1. Introduction
Statistics show that China discharges approximately 8 × 10⁸ m³ of industrial wastewater annually , containing not only highly toxic components such as cyanide but also metal ions like chromium, zinc, and nickel. Many wastewater treatment methods exist, primarily including chemical precipitation, electrolysis, and membrane treatment. This article introduces activated carbon adsorption. Activated carbon has a large surface area and high physical and chemical adsorption capabilities. Therefore, activated carbon adsorption is widely used in wastewater treatment , offering advantages such as high efficiency and good results.
2. Activated carbon
Activated carbon is a specially treated type of carbon with countless tiny pores and a huge surface area, ranging from 500 to 1500 square meters per gram. Activated carbon has strong physical and chemical adsorption capabilities, and also possesses detoxification properties. This detoxification effect utilizes its vast surface area to adsorb toxins within the micropores of the activated carbon, thus preventing their absorption. Simultaneously, activated carbon can bind with various chemical substances, thereby inhibiting their absorption.
2.1 Classification of Activated Carbon
There are many types of activated carbon used in production. They are generally made into powder or granules.
Powdered activated carbon has strong adsorption capacity, is easy to prepare, and is inexpensive, but it is difficult to regenerate and generally cannot be reused.
Granular activated carbon is more expensive, but it can be regenerated and reused, and the working conditions during use are better, making operation and management convenient. Therefore, granular activated carbon is more commonly used in water treatment.
2.2 Activated carbon adsorption
Activated carbon adsorption refers to the adsorption of one or more substances in water by the solid surface of activated carbon to purify water.
2.3 Factors Affecting Activated Carbon Adsorption
Adsorption capacity and adsorption rate are the main indicators for evaluating the adsorption process. Adsorption capacity is measured by the amount of substance adsorbed. Adsorption rate, on the other hand, refers to the amount of substance adsorbed per unit weight of adsorbent per unit time. In water treatment, the adsorption rate determines the required contact time between wastewater and the adsorbent.
The adsorption capacity of activated carbon is related to its pore size and structure. Generally speaking, the smaller the particles, the faster the pore diffusion rate, and the stronger the adsorption capacity of the activated carbon.
The pH and temperature of wastewater also affect the adsorption capacity of activated carbon. Activated carbon generally exhibits higher adsorption capacity under acidic conditions than under alkaline conditions. Adsorption reactions are typically exothermic, therefore lower temperatures are more favorable for the adsorption process.
Of course, the adsorption capacity of activated carbon is related to the concentration of wastewater. At a certain temperature, the adsorption capacity of activated carbon increases with the increase of the equilibrium concentration of the adsorbed substance.
3. Application of activated carbon in wastewater treatment
Because activated carbon has high requirements for water pretreatment and is expensive, it is mainly used in wastewater treatment to remove trace pollutants in order to achieve deep purification.
3.1 Activated carbon treatment of chromium-containing wastewater
Chromium is a metal raw material used in large quantities in electroplating. In wastewater, hexavalent chromium exists in different forms depending on the pH value.
Activated carbon has a highly developed microporous structure and a high specific surface area, giving it a strong physical adsorption capacity, effectively adsorbing Cr( VI ) from wastewater. The surface of activated carbon contains numerous oxygen-containing groups such as hydroxyl (-OH) and carboxyl (-COOH) , which have electrostatic adsorption capabilities, resulting in the chemical adsorption of Cr( VI ) . Therefore, it can be fully used to treat Cr( VI ) in electroplating wastewater , and the adsorbed wastewater can meet national discharge standards.
Experiments show that when the mass concentration of Cr( VI ) in the solution is 50 mg/L , pH=3 , and the adsorption time is 1.5 h , the adsorption performance of activated carbon and the removal rate of Cr( VI ) both reach the best effect.
Therefore, the process of treating chromium-containing wastewater with activated carbon is the result of the combined effects of physical adsorption, chemical adsorption, and chemical reduction of Cr( VI ) in the solution by activated carbon . Activated carbon treatment of chromium-containing wastewater exhibits stable adsorption performance, high treatment efficiency, and low operating costs, thus providing certain social and economic benefits.
