Nanofiltration and reverse osmosis membranes for treating nickel-plating wastewater

The process of depositing a nickel coating on the surface of metals or certain nonmetals by electrolytic or chemical means is known as nickel plating. Nickel plating encompasses both electroless nickel plating and electroplated nickel. In electroless nickel plating, metallic nickel serves as the anode, the workpiece acts as the cathode, and direct current is applied. Within an electrolyte composed of a nickel salt (the main salt), conductive salts, a pH buffer, and a wetting agent, a uniform and dense nickel coating is deposited onto the cathode (the workpiece). Bright nickel is obtained from a plating bath containing brightening agents, whereas dark nickel is produced in an electrolyte devoid of such agents.

Electroplating wastewater is harmful to the environment. If nickel in the wastewater enters the biosphere via surface water or groundwater, it can accumulate in the human body, posing health risks and even causing cancer. Many countries have developed a variety of technologies to treat and reduce such discharges. However, conventional industrial treatment methods for electroplating wastewater have certain limitations. In recent years, membrane separation technology has advanced rapidly. This process features no chemical reactions, no heating, no phase changes, and no secondary pollution. Nickel is widely used in the electroplating industry, with plating baths typically containing nickel sulfate or nickel chloride. Both nickel and its compounds are toxic substances. In response to increasingly stringent environmental regulations and the specific characteristics of nickel‑containing electroplating wastewater, two membrane separation techniques are commonly employed for its treatment.

I. Nanofiltration Membrane Treatment of Nickel-Plating Wastewater

As the operating pressure increases, the dynamic deviation in the membrane separation process grows, leading to a higher membrane flux and enhanced wastewater treatment capacity. For nanofiltration membranes, when the surface fouling level is significant, the rejection rate of Ni²⁺ decreases with increasing pressure. Consequently, during constant‑flux operation, once the operating pressure reaches a certain threshold, the membrane must be cleaned.

Under constant pressure, as the permeate flow rate increases, the rejection rate of Ni²⁺ ions by the nanofiltration membrane also rises, eventually reaching a steady state. At a given flow rate, increasing the operating pressure leads to a decrease in the rejection rate of Ni²⁺ ions. At low pressure (0.5 MPa), the removal efficiency for Ni²⁺ can exceed 99%, allowing for the recovery of most of the heavy metal. Thus, nanofiltration membranes are well suited for treating wastewater containing Ni²⁺.

II. Reverse Osmosis Membrane Treatment of Nickel-Plating Wastewater

The trends in membrane flux and operating pressure are similar to those observed in nanofiltration: as the operating pressure increases, the dynamic deviation of the membrane separation process grows, leading to a corresponding increase in membrane flux and enhanced wastewater treatment capacity. However, the increase in membrane flux is significantly less pronounced in reverse osmosis than in nanofiltration.

Like nanofiltration, reverse osmosis exhibits a very high rejection rate for Ni²⁺ ions, reaching over 99% once equilibrium is established. However, unlike nanofiltration, reverse osmosis is less sensitive to operating pressure, resulting in a relatively stable rejection performance. Both methods are highly effective for treating Ni²⁺‑containing wastewater; nevertheless, reverse osmosis membranes require higher operating pressures than nanofiltration membranes.

Nanofiltration membranes can operate at relatively low operating pressures, significantly reducing energy consumption, with recovery rates exceeding 50%—a performance that rivals industry standards. The permeate stream can be recovered according to specific customer requirements, and membrane element performance can be restored through cleaning, thereby lowering overall system operating costs. The system operates at ambient temperatures, without phase changes, thus avoiding adverse effects on the active components of the feed stream and yielding products with high concentrations of target constituents.

Reverse osmosis membranes offer high water flux, excellent desalination performance, stable product water output, and superior cost-effectiveness. The separation process involves no phase change, ensuring stable and reliable operation. They effectively retain organic matter, colloids, particulates, bacteria, and viruses, while exhibiting strong resistance to fouling, easy cleanability, and dependable performance. With low energy consumption and high water recovery rates, their operating costs are lower than those of other systems. Additionally, RO membrane systems are compact, simple to operate, easy to maintain, and highly practical.