Rui Lü ,Qing Xi ,Tan Li,Rui Li,Xiaochao Zhang,Jianxin Liu,Caimei Fan,*,Junqiang Feng,Lingyun Zhang,Zhihua Wang

1 College of Chemistry and Chemical Engineering,Taiyuan University of Technology,Taiyuan 030024,China

2 Shenzhen Batian Ecological Engineering Co.Ltd.,Shenzhen 518057,China

3 Tianji Coal Chemical Group Co.Ltd.,Changzhi 047507,China

Keywords:Ca2+and Mg2+ions Strong acid cation resin Adsorption equilibrium Kinetics Dynamic separation

ABSTRACT The separation of Ca2+and Mg2+ions from phosphoric acid-nitric acid aqueous solution is very signifciant for the neutralization process of nitrophosphate fertilizer.This paper studied the adsorption equilibrium,kinetics,and dynamic separation of Ca2+and Mg2+ions by strong acid cation resin,and the effects of phosphoric acid and nitric acid on the adsorption process were investigated.The results reveal that the adsorption process of Ca2+and Mg2+ions in pure water on resin is in good agreement with the Langmuir isotherm model and their maximal adsorption capacities are 1.86 mmol·g-1 and 1.83 mmol·g-1,respectively.The adsorption kinetics of Ca2+and Mg2+ions on resin fits better with the pseudo-first-order model,and the adsorption equilibrium in pure water is reached within 10 min contact time,while at the present of phosphoric acid,the adsorption rate of Ca2+and Mg2+ions on resin will go down.The dynamic separation experiments demonstrate that the designed column adsorption is able to undertake the separation of metal ions from the mix acids aqueous solution,but the dynamic operation should control the flow rate of mix acid solution.Besides nitric acid solution was proved to be effective to completely regenerate the spent resin and achieve the recyclable operation of separation process.

1.Introduction

Neutralization of mother liquor(the major ingredients:phosphoric acid and nitric acid)is an important and very complex step in the production process of nitrophosphate(NP)fertilizer[1,2].The Ca2+and Mg2+ions existed in the mother liquor always cause many problems[3,4],for example,aggravating viscosity of liquor,increasing the operational difficulty and running cost,reducing the water solubility of fertilizer.Consequently,the removal of Ca2+and Mg2+ions from the mother liquor will play a significant role for the practical production.The precipitation method by the addition of SO42-can effectively reduce Ca2+ions in the mother liquor,but will produce phosphogypsum.According to the requirements of green chemistry,the ideal solution should be to separate the Ca2+and Mg2+ions from the mother liquor and make full use of these Ca2+and Mg2+ions.Ion exchange technology using strong acid cation(SAC)resin has been proved to be competent for this task,by means of SAC resin,Ca2+and Mg2+ions considered as the impurities can be adsorbed from phosphoric acid-nitric acid aqueous solution and then eluted by nitric acid solution,which achieves the consecutive recycling operation.Moreover,the nitric acid solution is available in NP fertilizer plant,and when the regeneration ability of the nitric acid solution is exhausted,the solution containing the soluble Ca2+and Mg2+ions can be used for the production of another fertilizer,such as calcium ammonium nitrate fertilizer[5].Therefore,the purification of the mother liquor by SAC resin is really an environment-friendly process,not only solving the troublesome of the neutralization process,but also making the efficient use of Ca2+and Mg2+ions as the watersoluble fertilizers.

Actually,SAC resin is one of the popular materials with large adsorption capacity,rapid adsorption speed,great mechanical strength,and easily recyclable characteristic,which has been extensively applied in the water treatment,metallurgical industry and pharmaceutical separation[6-8].The application of SAC resin in the production of NPK fertilizer has been early studied and used for the production of chloridefree NPK-fertilizer[9].In that paper,SAC resin was used for absorbing the Ca2+ion from the nitrophosphate slurry and then was regenerated with the potassium chloride solution.Lim and J?rgensen optimized the SAC resin simulated-moving-bed process in terms of maximization of productivity for the NPK-fertilizer and minimization of wash-water consumption[10].Nevertheless,it should be noted that the adsorption process of metal ions on resin will be strongly affected by the reaction condition,solvent composition and nature of the resin[11-15].In phosphoric acid-nitric acid aqueous solution,the competition of massive hydrion for the adsorption site on resin and the formation of complexes of any phosphate species will suppress the adsorption of metal ions on resin[8].From a practical perspective,it is necessary to evaluate the effect of phosphoric acid and nitric acid on the adsorption behavior of Ca2+and Mg2+ions on SAC resin.

