxide and sulfuric acid in comparison to hydrochloric acid, arsenic removal with activated alumina is generally not pH dependent at the concentrations observed in the water evaluated in this study ( Figure 5 ).

Alkalinity was measured after pH adjustment before batch testing and after batch testing. Alkalinity decreased significantly post-pH adjustment with both mineral acids ( Figure 6 ). Further alkalinity measured after activated alumina treatment was lower for mineral acid pH adjusted tests. When initial pH was adjusted to 6, the alkalinity prior to activated alumina testing with sulfuric acid was 22 mg/L as CaCO₃, with hydrochloric acid was 27 mg/L as CaCO₃, and with carbon dioxide was 65 mg/L as CaCO₃ ( Figure 6 ). The observed percent difference in alkalinity between carbon dioxide and sulfuric acid treated samples was 195% and between carbon dioxide and hydrochloric acid was 141%. Alkalinity marginally decreased with carbon dioxide addition, and the observed alkalinity, post-treatment with activated alumina, was higher than the initial alkalinity of 70 - 75 mg/L as CaCO₃. This indicates that pH adjustment with carbon dioxide can enhance buffering capacity through changes in the carbonate buffer system (bicarbonate and carbonate ions) in the water. The changes in the buffering capacity depends on the raw water characteristics, such as pH, alkalinity and hardness.

Although alkalinity is not a primary or secondary parameter as part of the National Drinking Water Regulations, the recommended alkalinity level in drinking water ranges between 75 mg/L to 150 mg/L as CaCO₃. Alkalinity, pH, dissolved inorganic carbon (DIC) and corrosion inhibitors are the critical water quality parameters that impact lead and copper release in the distribution system. Further, alkalinity, pH, and DIC also impact the corrosion of pipes in the distribution system [50] . To prevent pipe corrosion, water treatment plants typically maintain alkalinity greater than 20 mg/L as CaCO₃ and pH ~ 8 [51] . Given that carbon dioxide may help maintain higher alkalinity, pre-treatment with carbon dioxide could help address corrosion control in the distribution network.

Post activated alumina treatment, prior to disinfection, chemicals are added to the water to maintain reasonable pH and alkalinity in the distribution system. This is achieved by addition of various chemicals (such as caustic, lime, calcium chloride) and EPA provides guidance documents on the choice of chemical selection based on the water quality [49] . Mimicking full-scale application, pH adjustment after activated alumina treatment was evaluated using a 0.1 N sodium hydroxide solution; the adjustment was performed for samples pretreated with sulfuric acid and carbon dioxide. Water pre-treated with hydrochloric acid was not chosen because it had a high percentage difference in alkalinity, between the pre and post activated alumina treatment.

Based on the initial pH, varying amounts of sodium hydroxide were required for post-stabilization, as expected ( Figure 7 ). When the initial pH is reduced to 5.5, 2 ml/L of 0.1 sodium hydroxide were required for sulfuric acid pre-treated samples while 9.6 ml/L of 0.1 sodium hydroxide was required for carbon dioxide pre-treated samples. But when the initial pH was 6, 1.6 ml/L of 0.1 sodium hydroxide was required for sulfuric acid pretreated samples, while 3.6 ml/L of 0.1 sodium hydroxide was required for carbon dioxide pretreated samples. Because carbon dioxide enhances buffering capacity, the amount of sodium hydroxide required for pH adjustment post-treatment was higher than waters pre-treated with sulfuric acid.

Activated alumina can remove other competing anions such as, nitrate, chloride, sulfate, etc. To elucidate the effects of competing anions, the removal efficiency of other anions, such as chloride, sulfate, nitrate, bicarbonate, and silica were also evaluated for the three different pretreatments, used for initial pH adjustment. For chloride, no significant removal was observed when initial pH was adjusted with sulfuric acid and hydrochloric acid, but with carbon dioxide, ~ 10% chloride removal was observed at a pH of 5.5. Chloride removal decreased with increasing pH ( Figure 8 ). The sulfate trend was like chloride where, when pH was adjusted with carbon dioxide, higher sulfate removal (45%) was observed at the lower initial adjusted pH of 5.5, its removal rate decreased with increasing pH. Twenty percent sulfate removal was observed at the lower initial pH of 5.5 with both hydrochloric acid and sulfuric acid, and sulfate removal decreased with increasing pH ( Figure 9 ).

In the case of nitrate, no removal was observed when pH was adjusted with hydrochloric acid or carbon dioxide, but a 10% removal was observed when the initial pH was adjusted to 5.5 with sulfuric acid. The nitrate removal rate decreased with increased initial adjusted pH, like that of chloride and sulfate ( Figure 10 ). The bicarbonate trend was not evaluated when pH was adjusted with carbon dioxide because increases in pH would increase the bicarbonate concentration. Both sulfuric acid and hydrochloric acid adjusted samples showed 50% - 55% bicarbonate removal at the lower initial adjusted pH of 5.5 and like other anions, the removal rate decreased with an increase in initial adjusted pH ( Figure 11 ).

Silica had a different trend compared to the other anions evaluated, which is expected because silica solubilizes at higher pH. The removal rate increased with an increase in initial adjusted pH from approximately 40% - 60%. Although all three acids indicated similar trends in anion removal, the overall removal rates were marginally higher with hydrochloric acid ( Figure 12 ).

Based on these results, it may be concluded that the influence of competing ions for active sites on activated alumina can be ranked as bicarbonate > silica > sulfate > nitrate > chloride with sulfuric acid [52] . For hydrochloric acid, bicarbonate and silica adsorption was similar and in case of carbon dioxide, silica showed higher adsorption in comparison to chloride and sulfate. The added advantage observed in these experiments is that along with arsenic and fluoride, 40 to 60% silica as SiO₂ can be removed from the water with activated alumina adsorption. Furthermore, it is important to understand the water quality before activated alumina treatment since the concentration levels of other competing ions can hinder removal rates for arsenic and fluoride.

Post treatment with blended phosphates and pH adjustment with NaOH will deliver a required finished water pH. However, to address the ever evolving lead corrosion chemical stability of treated water is estimated by alkalinity (in the range 40 to 80 mg/L as CaCO₃), tendency of the water to precipitate calcium carbonate (measured as Langelier Saturation Index, LSI; expected to be greater than “zero”), and concentration of soluble calcium ions in the water to be in the range of 50 to 120 mg/L as CaCO₃ [53] . These three targets can be achieved by post stabilization with lime versus NaOH.

To evaluate the cost implications associated with the choice of chemical employed for pH adjustment, four chemical supply vendors were contacted to provide proposal costs per pound for hydrochloric acid, sulfuric acid, and carbon dioxide. The cost of the chemicals varied marginally around the US and depended on the delivery distance from the production site to where the chemical was used. The average cost per pound (lb.) of chemical was $0.29 lb. for 37% hydrochloric acid: $ 0.24 lb for 93% sulfuric acid and $0.165/lb for liquid carbon dioxide. These costs were used to evaluate the chemical required for a 1 million gallons per day (MGD) water treatment plant with the same groundwater quality and the final adjusted pH of 6.0 prior to activated alumina treatment. Based on this evaluation, the cost of carbon dioxide is comparable to sulfuric acid, which is most used at water treatment plants ( Table 3 ). However, the cost of capital equipment required to deliver these chemicals into the treatment process has not been accounted in this evaluation.