Synthesis of zinc hydroxide nitrate complex by a sol-gel method for use as foliar fertilizers – a scale up approach

Zinc hydroxide nitrate complex is a new class of materials with outstanding characteristics promising its application as foliar nano-fertilizers. In this study, nano crystalline zinc hydroxide nitrate powder was synthesized by a sol – gel method using NaOH and Zn(NO3)2.6 H2O as precursors, yielding several grams products per batch. The products were characterized by XRD, FTIR, SEM and BET, indicating the initial molar ratio NaOH : Zn(NO3)2.6H2O = 1.6 and reaction time 1 hour as suitable. Under the indicated conditions, the particle size of products is in the range 50÷100 nm. The characteristics of products demonstrate their potential application as foliar nanofertilizer and the synthesis procedure might be further upgrading to production extents.


INTRODUCTION
Folia fertilizers have been applied in increasing extents and become indispensable to high-tech agriculture.Recently, zinc hydroxide nitrate Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O had been reported as a potential long-term zinc supplying foliar fertilizer owing to its appropriate characteristics, e.g.stable nano sized crystals with sheet-like morphology, positively charged surface and moderate solubility in water [1,2].Moreover, copper could be involved to provide foliar fertilizers functioning as dual micronutriens to foliars [3].In fact, Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O has been known for a long time as a representantive of layered hydroxide salts.Its preparation was newly patented concerning application as foliar fertilizer [4].However, only the procedure in laboratory scale was described and boundary parameters suggested, e.g. 1 hour reaction time, concentration 0.2 M NaOH, and initial molar ratio OH -/ Zn 2+ = 1.6 were suggested from the investigated ranges 1 hour ÷ 24 hour, 0.2 M ÷ 1.6 M NaOH, and 0.5 ÷ 1.6, respectively [1].In order to follow this procedure, additional investigation beyond the reported ranges, e.g.stirring time shorter than 1 hour, initial OH -/ Zn 2+ > 1.6 should be conducted.Moreover, the synthesis of Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O should be Trang 139 scaled up towards its possible application as foliar fertilizer.This paper describes our preliminary results of such attempts to scale up.First, both the concentrations of precursors' solutions were increased to 0.6 M and the initial molar ratio OH -/ Zn 2+ as well as aging time adjusted.Then both their volumes were increased accordingly 5-to 20-fold, resulting in its production of about 70g of Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O per batch.

EXPERIMENTAL
Zn(NO 3 ) 2 .6H 2 O (reagent grade 98%) and NaOH (reagent grade 99%) from China were used without further purification.Ultrapure water was obtained from a reverse osmosis system.Two stock solutions 0.6 M Zn(NO 3 ) 2 and 0.6M NaOH were prepared dissolution appropriate amounts of chemicals in water and stored at ambient conditions.Fig. 1 shows the procedure scheme of a typical laboratory scale experiment: 80 ml 0.6M NaOH were gradually added to 50 ml 0.6M Zn(NO 3 ) 2 under vigorous stirring at room temperature (28 o C ± 2 o C) for 1 hour.The resulting white precipitates were filtered, washed several times with ultrapure water, then dried at 50 o C for 24 hours.The investigated parameters were initial OH-/ Zn 2+ molar ratio (0.5, 1.0, 1.6, 2.0 corresponding to 25 ml, 50 ml, 80 ml and 100 ml 0.6 M NaOH vs. 50 ml 0.6 M Zn(NO 3 ) 2 ) and stirring time (15 min., 30 min., 45 min., 1 hour).
Scaleup experiments were conducted using 5-fold and 20-fold volumes of both the stock solutions at the initial OH -/ Zn 2+ molar ratio 1.6, i.e. adding 400 ml 0.6M NaOH to 250 ml 0.6M Zn(NO 3 ) 2 or 1600 ml 0.6M NaOH to 1000 ml 0.6M Zn(NO 3 ) 2 , respectively.Instead of magnetic stirrer, a blade mixer at around 180 rpm and room temperature was applied to assure extensive mixing for 1 hour.The filtering, washing and drying steps remained similar as in laboratory scale experiments.studied by a S-4800 instrument with an accelerating voltage of 10kV (Hitachi, Japan).
The specific surface areas of products were recorded in a Quantachrome Instrument version 10.0.

