Study On The Adsorption Performance And Mechanism Of Heavy Metal Ions By Halloysite Clay Nanotubes And Their Activated/Inorganic-modified Products

Toxic metal ions such as Cd（II）,Zn（II）and Pb（II）emitted by human activities are a threat to environmental ecosystems and human health.With considerable surface activity and good support function,halloysite nanotubes have great potential for adsorption of heavy metal ions and preparation of nanohybrid adsorbents.Some researches have been carried out to use raw halloysite nanotubes and their activation products or modified products as heavy metal adsorbents,and some problems have been exposed;for example,the heavy metal adsorption performance of raw and thermal-activated halloysite were still unclear or controversial,as well as the inorganic-modified halloysite as heavy metal adsorbents had not been well studied.To address these issues,a series of related studies were carried out in this thesis.Finally,the heavy metal adsorption properties and application values of raw and activated halloysite were clarified,and two heavy metal adsorbents with good adsorption performance,cheap and low toxicity potential were prepared through the modification of halloysite nanotubes with monometallic oxides and bimetallic oxides.The main contents and main conclusions of this study are as follows:（1）In order to clarify the adsorption performance of raw halloysite nanotubes on heavy metal ions,samples of halloysite from three different origins,including American Dragon Mine,Yunnan Province and Henan Province of China,were collected.They were characterized and analyzed,and their adsorption performance for heavy metal ions were determined under uniform experimental conditions.As a result,the adsorption capacities of the three halloysite samples varied slightly in the range of 7.8-10.2 mg/g,2.7-3.2 mg/g and 25.5-30.6 mg/g for Cd（II）,Zn（II）and Pb（II）,respectively.Yunnan halloysite had higher adsorption performance for heavy metal ions,which was related to its higher Al–OH density on the surface than one of American and Henan halloysite.Ultimately,it was concluded that the raw halloysite nanotubes had certain adsorption properties for heavy metal ions and can be directly used as an economical,cheap,and low-toxic heavy metal adsorbent.In addition,the Yunnan halloysite was also selected as the test material for the subsequent chapters of this thesis.（2）In order to clarify the adsorption performance of the activated halloysite nanotubes on heavy metal ions,halloysite was activated by heating,acid etching and combined heat/acid treatment.The activated products were characterized and analyzed,and their adsorption performance for heavy metal ions were determined under uniform experimental conditions.As a result,the heating treatment at 500-800°C reduced the adsorption performance of halloysite,and the acid treatment slightly enhanced the adsorption performance of halloysite.However,combined heat/acid treatment significantly increased the adsorption performance of halloysite nanotubes for Cd（II）,Zn（II）and Pb（II）by about 1-fold to～20 mg/g,～6 mg/g and～60 mg/g,respectively;this was related to the significant increase in both of specific surface area and Si–OH of halloysite after heat/acid treatment.The combined heat/acid treatment was finally concluded to be the best activation method to enhance the adsorption performance of halloysite nanotubes for heavy metal ions,and the resulting activated halloysite was a cheap and low-toxic heavy metal adsorbent with better adsorption performance than the raw halloysite.（3）In order to select a novel Fe_xO_y-modified halloysite nanotube as heavy metal adsorbent,three different Fe_xO_y-modified halloysite nanotubes,including aluminous hematite-modified halloysite Al He@HNTs,ferrihydrite-modified halloysite Fh@HNTs and siliceous ferrihydrite-modified halloysite Si Fh@HNTs,were obtained by aging the ferrihydrite/halloysite hybrids Fh/HNTs,respectively in acid,neutral and alkaline media.Among them,the Cd（II）adsorption performance of Si Fh@HNTs was significantly better;in particular,the Si Fh@HNT₄₀ with 40%clay content showed the Cd（II）adsorption capacity is up to 120.1 mg/g,which was about 11 times higher than that of the raw halloysite.The formation mechanism of Si Fh@HNT₄₀ was that the Si O₄^4-derived from clay was dopped into the ferrihydrite crystals,which inhibited the phase transformation of ferrihydrite and forms Si Fh nanoparticles（NPs）;meanwhile,Si Fh NPs were anchored on halloysite nanotubes,enhancing the dispersibility of Si Fh NPs and the exposure density of Fe–OH group.The main mechanism of Si Fh@HNT₄₀ adsorption of Cd（II）was the inner-sphere complexation of Cd（II）with Fe–OH.Thus,Si Fh@HNT₄₀ was an inexpensive and low-toxic heavy metal adsorbent with superior adsorption performance than activated halloysite.（4）In order to further elucidate the application potential of Si Fh@HNT₄₀,the effects of various environmental factors on its Cd（II）adsorption behavior on Si Fh@HNT₄₀were investigated.The results showed that Si Fh@HNT₄₀ is a fast adsorbent,can be recycled several times and has good adsorption effect on various heavy metal ions such as Cd（II）,Zn（II）and Pb（II）,although significant competitive adsorption occurs in mixed heavy metal systems.The elevated temperature and p H,the presence of anions SO₄^2-and CO₃^2-,and the coexistence of tetracycline were favorable to enhance the Cd（II）adsorption performance of Si Fh@HNT₄₀.However,the elevated ionic strength and the presence of anions Cl^-and NO₃^-conversely weakened the Cd（II）adsorption performance of Si Fh@HNT₄₀.Thus,it is concluded that Si Fh@HNT₄₀ had potential applications in the treatment of Cd（II）-containing wastewater with high concentrations of SO₄^2-and CO₃^2-（e.g.,mine wastewater）,and in the treatment of wastewater contaminated with a mixture of antibiotics and heavy metal ions（e.g.,intensive livestock and poultry farming wastewater）.（5）In order to prepare a novel bimetallic oxide-modified halloysite nanotube as heavy metal adsorbent,a series of Fe-Mn oxide-modified halloysite nanotubes Mn Fe₂O₄@HNTs were prepared by a hydrothermal co-precipitation reaction,along with changing the reactant mixing method（alkali dropwise or metal salt dropwise）and the doping ratio of clay.The results showed that the adsorption capacity of Mn Fe₂O₄@HNT₅₀ prepared by the metal salt dropwise method and by the 50%of clay doping ratio had the best adsorption capacity,and its adsorption capacities for Cd（II）,Zn（II）and Pb（II）were about 27%,42%and 5%higher than those of Si Fh@HNT₄₀,respectively.The main adsorption mechanism of Mn Fe₂O₄@HNT₅₀ was the adsorption of heavy metal ions with Fe-OH and Mn-OH by the inner-sphere complexation.The formation of abundant Fe-OH and Mn-OH on the surface of Mn Fe₂O₄@HNT₅₀ was mainly attributed to the enhanced exposure density of active hydroxyl groups on the surface of Mn Fe₂O₄ NPs by the supporting and dispersing effect of the halloysite nanotubes.Thereby,Mn Fe₂O₄@HNT₅₀ was an inexpensive and low-toxic heavy metal adsorbent with better adsorption performance than the Si Fh@HNT₄₀.