Effects of Phosphate on the Adsorption of Heavy Metal Ions onto TiO₂ Nanoparticles in Water and Mechanism Analysis

Nano-titanium dioxide (nTiO₂) is widely used to remove heavy metals from water. Phosphate, a common inorganic anion in the aquatic environment, can affect the adsorption characteristics of heavy metal ions on nTiO₂. However, the current state of knowledge on the influences of phosphate on the adsorption behaviors of heavy metal ions onto TiO₂ nanoparticles (nTiO₂) is inadequate. Herein, batch adsorption experiments were conducted to investigate the effects of phosphate on the adsorption of heavy metal ions (i.e., Zn²⁺ and Cd²⁺) onto suspended nTiO₂.

To elucidate the primary mechanisms controlling the adsorption behaviors of Zn²⁺ and Cd²⁺ on suspended nTiO₂ in the presence of phosphate under different solution chemistry conditions.

In order to determine the effects of phosphate on the adsorption of heavy metal ions onto nTiO₂, batch experiments were conducted by mixing background electrolyte ions, nTiO₂, and phosphate, which contained various concentrations of Zn²⁺ or Cd²⁺ in 20mL-amber glass vials at room temperature. The pH of solution was adjusted to target 7.0 using 0.1mol/L HCl or 0.1mol/L NaOH accordingly. Then, the mixtures were rotated end-over-end for 24h. After equilibration, the liquid and solid phases were separated by centrifugation at 15000r/min for 20min, and then the supernatants were filtered through 0.22μm pore-size cellulose ester membrane filter (the loss of metal ions can be neglected). The concentrations of Zn²⁺ or Cd²⁺ in the filtrate were measured by an inductively coupled plasma-optical emission spectrometry (ICP-OES). The adsorbed metal ions were then determined by the difference between the initial and final concentrations of metal ions in the aqueous phase. Furthermore, the classic Langmuir and Freundlich sorption models were used to correlate the adsorption isotherms.

(1) Adsorption isotherms showed that the presence of phosphate could enhance the adsorption of metal ions onto nTiO₂, the maximum adsorption capacity of Zn²⁺ and Cd²⁺ increased from 121.1mg/g and 84.7mg/g to 588.3mg/g and 434.8mg/g, respectively. We propose that phosphate probably enhance the adsorption of Zn²⁺ and Cd²⁺ onto nTiO₂ by the following mechanisms. Firstly, the ζ-potential of nTiO₂ surface becomes more negative with the increase of phosphate concentration in aqueous phase. Consequently, the electrostatic attraction between negatively charged nanoparticles and positively charged metal ions generally increases with increasing phosphate content. Secondly, the phosphate added into nTiO₂ suspension inhibits the aggregation of nanoparticles. In this case, more nTiO₂ could sufficiently contact metal ions, thus increasing the adsorption sites. Thirdly, phosphate could form an inner-sphere surface complex on the nTiO₂ surface, which can greatly influence the surface chemistry of nTiO₂^[55-57]. These products could strongly immobilize heavy metal ions^[40-41]. These results might account for enhanced Zn²⁺ and Cd²⁺ adsorption on nTiO₂ by forming metal-phosphate-surface ternary complexes in the presence of phosphate.

(2) The adsorption of heavy metals onto nTiO₂ decreased when concentration of NaCl increased from 0 to 10 mmol/L. It is likely that ionic strength can affect the attachment of nanoparticles via three major mechanisms. Firstly, the presence of competing cations (Na⁺) of salt reduces the adsorption of metal ions (i.e., Zn²⁺ and Cd²⁺) and this effect may have more significant roles with increasing Na⁺ concentration. Secondly, this could be related to the fact that an increase in ionic strength interferes with the electrostatic attraction between nTiO₂ and metal ions. Thus, adsorption of metal ions is suppressed. Thirdly, increasing ionic strength significantly enhances aggregation of nTiO₂, and consequently decreases the active surface sites of nTiO₂.

(3) The coexistence of competing anions (such as Cl^-, NO₃^- and SO₄^2-) weaken the enhancement effect of phosphate on the adsorption of metal ions onto nTiO₂, and the order of inhibition is: SO₄^2- >NO₃^->Cl^-. This may be because anions with higher ionic radii (i.e., SO₄^2-) may occupy more surface reactive sites. On the other hand, the competitive adsorption is related to the valence. It is well known that Cl^- and NO₃^- are more likely to form "outer sphere" complexes with binding surfaces. Meanwhile, the electrostatic adsorption and the ion energy of monovalent anions (e.g., Cl^- and NO₃^-) are weaker than those of divalent anions (e.g., SO₄^2-). For this reason, the competitive influence of Cl^- and NO₃^- during the adsorption of metal ions is negligible. In comparison, the divalent anion has a relatively stronger competitiveness on the adsorption of metal ions.

The research results show that phosphate can significantly enhance the removal efficiency of nTiO₂ to heavy metal ions, but the removal efficiency will be affected by the water chemical conditions in the background solution. Previous studies show that nTiO₂ is promising as an adsorbent for the removal of metal ions from aqueous solution. The present study demonstrates that phosphate plays an important role in adsorption of metal ions (e.g., Zn²⁺ and Cd²⁺) onto nTiO₂. Phosphate significantly enhances adsorption of Zn²⁺ and Cd²⁺ onto nTiO₂ by forming metal-phosphate-surface ternary complexes and increasing electrostatic attraction. The increase of ionic strength results in the low adsorption of metal ions in the presence of phosphate, resulting from electronic shielding of the negatively charged sites on the nTiO₂ surface and competition between Na⁺ and heavy metal ions for active surface sites. Moreover, the addition of competitive anions inhibits the adsorption of metal ions in the presence of phosphate following the order of SO₄^2->NO₃^->Cl^-. This phenomenon is mainly ascribed to the decrease of phosphate adsorption because of competition between anions and phosphate for adsorption sites on nTiO₂ surface, resulting in decreasing the amount of metal-phosphate-surface ternary complexes. Overall, the results obtained from this study indicate that the adsorption of metal ions onto nTiO₂ varies greatly with factors such as phosphate, ionic strength, and competitive anions. Therefore, these factors should be well considered to better understand the fate and toxicity of metal ions in the adsorption process for the treatment of wastewater.