most heavy metals are toxic and carcinogenic even at very low concentrations and usually cause a serious threat to the environment and the public health . for example , cr(vi ) is a toxic , carcinogenic substance to human and animals . cd(ii ) is also a potent carcinogen causing damage to the lungs , kidneys , liver , and reproductive organs [ 3 , 4 ] . therefore , us environmental protection agency   regulates at least ten metals , including chromium and cadmium , as primary contaminants that must be removed from drinking water . electroplating wastewater usually contains various heavy metals as cocontaminants ; however , few studies have been reported on simultaneous removal of cr(vi ) and cd(ii ) from wastewater . among the different treatments for removing heavy metals , adsorption has been developed as a simple , efficient , and cost - effective method . many of adsorbents such as clays , activated carbon , sewage sludge , and plant parts have been used for heavy metal removal [ 69 ] . but due to extremely small particle size and large surface area , iron nanoparticles ( fe nps ) with a high adsorption capacity are found to be one of the most effective adsorbents for removing heavy metals [ 1012 ] . moreover , fe nps have shown a high chemical reduction rate on several kinds of contaminants , including toxic metal ions [ 1315 ] . have found that fe nps acting as reductants could chemically reduce cr(vi ) to cr(iii ) . besides , fe nps can proactively attack contaminant plumes in the source zone and potentially reduce the remediation cost and time substantially . however , the applications of fe nps have been hindered by the key technical barrier that the nanoparticles tend to agglomerate and grow to micron scale or larger , thereby rapidly losing their mobility and chemical reactivity . at the same time , fe nps easily react with dissolved oxygen and water resulting in a reduction of their chemical reactivity owing to the formation of iron oxide . then , resin or carbon - supported fe nps [ 18 , 19 ] and polymer - coated fe nps [ 20 , 21 ] have been commonly prepared to enhance dispersion and antioxidationof nanoparticles . recently , nanoparticles with uniform size distribution and high antioxidation are obtained in virtue of sio2 coating . these core - shell nanomaterials may have an important application in cd(ii ) adsorption because cationic cd(ii ) ion can readily approach negative charged silica surface as a result of electrostatic attraction . in addition , these nanomaterials may reduce the cr(vi ) pollution because silica surface can strongly bind fe(iii ) and cr(iii ) via surface complexation [ 24 , 25 ] . however , such core - shell nanostructures are often generated by complicated processes , that is , the preparation of metal nps followed by the deposition of the inorganic material around them using appropriate synthetic methods . and surface - coupling agents [ 26 , 27 ] are often used to control the metal particle size or as the primer to facilitate the sio2 disposition . hence , it is significant to develop a simple method to coat fe nps with sio2 shells . in our earlier paper , a novel one - step method for the preparation of sio2-coated fe nanocomposites ( fe@sio2 ) was described . sio2 shell was directly precipitated on the fe nps surface through aqueous reduction combined with modified stber process . and the fe@sio2 were effective in cr(vi ) remediation . in this follow - up work , the specific objectives were ( 1 ) synthesis of the fe@sio2 using one - step method without using any of surface - coupling agents , ( 2 ) study of cr(vi ) and cd(ii ) simultaneous removal kinetics and mechanism , and ( 3 ) investigation of the influence of some experimental parameters on the removals of cr(vi ) and cd(ii ) , such as ph , iron dose , solution concentration , and coexisting ions . all the chemicals used in this research were analytical grade or higher and were used as received . and deionized water ( di ) water was used throughout the whole experiment . in a typical synthesis , 0.0436  g fecl3 was firstly dissolved in 30  ml , 0.1  ml tetraethylorthosilicate ( teos ) was added for the synthesis of silica , and thus theoretical fe contents of the finally prepared composite was about 37.5  wt.% . then freshly prepared kbh4 ( 2.69  mmoll ) was added dropwise to fecl3 solution at 7  :  1 molar ratio with vigorous stirring . next , fe nps were synthesized in the laboratory via the following reaction : 
