1. Introduction
Nowadays, as a result of growing industrialization and urbanization, water is becoming increasingly polluted, where large quantities of contaminants are being released into the environment as a result of physical, chemical, and biological processes. Heavy metals are amongst the most harmful pollutants due to their non-degradable properties. They are toxic and carcinogenic agents that accumulate over time and cause problems to human health and the ecosystem [
1].
Copper is one of these heavy metals. It has an atomic weight of 63.5 g/moL and a density of 8.96 g/cm
3. It is present in a range of physical and chemical forms in ecosystems, and some are discharged from industrial processes and then accumulate in the environment [
2]. Copper (II) in wastewater is discharged from many industries such as the electroplating industry, plastic industry, metal refining and industrial emissions [
1]. Long-term exposure to copper irritates the nose, mouth, eyes, headache, stomachache, dizziness and diarrhea. The maximum contamination level (MCLs) of Cu (II) that has been set by the World Health Organization (WHO) is 2 mg/L [
3].
Heavy metals such as copper should be eliminated from industrial wastewater before reaching the natural environment, and this can be accomplished through various treatment techniques including chemical precipitation [
4], a flotation process [
5,
6], ion exchange [
7,
8], electrochemical treatment [
9,
10], adsorption [
11,
12,
13,
14,
15] and membrane filtration [
16,
17,
18,
19,
20,
21,
22,
23]. Each of these methods has disadvantages and some limitations.
Membrane technology is an efficient process for wastewater treatment and has more advantages than other methods. It is a compact system, economically feasible and can be employed on large scales [
24]. Reverse osmosis (RO) membranes are considered the most effective process for heavy metal removal due to their higher degree of purification and high rejection level of contaminates. RO has a wide range of applications including drinking water desalination from seawater, wastewater recycling and industrial process water purification [
25].
Several studies have been conducted on the performance of the reverse osmosis membrane under different operating conditions in terms of pressure, flow rate [
26], concentration and temperature [
27]. Mostafa Ansari found that the permeate flux is directly proportional to feed pressure and negatively dependent on feed concentration [
28].
The removal of copper (II) ions using RO membrane under various operating conditions has been studied by many researchers. Ahmed Algureiri concluded that the removal efficiency and the permeate flux were directly proportional to applied pressure, pH, feed temperature and feed flow rate, but were inversely to feed concentration, and the maximum Cu (II) rejection obtained was 96% [
23]. Haider Aljendeel found that as the feed concentration increases, so does the permeate concentration, while water flow, recovery %, rejection % and mass transfer coefficient decrease, and the maximum Cu (II) rejection was 96.6%, while the maximum recovery percentage of 40.8% [
21].
Table 1 summarizes studies on copper (II) ions removal using the membrane technique.
The temperature of the feed water is an important factor in the performance of the reverse osmosis membranes [
29]. The higher the temperature, the higher the flow of permeate, this is due to the reduction in the solution’s viscosity and the increase in the diffusivity on the membrane surface [
30].
This paper aims to study the removal of copper (II) ions by reverse osmosis using SEPA CF042 Membrane Test Skid-TFC BW30XFR under different operating pressures, temperatures, feed concentrations and feed flow rates. The temperature correction factor (TCF) was studied. This important factor, which has rarely been investigated, is related to the alternating operating conditions in a novel approach. Moreover, mathematical models were developed to predict the impact of the tested parameters and were validated with the experimental data, which are considered the novelty of this work.
2. Materials and Methods
2.1. Artificial Wastewater Preparation
A copper stock solution of 2000 (mg/L) was prepared from copper sulfate pentahydrate salt (CuSO4·5H2O) pharma-grade assay 99.0–100% obtained from Panreac Co. (Milano, Italy), and then diluted copper solutions were prepared to concentrations of 25, 50, 100 and 150 (mg/L).
2.2. RO Membrane Setup
Experiments were conducted using (SEPA CF 042 Membrane Test Skid) from Sterlitech Co (Auburn, Al, USA)., which is a crossflow filtration unit that uses membranes for RO processes. The flat sheet membrane used in the device was Dow Polyamide TFC BW30XFR, with a pH range from 2–11, molecular weight cutoff (MWCO) of 100 Da with an effective membrane area of 140 cm
2 (
Figure 1).
2.3. Test Method
The experimental tests were performed on feed solutions of concentrations 25, 50, 100, and 150 ppm and operating pressures of 10, 20, 30 and 40 bar with regulated feed flow rates of 2, 3.2 and 4.4 L/min, while the temperature was gradually varied between 25, 35 and 45 °C. Every 10 min, the permeate water was collected and weighed to obtain the mass of permeate that would then be used to calculate the permeate flux and the metal rejection. The permeate and retentate water were continuously returned to the feed tank to maintain the feed concentration. A new membrane was replaced for each different feed concentration. The device was cleaned by running distilled water for at least 10 min before starting the new feed concentration. The product water samples were analyzed using a total dissolved solids (TDS) meter (VSTAR20, Thermo Scientific, Waltham, MA, USA).
2.4. Theoretical Calculations
The permeate flux is determined by Equation (1):
where:
= The permeate flux (kg/h·m
2),
= the permeate mass (kg),
= process time (h),
= the membrane surface area (m
2)
Heavy metal rejection is determined using Equation (2):
where:
= the metal rejection (percentage),
= the concentration in permeate (ppm),
= the concentration in feed solution (ppm).
4. Conclusions
The effects of operating pressure, feed temperature, feed concentration and feed flow rate on the permeate flux and removal of copper ions using reverse osmosis were experimentally and numerically investigated. It is concluded that the permeate flux and the copper ion removal % are directly proportional to operating pressure. For different feed concentrations, when the pressure increased from 10 to 40 (bar), the permeate flux increased by 180%, and the (Cu2+) removal % increased from 89.98 to 94.21%. Similarly, the permeate flux and Cu (II) removal % were found to be directly proportional to the feed temperature: when the temperature increased from 25 to 45 °C, the permeate flux increased by 45% due to the reduction in the solution viscosity and changes in the physical properties of the membrane. The permeate flux and copper ions removal % were found to be inversely proportional to the feed concentration. The feed flow rate, on other hand, showed negligible impact on the permeate flux and Cu (II) removal % for different feed concentrations. Moreover, the results showed that the temperature correction factor (TCF) is directly proportional to the temperature but inversely proportional to the operating pressure; however, the feed flow rate showed no effect on the TCF. Mathematical models were developed for the permeate flux and copper removal. The model of permeate flux showed a perfect match when compared with the experimental data; however, the copper removal model did not match well the experimental data.