The adsorption behavior of metal ions on resin also influences their dynamic separation process.The operation through fixed bed column,with high yield,excellent recyclability,and ease of operation and handling,has been widely used in the practical application[16-18].The operational parameter of the feed flow rate is the important factor to ensure efficiency of dynamic adsorption and elution process of metal ions,especially for the slow adsorption process[19,20].In order to achieve a good separation of Ca2+and Mg2+ions from phosphoric acid-nitric acid aqueous solution,the effect of flow rate on the dynamic adsorption and elution of metal ions should be further explored.

This paper is aimed to study the adsorption equilibrium,kinetics,and dynamic separation of Ca2+and Mg2+ions by SAC resin,evaluate its feasibility to remove these two metal ions from phosphoric acidnitric acid aqueous solution,and further provide a theoretical basis for the practical production of nitrophosphate fertilizer.Firstly,the adsorption isotherm was studied to understand the basic adsorption performance of Ca2+and Mg2+ions on this resin.Then in order to understand the adsorption process and explore the feasibility of SAC resin to separate Ca2+and Mg2+ions from phosphoric acid-nitric acid aqueous solution,their adsorption equilibrium and kinetics on resin in different acid solutions were examined.Finally,for the practical application,the dynamic adsorption and elution process of Ca2+and Mg2+ions were investigated at the different feed flow rates.

2.Experimental

2.1.Materials and chemicals

The SAC resin 001×7 was obtained from Tianjin Fu Chen Chemical Reagent Factory of China and its specifications were shown in Table 1.Prior to use,the resin was firstly dipped in deionized water for 24 h,then in 4 wt%NaOH solution for 4 h,and finally in 4 wt%HNO3solution for 4 h.After each dip,resin was washed with deionized water until the resulting water became neutral,and finally was dried at 60°C.

2.2.Adsorption isotherms

In batch experiments,a series of 100 ml pure water solutions with 50-300 mmol·L-1single Ca2+ion or single Mg2+ion were prepared by dissolving the analytical grade Ca(NO3)2·4H2O or Mg(NO3)2·6H2O into distilled water,and subsequently agitated with the dried resin at 298 K.After adsorption equilibrium,the supernatant was taken out toanalyze the metal ions concentration by atomic-adsorption spectrophotometer(AAS-990).The metal ion content on resin phase at equilibrium(qe,mmol·g-1)was calculated according to Eq.(1)as below:

Table 1 Characteristics of strong acid cation resin 001×7 supplied by manufacturer

where C0and Ceare the initial and equilibrium ion concentration in the aqueous phase(mmol·L-1),V is the volume of solutions(ml),and m is the mass of the dried resin(g).

The Langmuir isotherm model and the Freundlich isotherm model were used for analyzing the experimental results.The models can be expressed by following Eqs.(2)and(3),respectively:

where qmin Eq.(2)is the maximum adsorption capacity per unit mass of resin(mmol·g-1)and b is the Langmuir constant(g·mmol-1)that is exponentially proportional to the heat of adsorption and related to the adsorption intensity;kfin Eq.(3)is the Freundlich constant(mmol·L-1)incorporating all factors that affect the adsorption process,and n is the adsorption intensity.

2.3.Adsorption kinetics

Batch adsorption kinetics experiments were carried out via agitating the dried resin with 100 ml synthetic solution,and the solution samples at different time intervals(0～120 min)were taken out to analyze the metal ions concentration by AAS.The synthetic solutions were prepared by Ca(NO3)2·4H2O,Mg(NO3)2·6H2O,65 wt%HNO3,and 85 wt%H3PO4,presenting a series of synthetic acidic solutions(HNO3concentration:0-1.5 mol·L-1,H3PO4concentration:0-4.5 mol·L-1)containing 0.25 mol·L-1Ca2+ion or 0.25 mol·L-1Mg2+ion.

