磷酸盐对纳米二氧化钛吸附水体中重金属离子行为的影响和机理分析

Effects of Phosphate on the Adsorption of Heavy Metal Ions onto TiO 2 Nanoparticles in Water and Mechanism Analysis

纳米二氧化钛(n TiO_(2))被广泛应用于去除水体中的重金属。磷酸盐作为水体中普遍存在的无机阴离子,能够对重金属离子在n TiO_(2)上的吸附特征产生影响。本文聚焦磷酸盐存在条件下n TiO_(2)胶体颗粒对典型重金属离子(Zn^(2+)和Cd^(2+))的吸附行为,以电感耦合等离子体发射光谱法(ICP-OES)测定吸附平衡后水相中重金属离子的浓度。通过批量吸附实验考察不同水化学条件下(离子强度和共存阴离子),磷酸盐对n TiO_(2)胶体颗粒吸附水体中Zn^(2+)和Cd^(2+)特征的影响规律。采用经典吸附等温线模型对实验数据进行拟合,并结合纳米颗粒的Zeta电位和粒径变化等表征手段揭示了相关吸附机制。研究发现:①磷酸盐的存在能有效地增强重金属在n TiO_(2)上的吸附,Zn^(2+)和Cd^(2+)的最大吸附量分别由121.1mg/g和84.7mg/g增加至588.3mg/g和434.8mg/g,增加了3.8~4.1倍。这主要由于磷酸盐能够通过桥连作用形成金属-磷酸盐-n TiO_(2)三元络合物以及增加重金属离子和胶体颗粒之间的静电引力,从而增强n TiO_(2)对重金属离子的吸附。②背景溶液中离子强度的增加会降低n TiO_(2)对重金属离子的吸附效果。当背景溶液中离子强度(NaCl浓度)从0增加至10mmol/L时,n TiO_(2)与金属离子之间的静电吸引会减弱,同时Na+与重金属离子竞争n TiO_(2)表面吸附位点亦降低了n TiO_(2)对重金属离子的吸附。③共存竞争性阴离子(如Cl-、NO_(3)^(-)和SO_(4)^(2-))会削弱磷酸盐对n TiO_(2)吸附金属离子的增强作用,抑制顺序为:SO_(4)^(2-)>NO_(3)^(-)>Cl-,即与其离子半径的数量级成反比。这是由于共存阴离子与磷酸盐竞争n TiO_(2)表面的吸附位点所致。研究结果表明,磷酸盐可以显著地增强n TiO_(2)对重金属离子的去除效能,但是去除效能的大小会受到背景溶液中水化学条件的影响。

BACKGROUND:Nano-titanium dioxide(n TiO_(2))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 n TiO_(2).However,the current state of knowledge on the influences of phosphate on the adsorption behaviors of heavy metal ions onto TiO_(2)nanoparticles(n TiO_(2))is inadequate.Herein,batch adsorption experiments were conducted to investigate the effects of phosphate on the adsorption of heavy metal ions(i.e.,Zn^(2+)and Cd^(2+))onto suspended n TiO_(2).OBJECTIVES:To elucidate the primary mechanisms controlling the adsorption behaviors of Zn^(2+)and Cd^(2+)on suspended n TiO_(2)in the presence of phosphate under different solution chemistry conditions.METHODS:In order to determine the effects of phosphate on the adsorption of heavy metal ions onto n TiO_(2),batch experiments were conducted by mixing background electrolyte ions,n TiO_(2),and phosphate,which contained various concentrations of Zn^(2+)or Cd^(2+)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 Zn^(2+)or Cd^(2+)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.RESULTS:(1)Adsorption isotherms showed that the presence of phosphate could enhance the adsorption of metal ions onto n TiO_(2),the maximum adsorption capacity of Zn^(2+)and Cd^(2+)increased from 121.1mg/g and 84.7mg/g to 588.3 mg/g and 434.8mg/g,respectively.We propose that phosphate probably enhance the adsorption of Zn^(2+)and Cd^(2+)onto n TiO_(2)by the following mechanisms.Firstly,theζ-potential of n TiO_(2)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 n TiO_(2)suspension inhibits the aggregation of nanoparticles.In this case,more n TiO_(2)could sufficiently contact metal ions,thus increasing the adsorption sites.Thirdly,phosphate could form an inner-sphere surface complex on the n TiO_(2)surface,which can greatly influence the surface chemistry of n TiO_(2)[55-57].These products could strongly immobilize heavy metal ions[40-41].These results might account for enhanced Zn^(2+)and Cd^(2+)adsorption on n TiO_(2)by forming metal-phosphate-surface ternary complexes in the presence of phosphate.(2)The adsorption of heavy metals onto n TiO_(2)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.,Zn^(2+)and Cd^(2+))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 n TiO_(2)and metal ions.Thus,adsorption of metal ions is suppressed.Thirdly,increasing ionic strength significantly enhances aggregation of n TiO_(2),and consequently decreases the active surface sites of n TiO_(2).(3)The coexistence of competing anions(such as Cl-,NO_(3)^(-) and SO_(4)^(2-))weaken the enhancement effect of phosphate on the adsorption of metal ions onto n TiO_(2),and the order of inhibition is:SO_(4)^(2-)>NO_(3)^(-)>Cl-.This may be because anions with higher ionic radii(i.e.,SO_(4)^(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 NO_(3)^(-) 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 NO_(3)^(-))are weaker than those of divalent anions(e.g.,SO_(4)^(2-)).For this reason,the competitive in uence of Cl-and NO_(3)^(-) during the adsorption of metal ions is negligible.In comparison,the divalent anion has a relatively stronger competitiveness on the adsorption of metal ions.CONCLUSIONS:The research results show that phosphate can significantly enhance the removal efficiency of n TiO_(2)to heavy metal ions,but the removal efficiency will be affected by the water chemical conditions in the background solution.Previous studies show that n TiO_(2)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.,Zn^(2+)and Cd^(2+))onto n TiO_(2).Phosphate significantly enhances adsorption of Zn^(2+)and Cd^(2+)onto n TiO_(2)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 n TiO_(2)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 SO_(4)^(2-)>NO_(3)^(-)>Cl-.This phenomenon is mainly ascribed to the decrease of phosphate adsorption because of competition between anions and phosphate for adsorption sites on n TiO_(2)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 n TiO_(2)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.