 ( 1)fe(h2o)63++3bh4+3h2o  fe0(s)+3b(oh)3 + 10.5h2(g ) excess borohydride was added to accelerate fe nps synthesis and ensure silica formation . after 120  min of reaction , the resulting particles were collected by a magnet and then washed repeatedly with di water to get rid of the excess chemicals . the surface morphologies of the samples were characterized by a tem ( phillips tecnai f20 ) . details on tem and xrd analyses have been described elsewhere . to determine the isoelectric point of fe nps , the -potential of sample was measured as a function of solution ph using a zetasizer nanozs ( malvern instruments , uk ) . stock solutions of cr(vi ) and cd(ii ) ions ( 100  mgl ) were prepared by dissolving k2cr2o7 and cd(no3)2 in di water . batch experiments were performed in 125  ml glass vials capped with teflon mininert valves to minimize the oxidation of fe nps . next , a predetermined quantities of fe@sio2 were added into 100  ml mixed solution of cr(vi ) and cd(ii ) and then placed on a rotary shaker with 180  rpm . at given interval , samples were withdrawn by a 1  ml - syringe , filtered through a filter ( 0.22  m ) , and tested for cr(vi ) and cd(ii ) concentration , respectively . in addition , to examine the role of silica shell , an experiment was designed to mix fe@sio2 with cr(vi ) and cd(ii ) independently at the same reaction condition . to study the effect of initial iron dose on the cr(vi ) and cd(ii ) removals using fe@sio2 , 0.1 , 0.15 , 0.2 , and 0.3  mgl fe nps were added to the reactor vials , respectively , with initial concentrations of both metal ions at 50  mgl and solution ph at 6 . the effects of initial cr(vi ) and cd(ii ) concentrations were investigated by varying the cr(vi ) and cd(ii ) concentrations from 50 to 80  mgl and reacting with 0.15  gl fe nps at ph 6 . to examine the effect of initial ph the effect of competitive cations ( ca , mg , k , and na ) was investigated by adding 10  mmoll of a cation to the reactor vial , respectively . it was also conducted to investigate the effect of 10  mgl humic acid ( ha ) on the heavy metal removal . all experiments were performed at room temperature ( 25c ) . to assure data quality , the cr(vi ) concentration was tested using the diphenylcarbohydrazide method , and the concentration of cd(ii ) was measured by an icp - aes ( icp - optima 2001dv , perkin - elmer , usa ) . the ph was measured throughout the experiments by using a ph meter ( pb-10 , sartorius , china ) . tem analysis indicated that the synthesized sample had a clearly distinguished core - shell structure : the dark cores were attributed to fe and the grey shells were attributed to sio2 . the mean particle size of fe nps was calculated to be 25  nm . and the sio2 shell had series of small pores . the isoelectric point of bare fe nps was about 8~8.5 [ 31 , 32 ] . however , with the presence of silica shell on fe nps surface , a much lower isoelectric point 5.2 was observed for fe@sio2 . figure 2(a ) shows the removal capacity of cr(vi ) by fe@sio2 at an fe dose of 0.15  gl and initial cr(vi ) concentration of 70  mgl and ph of 6.0  0.1 . the normalized residual concentration ( c / c0 ) was used to describe the removal rate . after 120  min of contact , almost 100% cr(vi ) was removed by fe@sio2 and cr(vi ) removal capacity was calculated to be 467  mg  cr / g fe . the control reactor showed no loss of cr(vi ) during the whole experiment . compared with that of reported stabilized fe nps [ 13 , 17 ] , a significant increase in cr(vi ) removal this was because cr(vi ) ions could readily approach small fe nps surface by passing through the porous sio2 shell . the xps analysis in our previous research indicated that fe was primarily oxidized to fe(iii ) and cr(vi ) was reduced to cr(iii ) . at the same time , silica surface can strongly bind fe(iii ) and cr(iii ) via surface complexation [ 24 , 25 ] . obviously , there was an initial sorption phase along with the quick redox reaction during the first 2  min . this was probably due to the formation of cr - fe hydroxides on the fe surface . similar investigations were conducted for studying the removal efficiency of cd(ii ) ions by fe@sio2 . as shown in figure 2(b ) , around 72% of cd(ii ) was removed by fe@sio2 in 120  min . ph rose to about 8.4 after reaction because of oh release when fe reacted with h2o . it was calculated that the removal capacities for cd(ii ) by fe@sio2 were approximately 336  mgg , while it was only 242  mgg for the uncoated fe nps . therefore , the cd(ii ) removal rate was improved greatly by fe@sio2 . as the standard reduction potential of cd ( ecd / cd = 0.40  v ) is very close to that of fe ( efe / fe = 0.44  v ) and sio2 shell was porous , cd(ii ) was removed mainly through adsorption on the fe nps surface . the adsorption of cd(ii ) on fe nps surface was assumed to occur through the following surface reactions : ( 2)cd2++sohsocd++h+(3)cd2++2(soh)(so)2cd+2h+(4)cd2++soh+h2osocdoh+2h+ 