3.2 Activated carbon treatment of cyanide-containing wastewater
In industrial production, cyanide or by-product cyanide is used in industries such as wet extraction of gold and silver, production of chemical fibers, coking, synthetic ammonia, electroplating, and coal gas production. Therefore, a certain amount of cyanide-containing wastewater will inevitably be discharged during the production process.
Activated carbon has been used for wastewater purification for a considerable period of time, and there are increasing reports in the literature on its application in treating cyanide-containing wastewater. However, due to the small adsorption capacity of CN- and HCN on activated carbon, generally 3 mg CN/g AC to 8 mg CN/g AC ( depending on the type ) , it is not cost-effective in terms of treatment costs.
3.3 Activated carbon treatment of mercury-containing wastewater
Activated carbon has the ability to adsorb mercury and mercury-containing compounds, but its adsorption capacity is limited, making it suitable only for treating wastewater with low mercury content. If the mercury concentration is high, chemical precipitation can be used first, resulting in a mercury content of approximately 1 mg/L , or up to 2-3 mg/L in some cases , before further treatment with activated carbon.
3.4 Activated carbon treatment of phenol-containing wastewater
Phenolic wastewater widely originates from petrochemical plants, resin plants, coking plants, and oil refineries. Experiments have shown that activated carbon has good adsorption performance for phenol; however, increasing temperature is detrimental to adsorption, reducing the adsorption capacity ; but increasing the temperature shortens the time to reach adsorption equilibrium. There are optimal values for the amount of activated carbon and the adsorption time. Under acidic and neutral conditions, the removal rate does not change significantly ; under strongly alkaline conditions, the phenol removal rate drops sharply, and the stronger the alkalinity, the worse the adsorption effect.
3.5 Activated carbon treatment of methanol-containing wastewater
Activated carbon can adsorb methanol, but its adsorption capacity is not strong, and it is only suitable for treating wastewater with low methanol content. Engineering operation results show that the COD of the mixed solution can be reduced from 40 mg/L to below 12 mg/L , and the methanol removal rate reaches 93.16% –100 % . The effluent quality meets the water quality requirements for reuse in the boiler demineralization system.
3.6 Deep Processing in Refineries
Oily wastewater from the refinery undergoes oil separation, flotation, and biological treatment, followed by further treatment through sand filtration and activated carbon filtration. The phenol content decreased from 0.1 mg/L ( after biological treatment ) to 0.005 mg/L , cyanide from 0.19 mg/L to 0.048 mg/L , and COD from 85 mg/L to 18 mg/L.
4. Prospects
With the advancement of science and technology and the special requirements of wastewater treatment, the research on activated carbon has gradually developed from its pore structure and specific surface area to the study of the influence of surface functional groups on the adsorption performance of activated carbon.
For example, activated carbon fiber ( ACF ) has received attention from researchers in wastewater treatment in recent years. Its diameter is generally 5 to 20 μm , and its preparation principle is the same as that of traditional activated carbon, that is, fibrous carbon is activated with steam or carbon dioxide at a temperature above 800 ° C.
The pore structure of fibrous activated carbon is mainly micropores with very few mesopores and almost no macropores, and the specific surface area can reach 2500 m2/g. It has the characteristics of high adsorption and desorption rates, large adsorption capacity and high conductivity.
Experiments show that ACF has an adsorption capacity of 248 mg/g for phenol , and the adsorption capacity remains almost unchanged after multiple regenerations after adsorption saturation, demonstrating better adsorption performance than activated carbon. At room temperature, under acidic or neutral conditions, adding 0.5 g of activated carbon fiber to 100 mL of simulated phenol-containing wastewater with a concentration of 282 mg/L and shaking at a constant temperature for 30 min resulted in a phenol removal rate of up to 91% .
Recently, it has been discovered that activated carbon not only possesses adsorption properties but also exhibits catalytic properties. As a result, catalytic oxidation methods have gained increasing attention, and research on these methods continues to deepen. To improve treatment efficiency, research is being conducted on the catalytic oxidation mechanism, modifying the surface structure of activated carbon to enhance its capacity and identify ideal adsorbents.