The kinetics experimental results were analyzed by the pseudo-firstorder model and the pseudo-second-order model,which can be expressed by following Eqs.(4)and(5),respectively:

where qeis the equilibrium adsorption capacity of resin(mmol·g-1),qtis the amount of metal ion adsorbed at time t,k1and k2are the rate constants of the pseudo-second-order model(min-1)and the pseudosecond-order model[g·(mmol·min)-1].

2.4.Dynamic adsorption and elution

The dynamic column adsorption and elution of Ca2+and Mg2+ions were carried out in a glass column(internal diameter:2 cm,length:50 cm)packing with 100 g filtered resin.The operation parameters in the adsorption and elution processes are listed in Table 2.The adsorption-elution operations were performed as described below:

Table 2 The dynamic adsorption and elution operation parameters in 6 consecutive processes

Fig.1.Adsorption isotherm of single Ca2+ion and single Mg2+ion in pure water,5 g dry resin,50～300 mmol·L-1 initial metal ion concentration.

(1)The pretreated fresh resin was firstly loaded into the column,and then 250 ml 4 wt%HNO3solution was passed through the column upward at 3 ml·min-1flow rate.The flow rate was controlled by a peristaltic pump(SHENCHEN,Lab 2015).After that,the deionized water was fed to the top of the column to rinse the HNO3solution retained.

(2)The feed solution with 0.25 mol·L-1Ca2+ion and 0.25 mol·L-1Mg2+ion was passed through the column upward at different flow rate as shown in Table 2.The samples in the effluent were taken out at regular intervals and analyzed with AAS.The deionized water was fed to the top of the column to rinse the synthetic acidic solution retained.

(3)The 7 mol·L-1nitric acid solution was then flowed through the column in up-flow mode at the flow rate as shown in Table 2.The samples in the effluent were taken out at regular intervals and analyzed with AAS.The deionized water was fed to the top of the column to rinse the nitric acid solution retained and adjust pH to neutral.

(4)The next adsorption-elution processes were run as described in(1)and(3)at the operation parameters as shown in Table 2.

Table 3 The correlation coefficients of the Langmuir model and the Freundlich model for the adsorption isotherm of single Ca2+ion and single Mg2+ion in pure water

Thomas model was used to predict the breakthrough curve of metal ions.This model assumes that the adsorption process matches the Langmuir isotherm and the second-order reversible reaction kinetics,and no axial dispersion occurs[21,22].The model can be represented by Eq.(6):

where C0is the inlet metal concentration(mol·L-1),C is the outlet concentration at different effluent volume,kTis the Thomas rate constant[L·(mol·min)-1],Q is the volumetric flow rate(ml·min-1),qmis the maximum adsorption capacity of metal ion per unit mass of resin(mmol·g-1),W is the resin dosage in the fixed bed column(g),Voutis the effluent volume(ml).

3.Results and Discussion

3.1.Batch adsorption isotherms

Adsorption isotherm is an important parameter to evaluate the adsorption process of metal ion on resin.The adsorption behavior of Ca2+ion and Mg2+ion on resin in pure water was studied and shown in Fig.1.As initial ion concentration increasing from 50 mmol·L-1to 300 mmol·L-1,the adsorption capacity of resin to Ca2+ion and Mg2+ion slightly increases and eventually reaches the maximum.The maximal adsorption quantities with unit mass dry resin are 1.80 mmol·g-1of Ca2+ion and 1.78 mmol·g-1of Mg2+ion.The variation in the adsorption affinity of different metal ions on resin is because of its preference for the greater ionic radius metal ion that has the lower hydration energy and the greater tendency to migrate into the resin[15].

Fig.2.The Langmuir model fitting(a)and the Freundlich model fitting(b)for the adsorption isotherm of single Ca2+ ion and single Mg2+ ion in pure water,5 g dry resin,50-300 mmol·L-1 initial metal ion concentration.

Fig.3.The adsorption kinetics of single Ca2+ion(a)and single Mg2+ion(b)in pure water and mix acid solution(1.1 mol·L-1 HNO3and 4.4 mol·L-1 H3PO4),5 g dry resin,0.25 mol·L-1 initial metal ion concentration.