 where soh represents a surface hydroxyl group . in fact , the maximum removal efficiency of cd(ii ) ions by fe@sio2 was achieved within the first 30  min . it might be due to the large amount of adsorptive sites available at the beginning , and the adsorption became slower as the adsorption sites were gradually filled up . the initial ph value of 70  mgl cr(vi ) and cd(ii ) mixed solution was around 6 . as shown in figure 3 , simultaneous removal of 82% cr(vi ) and 62% cd(ii ) by fe@sio2 was observed after 120  min reaction . and a more efficient removal of cr(vi ) than that of cd(ii ) this was because the standard reduction potential of cr(vi ) was more positive than that of cd(ii ) , meaning that cr(vi ) was much easier to be reduced compared with cd(ii ) . at the same time , the positive cd(ii ) was more easily adsorbed on the negatively charged fe nps surface than the negative cr2o7 as a result of electrostatic interactions and specific surface bonding . however , in the acidic medium ( ph < 7 ) , the speed of cr(vi ) reduction was faster than that of cd(ii ) adsorption . as the cr(vi ) reduction reaction proceeded , fe nps were dissolved and the active adsorption sites were consequently decreased . therefore , the adsorption of cd(ii ) had declined compared with that of cd(ii ) removal individually using fe@sio2 . while the reduction of cr(vi ) with fe@sio2 was also hindered by the adsorption of cd(ii ) . in a word , adsorption could be dominated in the removal of cd(ii ) , while reduction may play a main role in the removal of cr(vi ) . according to the icp - aes analysis , the influence of fe nps dose on the removal of 70  mgl cr(vi ) and 70  mgl cd(ii ) ions by fe@sio2 was investigated . as shown in figure 4 , when fe dosage increased from 0.1 to 0.2  gl , the removal rate of cr(vi ) increased from 48% to 100% , and cd(ii ) removal rate increased from 47% to 75% . this was on account of the greater surface area and the availability of more active sites at higher dosages of fe . the effect of initial cr(vi ) and cd(ii ) concentrations which ranged from 50 to 80  mgl on degradation rates by 0.15  gl fe@sio2 at ph 6 was studied . as shown in figure 5 , the removal rate for both ions decreased as the initial concentrations increased . with the increase in initial ions concentration from 50 to 80  mgl , the cr(vi ) removal capacity decreased from 100% to 54% , and the cd(ii ) removal capacity decreased from 100% to 45% , respectively . this was attributed to the fact that the removals of both cr(vi ) and cd(ii ) were a surface - mediated process . at the lower ion concentration , however , when more heavy metal ions approached the fe surface , the available active sites for per mole ions decreased . a quickly formed passivation layer on the surface of nzvi would lead to the degeneration of reactivity . on the other hand , compared to cr(vi ) , a lower removal rate of cd(ii ) was obtained at the same reaction condition , which was consistent with the previous section . for instance , when the initial concentrations of two metal ions were 60  mgl , 100% cr(vi ) and 90% cd(ii ) were removed , respectively . the effect of initial ph on 70  mgl cr(vi ) and cd(ii ) removal using 0.15  gl fe@sio2 under temperature of 25c was shown in figure 6 . after reaction for 120  min , the final ph was all about 9.2 as a result of the oh release . as shown in figure 6 , the cr(vi ) the cr(vi ) removal rate was reduced by 33% with the ph rising from 4 to 9 . according to 
 ( 5)cr2o72+2fe0 + 7h2o  2cr3++14oh+2fe3 + the high ph also can accelerate the formation of fe(iii)-cr(iii ) precipitates on fe surface , which lowers the reducing power of fe nps . although chemisorption between cd(ii ) and fe surface was likely to be the major mechanism for cd(ii ) adsorption , solution ph strongly influenced the cd(ii ) adsorption processas indicated in figure 6 . when ph was 5 , the cd(ii ) adsorption on fe@sio2 was 57% . this was because the solution ph affected the surface charge of fe@sio2 and the degree of ionization , as well as the speciation of the metal contaminant . at ph below the isoelectric point , the adsorbent surface was protonated , and an electrostatic repulsion existed between the