Investigating the Effects of Lead on aquatic environments and its removal by electrocoagulation process

 
Introduction
Population growth and development of various industries have turned water pollution into one of the most fundamental problems in the world. Researches indicate that, today, underground aquifers, especially in the large and populous cities of the world, are faced with many problems caused by infiltration of industrial wastewater, presence of absorption wells for disposal of human sewage, and infiltration of chemical fertilizers and manure deep into the ground. Rivers, wells, and lakes are contaminated with pollutants produced by man, and their treatment requires a complicated and costly technology. In general, today, most of the rivers, lakes, and surface waters are exposed to contamination by the lead originated from industries, mining operations, and agriculture activities. Presence of lead in drinking water is a serious hazard as it damages human intelligence, accumulates in bones and prevents from hemoglobin synthesis. Knowing that its permissible level in water is 0.01 to 0.05 mg/L, various studies have shown that increased levels of lead weaken the body’s immune system and interfere with the activities of many enzymes. Children are more vulnerable to lead pollution and, if exposed to lead, exhibit symptoms such as anemia, digestive problems, or brain inflammation. One of the methods used for removing heavy metals is electrocoagulation which has recently become popular in water and wastewater treatment. In the process of electrocoagulation, metal ions produced from the dissolution of anodes act as a coagulant. Electric field facilitates the movement of small colloidal particles and results in coagulation. Studies on removing fluoride, organic pollutants, dyes, heavy metals, turbidity, suspended solids, COD, and BOD₅ from water and wastewater of pharmaceutical industries, tannery, plating, slaughterhouses, and paper mill have proved the effectiveness of the electrocoagulation process in eliminating the pollutants. This study investigates the removal of lead pollutant using the electrocoagulation method.
 
Materials & Methods
Contaminated water containing lead ions with the concentration of 10 mg/L was poured into an electrical coagulation chamber made of plexiglass, and investigated for the removal of mentioned pollutant. Lead nitrate, sodium hydroxide, and nitric acid (Merck Company, Germany) were used in this study. All the experiments were done at 25 °C. Before and after the electrocoagulation experiment, samples were examined to determine lead (II) based on standard water and wastewater tests manual. Lead concentration was determined using atomic absorption spectrophotometry (AAS) (GBC model). German-made IKA RCT basic magnetic stirrer, a DAZHENG DC POWER SUPPLY PS-305D current transformer, and a Swiss-made 691 pH Meter-Metrohm were used. The electrocoagulation reactor chamber included a tank made of plexiglass having four iron electrodes, in a bipolar arrangement with a cross section of 96 cm² and thickness of 0.2 cm, placed at the spacing of 2 cm from each other.
 
Discussion of Results
In this study, magnet rotation speed, test time, voltage, and pH were tested to achieve the optimum experimental conditions for the initial lead concentration of 10 mg/L. To find the suitable speed of stirrer, the tests were conducted at 50, 100, 150, and 200 rpm. In these tests, the voltage and test time were 20 V and 20 min, respectively. The results show 100 rpm as a suitable speed, due to the fact that metal cations react with the OH^- ions, form a metal hydroxide with a high absorption and form bonds with the pollutants. Since ions contact and floc formation are targeted, the higher speeds of stirrer break up the flocs and release the pollutant. It was also found that lower speed of stirrer cannot facilitate the required contact rate between onions and cations. Therefore, the removal rate in lower stirrer speed is lower than in suitable stirrer speed.  
To reach the optimum time for the reactions, experiments were conducted at durations of 10, 15, 20, and 25 min. In these experiments, the voltage and stirrer speed were 20 V and 100 rpm, respectively. According to the results, 20 min was selected as the optimum time for testing other parameters. Increase in the duration of experiments increased the percentage of pollutants elimination, but reduced the voltage due to the precipitation that happened on the cathode.
High voltages increased the temperature of the system and led to the passivation. On the other hand, low voltages increased the time required to reach the desired elimination rate. Therefore, to determine the suitable voltage, the experiments were conducted at the optimum stirrer speed of 100 rpm and the optimum test time of 20 min (obtained in earlier experiments). The results from these experiments indicated that 20 V was a suitable voltage. Results showed that at higher voltages, the rate of cation production and the extent of the cation hydrolysis reaction increased and a high percentage of lead pollutant was eliminated.
To attain the suitable pH value, the experiments were done at stirrer speed of 100 rpm, time of 20 min, and voltage of 20 V.
As can be seen in the results in Table 1, the lead removal efficiency increased at higher pH values, because iron hydroxides were rapidly produced at high pH values and these hydroxides eliminated lead particles.
 
Table1. The effect of pH and final Lead amount after electrocoagulation





 (mg/L)Final Lead


 (mg/L)Initial Lead


pH




0.09


10


3




0.036


10


5




0.008


10


7




0.01


10


9





 
Considering the standards available for drinking water, pH of about 7 was selected as the optimum pH in this study.
Finally after specifying the optimal conditions, amounts of the iron released and the sludge produced by the process were determined to be 0.16 mg/L and 0.174 g respectively. As can be seen, the amount of released iron falls within the standard limits. Subsequently, in order to further evaluate the process, the energy consumed during the tests was calculated by Eq. (1).
 
E = U.I.t.V^-1                     (1)
 
E represents the consumed energy (kWh/m³), U is used voltage (V), I is current density (A), t is test time (h), and V is volume of the treated fluid (L). In this study, using Eq. 12, the energy consumed during the tests was estimated to be 0.66 kWh/m³.
 
Conclusion
Presence of lead in drinking water is harmful as it can cause serious problems for human. Therefore, it was attempted to treat the lead-containing water using the new method of electrocoagulation. Results from the experiments showed the appropriateness of electrocoagulation method for the removal of lead from water. In this study, the best pH was 7, because at this pH metal hydroxides were produced in sufficient quantities and also iron co-precipitation with lead occurred. Thus, pH was found to be the parameter which had a direct effect on the reactions taking place in electrocoagulation. Metal cation resulting from electrode corrosion formed a hydroxide with OH^- ions which had a high absorptive capability and also formed bonds with pollutants. At pH levels ranging from 5 to 7, iron hydroxide was formed and precipitation of lead hydroxide flocs was started. Also, a little amount of consumed energy was observed. In the electrocoagulation process, electric energy initiates the corrosion of electrodes. Since the electrodes used in the tests were made of iron, the aquatic environment was investigated to determine the amount of iron receptors after the tests. The results showed that the amount of iron released to the environment is within the standard limits.