The Langmuir isotherm model and the Freundlich isotherm model were used for fitting the adsorption process of metal ion on resin.The Langmuir isotherm model assumes uniform energies of ion exchange and is applied to adsorption on a completely homogenous surface with negligible interaction between adsorbed molecules[12].The Freundlich isotherm model described as a fairly satisfactory empirical isotherm used for non-ideal adsorption is related to heterogeneous process as well as multilayer adsorption[14].The fitting plots using the Langmuir model and the Freundlich model are shown in Fig.2 and the correlation coefficients are summarized in Table 3.It can be found that the Langmuir isotherm model provides the excellent fit for the adsorption process of Ca2+ion and Mg2+ion on resin in pure water,and the correlation coefficients are 0.998 and 0.999 for Ca2+ion and Mg2+ion,respectively.The maximal adsorption capacities evaluated from the Langmuir isotherm fitting are 1.86 mmol·g-1of Ca2+ion and 1.83 mmol·g-1of Mg2+ion,which are much closer to the values determined by experiments.

3.2.Batch adsorption kinetics

The adsorption kinetics is another important factor to define the efficiency of adsorption process.In order to understand the adsorption process of metal ion on resin and explore the feasibility of SAC resin to separate the metal ions from phosphoric acid-nitric acid aqueous solution,the adsorption kinetics of Ca2+ion and Mg2+ion on resin in pure water and mix acid solution was investigated and shown in Fig.3(a)and(b).Meanwhile,the pseudo-first-order model and the pseudo-second-order model were employed to fit the experimental results.The correlation coefficients are listed in Tables 4 and 5.

Table 4 The correlation coefficients of the pseudo-first-order model and the pseudo-second-order model for the adsorption kinetics of single Ca2+ion in different acid solutions

As shown in Fig.3(a)and(b),the adsorption rate of Ca2+ion and Mg2+ion in pure water is very rapid,and the adsorption equilibriums are attained within 10 min contact time.However,Ca2+ion and Mg2+ion in phosphoric acid-nitric acid aqueous solution present the slow adsorption rate and their adsorption equilibriums on resin are attained after 60 min contact time.These indicate that due to the presence of mix acids,the adsorption rate of metal ion on resin will go down.It is noticed from Tables 4 and 5 that with the decreasing of adsorption rate,the kinetics correlation coefficient k1reduces while the k2increases[23-25].Such as,the k1values for the adsorption behavior of Ca2+ion in pure water and mix acid solution are 0.55 min-1and 0.11 min-1and the k2values are 1.08 g·(mmol·min)-1and 7.36 g·(mmol·min)-1.Besides,it can be expected that when phosphoric acid-nitric acid is present,the adsorption capacity of metal ions on resin will reduce[8,15].As shown in Fig.3(a)and(b),the adsorption capacity of Ca2+ion and Mg2+ion on resin in pure water is larger than that in mix acid solution.And the Qein Tables 4 and 5 also presents the similar variation.

3.3.Effect of phosphoric acid and nitric acid

In order to further explore the effect of the presence of phosphoric acid-nitric acid on the adsorption process of metal ion on resin,the adsorption equilibrium and kinetics of Ca2+ion and Mg2+ion on resin in different concentration phosphoric acid solutions and nitric acidsolutions were investigated.The experiment results and the fitting plots by the pseudo-first-order model and the pseudo-second-order model are shown in Fig.4.The correlation coefficients for the adsorption process of Ca2+ion and Mg2+ion are also listed in Tables 4 and 5,respectively.

Table 5 The correlation coefficients of the pseudo-first-order model and the pseudo-second-order model for the adsorption kinetics of single Mg2+ion in different acid solutions

Fig.4.The adsorption equilibrium and kinetics of single Ca2+ion(a,c)and single Mg2+ion(b,d)in different concentration phosphoric acid solutions(1.5 mol·L-1,3.0 mol·L-1,and 4.5 mol·L-1)and nitric acid solutions(0.5 mol·L-1,1.0 mol·L-1,and 1.5 mol·L-1),5 g dry resin,0.25 mol·L-1 initial metal ion concentration.

Fig.5.Breakthrough behaviors of the total metal ions in pure water(a)and mix acid solution(b),0.5-3.0 ml·min-1 of flow rate,0.25 mol·L-1 Ca2+ion and 0.25 mol·L-1 Mg2+ion,50 cm bed height.

