Treatment of iron bearing groundwaters by the simultaneous addition of sodium silicate and sodium hypochloride

Author

Joseph M. Holden

Date of Award

6-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

R. Bruce Robinson

Committee Members

Roger Minear, Gregory Reed

Abstract

With conventional treatment iron bearing groundwaters are made potable by first oxidizing the ferrous iron followed by removal of iron floc by filtration. While this is a reliable treatment method the costs of constructing and operating a conventional iron removal plant can be prohibitive for many communities. The simultaneous addition of sodium silicate and sodium hypochlorite to iron bearing groundwaters has been demonstrated to effectively prevent iron precipitation and offers a relatively inexpensive alternative to conventional treatment. This method might gain wider acceptance if the effects of various species found in groundwater such as Ca²⁺,Na⁺, Cl^- SO₄^2- ,and alkalinity could be quantified. A statistical model developed from this information could be used to predict the suitability of actual ground waters to this treatment method and the dosage requirements for effective treatment. This study was undertaken to quantify the effects of the above mentioned species on a model iron bearing groundwater that contained 2 ppm Fe²⁺,0 ppm dissolved oxygen, and had a pH of 7and an alkalinity ranging from 50to 300 ppm as CaCO₃. Various concentrations of CaCl₂,CaS0₄,NaCI,Na₂S0₄,and NaHCO₃ were added to the model groundwater in a series of 10 day experiments. Treatment consisted of dosing these ferrous solutions with freshly prepared mixtures of dilute silica and sodium hypochlorine. The 0.1% sodium hypochlorite solution was added in slight excess of that required to oxidize the iron and the silica doses ranged up to 45 ppm as Si0₂. Typically,40-42° Be technical grade sodium silicate was diluted to 7.8 mg/ml less than an hour before it was used to dose the iron solutions. However, one experiment used silica solutions that ranged from 50 times more concentrated to 5 times as dilute as the one typically used. This experiment was conducted to see if initial silica concentration affected treatment results. After preparing and dosing these iron solutions under an N₂-CO₂atmosphere the capped jars were stored at 2Q°C in a mechanical shaker. Data collected from these experiments at regular intervals included pH, turbidity, dissolved oxygen, and total and filterable iron (that which passed through a membrane filter with a pore size of0.1 µm).

The results of these experiments showed the following:

1. When sufficient silica was added to the reduced iron solution simultaneously with excess sodium hypochlorite, iron precipitation was retarded or prevented.

2. The presence of dissolved calcium in the reduced iron solution increased the amount of silica required to prevent iron precipitation. Similar results were obtained when CaS0₄ and CaCl₂ were used indicating that the cation Ca2⁺was responsible for increased silica requirements.

3. Sodium salts, added to the iron solution at much higher concentrations than the calcium salts increased the silica required for iron stabilization, however, the effect was not as pronounced as that observed when calcium salts were used. Similar results were obtained with NaCI, Na₂S0₄ , and NaHCO₃.

4. The similarity in the results obtained when the iron solution contained CaS0₄ or CaCl₂ or when it contained Na2SO₄,NaCI, or NaHCO₃, seems to indicate that the anions SO₄^2-, Cl^-, and HCO_3-,do not contribute greatly to any observed increase in the silica required to prevent iron precipitation.

5. For the same dose, concentrated silica seemed to stabilize the iron better than dilute. More work is required to develop a statistical model that could be used for predicting the suitability of a given groundwater to this treatment method. This additional work should include a wider variety of species and concentrations typically found in ground waters. Ultimately, statistically significant models should be tested on actual groundwaters to see how reliable these models are.

Recommended Citation

Holden, Joseph M., "Treatment of iron bearing groundwaters by the simultaneous addition of sodium silicate and sodium hypochloride. " Master's Thesis, University of Tennessee, 1985.
https://trace.tennessee.edu/utk_gradthes/14020