Laboratory investigation and modeling of nickel removal from artificial wastewater with modified rice paddy adsorbent in a continuous reactor using the response surface method (RSM).

Master's Thesis in Civil Engineering (Environmental Engineering)

Abstract

The surface absorption method is one of the effective methods for removing metals, including nickel metal. In this research, modified rice paddy has been used as absorbent material. Since this adsorbent is a natural material with a high contact surface, in addition to its high ability to absorb metal, it is cheap and available and makes the absorption operation cost-effective. Also, experiments have been carried out in the form of a continuous column system for metal removal.

In order to investigate the effects and optimization of the effective parameters on the absorption efficiency, including the height of the absorber, the inlet flow rate to the column and the initial concentration of nickel, the test plan was carried out using the response surface method and using the central composite test plan. The results of the experiments were analyzed by Design Expert software and the most appropriate model was determined to draw the response surface curves, which was the second order method. Also, Adam-Bohart and Thomas experimental models were used in order to compare the results of the experiments. According to the obtained results, the height of the absorber has the greatest effect on the absorption efficiency and has a positive effect. The two factors of flow rate and input concentration of nickel have less effects on absorption and the efficiency decreases with the increase of each of them. Also, the highest absorption efficiency was equal to 69.8% and the highest absorption capacity of the column was equal to 16.33 milligrams of nickel per unit mass of the absorber. Chapter 1 1-1 Generalities Nowadays, human activities have caused more and more pollution of the environment. Of these, industrial activities create a large part of these pollutions, so that heavy metals from the effluents of these industries accumulate in the environment. This is despite the fact that these metals are not biodegradable and when they enter the food cycle, they accumulate in the body of living organisms and cause dangerous diseases (Ozer, 2007). Several methods have been used to clean heavy metals from the environment, among which the following can be mentioned: Reverse osmosis: in this method, separating membranes are used that allow water molecules to pass through, but Molecules of dissolved substances do not have the ability to pass through this membrane. In this method, pure water is introduced from the dilute part to the thick part by osmotic pressure, and this process continues until the chemical potential of both sides is equal. This process is called osmosis. Therefore, in the state of equilibrium, the pressure difference between the two sides of the membrane is equal to the osmotic pressure difference (Mozaffarian et al., 2005). Sedimentation method: In this method, a wide range of heavy metals are removed, and anions that play the role of special precipitants and are used in precipitation can be obtained. This method is cheaper than other methods, and the construction of new units does not require exorbitant costs (Khosravi, 2016). Ion exchange method: In this method, non-toxic cations replace heavy metal cations in the solution. In this case, resins are used that can receive cations and anions according to their active capacity and carry out ion exchange. To remove heavy metals, cationic resins with proton coating or another metal must be used as an exchange agent (Khosravi, 1386).

In this method, if a high volume of resin is used, heavy metals can be purified up to ppb. Also, this method is relatively cheap and its equipment is available (Ziodar, 2013).

•      Electrodialysis method: This method is the most common electrochemical purification method in which an electrochemical cell is used to reduce metals to a zero oxidation state. A simple cell consists of two positive (anode) and negative (cathode) electrodes, using an external power source such as a battery, a constant potential is applied to the electrodes, and due to the electrostatic attraction between non-identical charges, the ions in the solution are attracted to the electrodes. Usually, consecutive cells are created in such a way that the anode and cathode blades are placed in the middle, and the positively charged heavy metal interstitials move towards the cathode and are reduced to zero oxidation number there. If there is anion in the water, the anion is absorbed by the anode.

If using electrochemical cells, a metal can be selectively removed and the absorbed metal can often be recycled.Also, with this method, concentrations of 1 ppm or even less were reached (Ziodar, 2013).

•      Surface absorption method: absorption is a term that refers to the accumulation of a substance at the interface between two different phases such as solid and liquid or solid and gas. The substance that accumulates in the interface is called the absorbing substance and the solid on which the absorption process takes place is called the absorbing substance (Bhatnagar & Sillanpaa, 2010). This method has an easy and effective application in which it is possible to remove organic and mineral substances even in low concentrations. It can also be used in different situations without the need for complex equipment if not only sludge is not produced, but it is also possible to recover the adsorbent material and reuse it, and it will cost little if natural adsorbents are used (Hu et al., 2009; Zhao et al., 2009; Wang et al., 2007; Yang et al., 2009; Ozer, 2007; Gao et al., 2009 Rao et al. 2007).

One of the heavy metals that there are many concerns about its destructive effects is nickel. Nickel salts are usually used in casting, silver refining, battery industries, electroplating, printing and making some alloys. One of the problems caused by this metal is skin swelling and sensitivity. Also, if the concentration of nickel is high, it causes lung and bone cancer, nausea and vomiting, shortness of breath, increased heart rate, headache and dizziness. (Saadat, 2011; Kandah & Meunier, 2007; Lu et al., 2008).

According to the latest standard published by the Iranian Environmental Organization, the maximum allowable concentration of nickel metal in the effluent is 2 mg/liter (Environmental Protection Organization, 2013). Also, the World Health Organization has declared the maximum permissible concentration of this metal in drinking water to be less than 0.1 mg/liter (Kandah & Meunier, 2007).

2-1- Necessity of conducting research

Effluents of some industries such as battery making, casting and printing have high concentrations of nickel metal. When this wastewater enters the surface or underground water, it becomes polluted and causes serious problems for the environment and humans. As mentioned in the previous section, there are several methods for removing heavy metals. But among these methods, in the reverse osmosis method, osmotic pressure is applied to the thick part, and pure water is introduced from the thick part to the dilute side, and the ions do not have the ability to pass through the membrane. Also, in the precipitation method, the efficiency of the precipitation operation is affected due to the presence of acid and anionic salts. Also, chemicals must be added to water or wastewater to perform the purification process. In the ion exchange method, the resins may be contaminated by organic matter in the water or other solids. Also, very concentrated solutions cannot be purified in this way. In the electrodialysis method, the operating cost of electrochemical cells is very high and requires constant control.

Surface absorption occurs in both physical and chemical ways. Physical absorption, which occurs due to the presence of van der Waals forces of attraction between pollutant molecules and the absorbent surface, is one of the most common types of absorbents and has the property of reversibility. Chemical absorption occurs as a result of a chemical reaction between the pollutant and the absorbent surface and has a low speed and is irreversible unlike physical absorption. The probability of this type of absorption occurring in adsorbents is low (Raynolds & Richards, 1996). So far, the use of various adsorbents to remove metal ions has been studied, including activated carbon, sawdust, rice husk, walnut shell, and carbon nanoparticles (Sadat, 2013). This method is one of the common processes for removing heavy metals, and the absorbent material is the most important part of this process. Practically, there are problems in the use of conventional adsorbents, such as limitations in absorption and regeneration capacity and low absorption speed. In recent years, the attention of researchers has been directed towards the use of plant waste and agricultural products as adsorbent material because natural materials are an economic solution for the removal of heavy metals and the process of surface absorption. The absorption of metals by these materials is done by proteins, carbohydrates and their phenolic compounds. These compounds have sulfate, hydroxyl, amino, phosphate and carboxyl groups, which have the ability to combine with metal ions.