Table 6 The fitting results of Thomas model for the dynamic separation of Ca2+and Mg2+ions from pure water and mix acid solution in fixed bed column at different flow rates

The results in Fig.4(a)and(b)reveal that phosphoric acid interferes with the adsorption process of metal ion on resin.The adsorption rates of Ca2+ion and Mg2+ion on resin decrease with phosphoric acid concentration increasing,which may be due to the formation of phosphate complexes species with metal ions[8].The adsorption capacities of Ca2+ion and Mg2+ion on resin also gradually reduce with phosphoric acid concentration increasing,which should be owe to the competition of hydrion for the adsorption site on resin[26,27].The Qein Tables 4 and 5 presents slightly reducing tendency,such as,the Qevalues fitted by the pseudo-first-order model for the adsorption of Ca2+ion in 0 mol·L-1,1.5 mol·L-1,3.0 mol·L-1,and 4.5 mol·L-1phosphoric acid solutions are 1.78 mmol·g-1,1.59 mmol·g-1,1.44 mmol·g-1,and 1.32 mmol·g-1,respectively.Fig.4(c)and(d)displays that the adsorption rate of metal ions in nitric acid solution is fast,and the adsorption equilibriums are reached within 10 min contact time,but there is a declining trend in the adsorption capacity of metal ions on resin with the nitric acid concentration increasing,the Qevalues(Tables 4 and 5)fitted by the pseudo-first-order model for the adsorption behavior of Ca2+ion in 0 mol·L-1,0.5 mol·L-1,1.0 mol·L-1,and 1.5 mol·L-1nitric acid solutions are 1.78 mmol·g-1,1.32 mmol·g-1,0.95 mmol·g-1,and 0.66 mmol·g-1,respectively.Therefore,based on the above results,it can be concluded that the phosphoric acid has a great influence on the adsorption rate of Ca2+ion and Mg2+ion on resin while nitric acid has almost no influence,and both phosphoric acid and nitric acid suppress the adsorption capacity of resin to metal ions,especially nitric acid has a greater suppression.

Besides,the pseudo-first-order model presents a more excellent fit for the adsorption of Ca2+ion and Mg2+ion on resin in different acid solutions.Most of the correlation coefficients R2by this model are larger than that by the pseudo-second-order model.Moreover,it can be found in Tables 4 and 5 that with the adsorption rate decreasing the k1values will decrease while the k2values will increase[23-25],such that the k1values for the adsorption behavior of Ca2+ion in 0 mol·L-1,1.5 mol·L-1,3.0 mol·L-1,and 4.5 mol·L-1phosphoric acid solutions are 0.55 min-1,0.46 min-1,0.31 min-1,and 0.20 min-1,respectively,and the k2values are 1.08 g·(mmol·min)-1,1.33 g·(mmol·min)-1,2.17 g·(mmol·min)-1,and 3.62 g·(mmol·min)-1,respectively.

3.4.The dynamic adsorption

Fig.6.Elution curves of the total metal ions from the spent resin at different flow rate of 7 mol·L-1 nitric acid solutions,0.5-3.0 ml·min-1 flow rate,50 cm bed height.

Table 7 The adsorption quantities and elution quantities of metal ions calculated by the breakthrough and elution curves for the 6 adsorption and elution processes

In the practical application,dynamic column adsorption is widely used as an operating mode for SAC resin to remove the metal ion.In order to prove the applicability of SAC resin to separate Ca2+and Mg2+ions from phosphoric acid-nitric acid aqueous solution,their dynamic adsorption behavior in the pure water and mix acid solution was investigated.The experimental results and the fitting plots using Thomas model are shown in Fig.5(a)and(b),and the fitting correlation coefficients are listed in Table 6.The results reveal that dynamic column adsorption is able to separate simultaneously the Ca2+and Mg2+ions from the pure water and mix acid solution.As shown in Fig.5(a)and(b),the break volumes(C/C0＞0)for the pure water and mix acids are 225 ml and 150 ml,respectively.Similar to the batch adsorption,the adsorption capacity of metal ions on resin in pure water is larger than that in mix acid solution,but this difference is declining for the dynamic column adsorption,such that the decrease degrees of the adsorption capacity of resin to metal ions between pure water and mix acid solution for the batch adsorption and dynamic column adsorption are(1.78-0.9)/1.78×100%=49.4%(Table 4)and(1.2-0.85)/1.2×100%=29.2%(Table 6),respectively.This further proves the advantage of dynamic column adsorption and its necessity for the scale-up industrial production.