positively charged surface and cd(ii ) ions , resulting in the reduced cd(ii ) adsorption . in addition , in the highly acidic medium , the high concentration of h ions in the solution may compete with cd(ii ) for the adsorption sites , inhibiting the adsorption . at ph above the isoelectric point , the fe@sio2 surface was negatively charged , making the surface electrostatically favorable for adsorption of positive cd(ii ) . in the present study , the isoelectric point of fe@sio2 was about 5.2 because the fe nps was coated by a highly negative silica shell . at ph of 6.0 , the final cd(ii ) removal rate by fe@sio2 was 62% . however , only 5% cd(ii ) adsorption on bare fe nps was obtained at ph of 6.0 as reported by boparai et al . . this was because the bare fe nps surface was positively charged at ph of 6 . moreover , with the simultaneous removal of cr(vi ) by fe@sio2 , the solution ph immediately increased to about 9.2 and thus promoted cadmium hydrolysis / precipitation ( i.e. , cdoh , cd2(oh ) , cd(oh)2 , cd(oh)3 , and cd(oh)4 ) . therefore , most of the cd(ii ) was adsorbed on fe@sio2 event at low initial ph . anyway , this core - shell fe@sio2 showed a high activity in a broad range of ph which was very suitable for the practical application . alkali and alkaline - earth metal cations such as na , k , mg , and ca are commonly present with metal contaminants in polluted waters . thus , it was necessary to study the effects of these metal ions on cr(vi ) and cd(ii ) removal by fe@sio2 . meanwhile , the effect of natural organic matter ( ha ) on cr(vi ) and cd(ii ) removal was also investigated . a sample containing a mixture of 70  mgl cr(vi ) and cd(ii ) with interfering substance was prepared at ph of 6 and 25c . as shown in figure 7 , compared with di water , the cr(vi ) removal ability was reduced by 6.1% , 8.61% , 32.02% , 25.63% , and 17.87% in the presence of 10  mmoll na , k , mg , ca , and 10  mgl ha , respectively . compared with di water , the cd(ii ) removal ability was reduced by 10.5% , 12.38% , 40.2% , 28.37% , and 20.05% in the presence of 10  mmoll na , k , it was apparent that divalent metal ions exerted more impact than monovalent metal ions on the cr(vi ) and cd(ii ) removals . this was attributable to two effects : divalent metal ions can effectively compete for sorption sites and decreased the electrostatic repulsions , resulting in serious aggregation of fe@sio2 . the deterioration effect of ha may be in that the adsorbed ha on silica surface can decrease the active sites of fe nps . in conclusion , a simple method of synthesizing uniform sio2-coated fe nanoparticles ( fe@sio2 ) can be established in a one - pot system . the resultant sio2 shell not only suppressed the growth of the fe nps but also prevented it from aggregation . and the removals of cr(vi ) and cd(ii ) by fe@sio2 under ambient condition were evaluated . the result showed that cd(ii ) was adsorbed on fe surface while cr(vi ) was reduced to cr(iii ) . further , the removal rate rose as the initial concentrations of heavy metals decreased and fe dose increased . the ph had complex effect on the cr(vi ) and cd(ii ) removals by fe@sio2 . the acidity of system had been found to play a major role in the reduction of cr(vi ) . however , cd(ii ) adsorption increased with the increased solution ph . the presence of alkali and alkaline - earth metal cations had effect on the cr(vi ) and cd(ii ) removals . summarily , the core - shell fe@sio2 may be considered as an effective material for the removals of cr(vi ) and cd(ii ) from aqueous solutions .
core - shell silica - coated fe0 nanoparticles ( fe@sio2 ) were prepared in one - step synthesis by aqueous reduction combined with modified stber method . 
 the as - prepared fe@sio2 were then used for simultaneous removal of cr(vi ) and cd(ii ) from aqueous solution . 
 batch tests indicated that fe@sio2 exhibited high removal capacity toward cr(vi ) and cd(ii ) . 
 cr(vi ) was removed by fe@sio2 through reduction rather than adsorption , while cd(ii ) removal was mainly through adsorption . the removal rate increased with increasing initial 
 fe nps dose and decreased with increasing initial cr(vi ) and cd(ii ) concentrations . 
 cd(ii ) adsorption was also strengthened by cr(vi ) reduction with the release of oh. the removals of cr(vi ) and cd(ii ) were weakened in the presence of cations or humic acid , as a result of aggregation and less active site of fe@sio2 . 
 overall , the simply prepared fe@sio2 were potential material for the heavy metals removed from water .