Nitric Acid Modified Activated Carbon for Metal Ions Adsorption

The chemical interaction between the organic molecules and the activated carbon surface is a function of three factors: (1) the structure of the target molecule; (2) the surface chemistry of the activated carbon; (3) the solution chemistry. The functional groups on the surface of activated carbon have a significant impact on its adsorption properties. At present, the surface modification of activated carbon mainly focuses on improving the surface acidity through oxidation, especially by HNO₃.

Activated carbon can not only adsorb organic matter from water, but also remove inorganic pollutants such as heavy metals from water through various mechanisms. Activated carbon can effectively remove iron and radioactive iodine through catalytic oxidation and physical adsorption, mercury and free chlorine through catalytic reduction. It can also reduce Chromium(VI) to less toxic chromium(III).

Effect of activated carbon surface modification by nitric acid on its physical and chemical properties

Chemical parameters like surface functional groups, pHPZC, and physical parameters such as specific surface area, iodine number, methylene blue value of the activated carbon samples modified by different concentrations of nitric acid, as well as the one modified by concentrated nitric acid and then concentrated ammonia, are tested. (The samples are rinsed to be neutral.)

As the oxidation degree of increases, the content of acidic surface functional groups, especially carboxyl groups, of activated carbon gradually increases, while the basic groups, pHPZC, and specific surface area gradually decrease. After the concentrated nitric acid modified activated carbon is further modified by ammonia, the carboxyl group content significantly reduces, while the weakly acidic and basic groups such as phenolic hydroxyl groups, as well as pHPZC and specific surface area increase. For the three physical parameters (specific surface area, iodine number, methylene blue value), wood-based activated carbon is generally higher than coal-based activated carbon.

Effect on the adsorption of metal ions

Adsorption of calcium ions

The content of Ca²⁺ and Mg²⁺ is much higher than heavy metal ions in both surface water and groundwater. Because of their chemical properties similar to heavy metal ions, they may influence the adsorption of heavy metals by activated carbon. The test results show that except for the two activated carbon samples which are oxidized by concentrated nitric acid, the adsorption capacity of other activated carbon samples for Ca²⁺ is small. And the concentration of surface acid groups and the adsorption capacity for Ca²⁺ of the two samples are close to those of ordinary ion exchange resins, indicating that they have a strong ability to adsorb metal ions.

Under a neutral pH condition, calcium in water mainly exists in the form of Ca²⁺. The correlation between the content of carboxyl groups on the activated carbon surface and the amount of Ca²⁺ adsorption indicates that one of the adsorption mechanisms of Ca²⁺ by activated carbon is the complexation with surface groups like carboxyl groups.

Adsorption of lead(II) ions

At different pH values, the main existing forms of Pb(II) are also different. In order to avoid the influence of different forms of Pb(II) on the activated carbon adsorption performance, this test regarding the adsorption of lead(II) is under an acidic condition of pH 2, where Pb(II) mainly exists in the form of Pb²⁺.

The adsorption capacity of the tested activated carbon samples for Pb²⁺ is similar to that for Ca²⁺. The activated carbon sample oxidized by concentrated nitric acid has a significantly higher adsorption capacity for Pb²⁺ than other samples. According to the calculation of the surface carboxyl group concentration, its maximum adsorption capacity for Pb²⁺ is 10⁴ times that of unmodified ones.

The presence of other ions such as Ca²⁺ and Mg²⁺ did not seriously affect the adsorption of Pb²⁺ by activated carbon, although the concentration of these ions was much higher than that of Pb²⁺ (3-4 orders of magnitude). This may be the reason that the hydration radius of Pb²⁺ is smaller than those of Ca²⁺ and Mg²⁺. So Pb²⁺ is preferentially adsorbed by activated carbon.

Summary

The content of acidic groups on the surface of activated carbon can be increased by nitric acid oxidation, resulting in increased hydrophilicity of the carbon surface and a decrease in pHPZC value. Meanwhile, concentrated nitric acid oxidation can also affect the structure of activated carbon and lead to the decrease of the specific surface area, thereby decreasing the physical adsorption performance of activated carbon.
Nitric acid oxidation significantly increases the adsorption capacity of activated carbon for Pb²⁺. And the presence of ions such as Ca²⁺ and Mg²⁺ has little influence on the Pb²⁺ adsorption.