Besides,it should be noted that dynamic adsorption behaviors of metal ions in pure water remain almost the same at different flow rates,which the break point appears at 225 ml feed solution and the saturation point(C/C0=1)rapidly appears at 250 ml feed solution.However,the adsorption behaviors of metal ions in mix acid solution present significant difference.As shown in Fig.5(b),at the 0.5 ml·min-1flow rate the break volume is 150 ml and the saturation volume is 200 ml,and with the flow rate increasing to 3.0 ml·min-1,the break volume becomes smaller(75 ml)while the saturation volume increases to 225 ml.This phenomenon suggests that the less residence time as a result of the increased flow rate will make the diffusion of metal ion on resin worse and result in the less treatment efficiency[28,29].In addition,combined with the batch adsorption results,the presence of nitric acid and phosphoric acid led to a decrease in the adsorption rate of metal ions on resin,thus,it can be concluded that the flow rate has a great influence on dynamic adsorption behavior of Ca2+and Mg2+ions in mix acid solution.In the practical application,the suitable flow rate should be controlled as the key operational parameter to ensure the enough residence time for the diffusion of metal ion in resin and improve the treatment efficiency.

3.5.The dynamic elution

As for the industrial application,the elution process of metal ions from spent resin is the key aspect to achieve its recyclable ability and the consecutive separation process[19].Based on our previous research,7 mol·L-1nitric acid solution is chosen and used as regeneration solution to study the effect of flow rate on the elution process of metal ions from spent resin in this study.The elution curves of metal ions from the spent resin column are shown in Fig.6 and the corresponding elution quantities of metal ions are listed in Table 7.It can be observed that the elution flow rate has a smaller influence on dynamic elution behavior of metal ions.The elution processes present the almost same curve at 0.5 and 1.0 ml·min-1flow rate while a slight poor at 3.0 ml·min-1.The peak points of elution curve at different flow rates(0.5 ml·min-1,1 ml·min-1,3 ml·min-1)are 1.34 mol·L-1,1.35 mol·L-1and 1.09 mol·L-1and the elution quantities of metal ions are 84.85 mmol,82.85 mmol,and 78.18 mmol,respectively.

Furthermore,the results in Table 7 reveal that the elution of metal ions using 7 mol·L-1nitric acid solution can regenerate effectively the spent resin.The adsorption ability of the regenerative resin to metal ions presents the constant tendency just like the new resin.As shown in Table 7,the adsorption quantities of metal ions in the pure water and mix acid solution remain the steady values,122.12-122.88 and 87.70-88.91 mmol,respectively.Besides,it should be noted that the elution quantities of metal ions for the processes 1-6 are lower than the corresponding adsorption quantities,which is due to the influence of washing resin by deionized water.

4.Conclusions

This study investigated the adsorption equilibrium,kinetics,and dynamic process of Ca2+and Mg2+ions on the SAC resin and evaluated its feasibility to separate these two metal ions from phosphoric acid-nitric acid aqueous solution.The results demonstrate that SAC resin is suitable to separate the Ca2+and Mg2+ions from phosphoric acid-nitric acid aqueous solution.The adsorption process of Ca2+and Mg2+ions on resin is in good agreement with the Langmuir isotherm model and the pseudo-first-order model.The adsorption capacities of Ca2+and Mg2+ions in pure water on resin are 1.8 mmol·g-1and 1.78 mmol·g-1,and the adsorption equilibriums are reached within 10 min contact time.But when phosphoric acid and nitric acid are present,the adsorption capacity and the adsorption rate of Ca2+and Mg2+ions on resin will go down.Besides,the separation of Ca2+and Mg2+ions from mix acid solution has been successfully performed by the dynamic column operation,and 7 mol·L-1nitric acid solution chosen as the regeneration solution can effectively regenerate the spent resin and achieve the recyclable operation.Moreover,it should be noted that the flow rate plays a vital role in dynamic adsorption behavior of Ca2+and Mg2+ions from the mix acid solution,thus in the practical application the flow rate should be optimized to ensure the treatment efficiency.