REMOVAL OF DIVALENT IRON AND MANGANESE IONS AND HYDROGEN SULFIDE FROM GROUNDWATER

Processes of removal of divalent ions of iron and manganese and hydrogen sulfi de from groundwater at various pH values and temperature were studied. Obtained results have been used in order to elaborate a process of groundwater purifi cation from the mentioned pollutants. The use of elaborated process for natural water leads to the decrease of the content of iron, manganese and hydrogen sulfi de below the maximum allowable concentrations.


Experimental
The chemical composition of groundwater from village Sculeni (Ungheni district) and the maximum allowable concentrations (MAC) of detected chemical components, according to the current regulations, are presented in Table 1.
As shown in Table 1, the content of iron exceeds the maximum allowable concentration by 4.3 times, manganese -by 9 times and hydrogen sulfi de slightly exceeds the MAC.
The treatment processes (adsorption/catalytic oxidation and precipitation) of water polluted by the bivalent ions of manganese and iron, as well as hydrogen sulfi de were carried out using the installation presented in Figure 1. The model solution, which contained Fe -1.24 mg/dm 3 , Mn -0.47 mg/dm 3 and H 2 S -0.12 mg/dm 3 , appropriate to the real water from the village Sculeni, was used in the experimental tests. In order to oxidize only hydrogen sulfi de to sulfate, the 1.8 cm 3 of 35% H 2 O 2 was added to the volume of 10 dm 3 , by mixing for 10 minutes. The value of pH was then adjusted to 10.25. The removal process of iron and manganese ions was carried out by stirring for 20 minutes at a temperature of 15°C. During this time the precipitation fl occules appear which representing three valence iron hydroxide and four valence manganese hydroxide. The water from the reactor was passed through a sand fi lter, fraction 0.8 to 1.3 mm by means of a pump. The fi ltration speed was 7 m/h. After fi ltration, the water was subjected to laboratory analysis in order to establish the concentrations of iron, manganese and sulfur.

Results and discussion
The results of the analyzes of purifi ed water is as follows: (i) the concentration of sulfate ions initially was 60.1 mg/dm 3 and after the treatment increased to 63.3 mg/dm 3 due to the oxidation of 1.2 mg/dm 3 of hydrogen sulfi de; (ii) the concentration of hydrogen sulfi de after treatment became 0; the initial pH of the water was 10.25 and after fi ltration became 9.9; (iii) the concentration of iron and manganese after treatment was 0.12 mg/dm 3 and <0.05 mg/dm 3 , respectively. Test results allow us to conclude that the developed method allows treating the groundwater contaminated with bivalent ions of iron, manganese and hydrogen sulfi de. These processes can be described by the following chemical reactions: Groundwater is used daily throughout the year at various temperatures. In these conditions, it is the necessary to perform the scientifi c research, regarding the infl uence of temperature and the value of pH on the process of removal of iron and manganese ions and hydrogen sulfi de, as these factors are changing in dependence on the season.
The data on the removal of iron, manganese ions and hydrogen sulfi de from groundwater at various values of pH and temperature are presented in the Table 2. The data presented in this table show that the optimal conditions for removal of such pollutants from water are: temperature of 10 -15 o C, the hydrogen peroxide concentration -1.8 mg/dm 3 , and its initial pH is adjusted to 9.45. The value of pH after treatment is equal to 8.85, which is below the drinking water standard, according to current regulations in the Republic of Moldova. The technological fl owsheet for the treatment of groundwater in the village Sculeni is shown in Figure 2.
The total content of sulfi des in many places of Moldova is more than 10 mg/dm 3 . Taking this fact into account, the identifi cation of the treatment solutions for groundwater with a high content of sulfi des is of interest. During the removal of hydrogen sulfi de, into the 10 dm 3 of water, which contained the 10 mg/dm 3 H 2 S and 70.1 mg/dm 3 SO 4 2-, the 4 cm 3 of 35% H 2 O 2 were added and stirred. Samples were removed every 5 min for the determination the sulfi des concentration. The kinetics of sulfi des removal from the studied water is illustrated in Figure 3.
The presented data show an increase of sulfates concentration from 70.1 mg/dm 3 to ~ 100 mg/dm 3 in a short time (~ 8 min), which proves that the oxidation of hydrogen sulfi de occurred and the all hydrogen sulfi de present in water was transformed to sulfates.

Conclusions
Processes and mechanisms of divalent ions of iron and manganese and hydrogen sulfi de removal from groundwater have been studied.
The maximum effi ciency of the removal of divalent ions of iron and manganese and hydrogen sulfi de from the studied water was established at a temperature of 10 -15 ○ C and pH adjusted to 9.45.
After fi ltration of purifi ed water, the value of pH is 8.85, this value being at the upper limit of pH of drinking water standard.
Hydrogen sulfi de and sulfi des are oxidized to sulfates using as oxidant concentrated hydrogen peroxide.