Adsorption characteristics of ball clay for mono and multi-component heavy metal ions

Heavy-metal-contaminated water has posed a serious risk to health and survival in recent years. It is urgent to prevent and treat heavy metal water pollution. Adsorption can be an economically feasible and environmentally friendly way for the removal of heavy metal ions from water. Clay minerals can be directly used for natural adsorption, due to their high adsorption properties. Among them, ball clay is characterized by fine particles, high content of disordered kaolinite, and high activity, indicating high potential adsorption. However, it is still lacking in the adsorption performance at present. This study aims to investigate the adsorption properties and competitive adsorption relationship of ball clay for Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Cr⁶⁺. Batch adsorption tests were performed on the mono and multi-heavy metal ions. The mono-component heavy ion adsorption tests showed that the adsorption capacity of ball clay for the heavy metal ions followed a pattern of rapid increase followed by slow increase with the increase of pH value and the initial concentration, and finally reaching adsorption saturation. The optimal duration of adsorption equilibrium was 90 min, and the optimal solution pH value was 5. The influence level of pH value on the adsorption was ranked in the descending order of Zn²⁺, Cd²⁺, Cr⁶⁺, Cu²⁺, Pb²⁺. The adsorption capacity for Zn²⁺ (the most influential ion) increased by 184% when pH value varied from 2 to 5. The weights of saturated adsorption for the five ions Pb²⁺, Cr⁶⁺, Cd²⁺, Zn²⁺, and Cu²⁺ were 311.847, 301.437, 263.213, 195.435 and 179.635 mg/g, respectively. The kinetic and isotherm models displayed that the adsorption data was more consistent with the pseudo-second-order kinetic and Langmuir isothermal adsorption models, indicating the single-layer and chemical adsorption. The maximum adsorption was ranked in the order of the Pb²⁺ (497.593 mg /g) > Cr⁶⁺ (442.868 mg/g) > Cd²⁺ (339.376 mg/g) > Zn²⁺ (290.039 mg/g) > Cu²⁺ (194.941 mg/g). Multi-component adsorption tests showed that the adsorption capacity of ball clay for the four types of ions also shared a trend of rapid increase followed by slow increase until it was stable. The intensities of four heavy metal ions on the active sites of ball clay were ranked in the descending order of Cr⁶⁺, Cd²⁺, Zn²⁺, and Cu²⁺. Isotherm fitting found that the multi-component heavy metal ion adsorption was more consistent with the Langmuir isothermal adsorption model. The adsorption mode and process were consistent with those of the mono-component heavy metal ions. There was a decrease in the maximum adsorption capacity of ball clay for each ion in the multi-component heavy metal ion solutions. However, the total amount of adsorption increased greatly in the ball clay, indicating the better adsorption capacity for the multi-metal ions and the synergistic effect among heavy metals. Therefore, the ball clay can be expected to serve as the greater adsorption for the heavy metal ions. This finding can provide the theoretical basis and data support to the ball clay as an adsorbent, The treatment efficiency of industrial wastewater can be improved to reduce the secondary pollution to rivers and agricultural products, particularly with the great practical significance.