Scaling-up the initial concentration of precursors
The synthesis procedure presented in Fig. 1 resembles the one described by Li et al [1], except for higher concentrations of precursors (both 0.6 M instead of 0.2 M) and slightly higher ambient temperature (28 ± 2 0 C instead of 25 0 C).Li et al [1] also tried with concentration 1.6 M NaOH but concluded the smaller onei.e.0.2 Mis more suitable, though the concentration of Zn(NO 3 ) 2 in the former case was not clearly specified.In fact, Newman et al. [5] synthesized the same product dropping 50 ml 0.75 M NaOH into 20 ml 3.5 M Zn(NO 3 ) 2 at room temperature, followed by an immediate filtration step.They did not recommend initial molar ratios OH -/ Zn 2+ higher than 0.5 because ZnO would appear as impurities in products [5].However, Li et al [1] did not detect such impurities even with the initial molar ratio OH -/ Zn 2+ = 1.6 using lower concentrations of both precursors (0.2 M).The main reason of this discrepancy is the rather slow kinetics of reaction: If some local concentrations of "free" OH - in the mixture were high enough during the reaction course, they could even attack the freshly formed Zn 5 (OH) 8 (NO 3 ) 2 , resulting in its structural changes to ZnO.Therefore, this work just scaled up the initial NaOH concentration to 0.6 M and re-checked the effect of initial molar ratio OH -/ Zn 2+ .In order to maximize the material effectiveness, the initial molar ratio OH -/ Zn 2+ = 1.6 was chosen for further study.However, much attention should be paid to the mixing condition to avoid local increase of pH, as a small pH increase at the initial OH -/Zn 2+ = 2.0 resulted XRD pattern containing only characteristic peaks for ZnO.

Adjusting the aging time
As longer aging time than 1 hour is proved to be unsuitable [1], possible effects of shorter aging times were investigated.Fig. 2  H2O molecules in the interlayer space or adsorbed in the surface resulted a shoulder and a peak at around 3300 cm -1 and 1630 cm -1 , respectively.Also, vibrations of the nitrate groups are represented by a very strong peak at around 1380 cm -1 and two weak peaks at around 1050 cm -1 and 840 cm -1 .
Although the XRD paterns and also the FTIR spectra for products with aging times 30 ÷ 60 minutes do not significantly differ from each other, the aging time of 60 minutes was chosen for further experiments as this process should be scaled up further.It's worth to note that longer aging time is not desirable due to a non-optimized mixing regime caused an additional peak at 2θ = 100 in the XRD pattern of products from 5-fold scaled-up approaches, while such "strange" peaks did not appear in case a good mixing regime was applied in our 20-fold scaled-up approach.The FT-IR spectra in Fig. 7 also confirms that characteristic groups of the Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O structure are conserved in the products of our scaled-up approaches.
In addition, Fig. 8 shows that products of our 5-fold and 20-fold volume scaled-up products conserves the desired sheet-like morphology with thickness less than about 50 nm.Comparing with Fig. 5, one can see that our volume scaled-up approach did not affect the morphology of products, just increased their mass to about 70 g per batch.Từ khóa: Phân bón lá, phân nano, qui trình nâng cấp, kẽm hydroxo-nitrat, phương pháp sol-gel.

Figure 1 .
Figure 1.Procedure of the typical laboratory scale experiment All products were characterized under the same conditions and results compared with each other.The XRD patterns were collected using a D8 Advance Diffractometer (Bruker AXS), Ni MultiFlex X -ray diffraction and Cu K(α) ( λ = 1.54184Å) radiation.The beam voltage and beam current are 40kV and 40mA, respectively.A two theta range of 5 -70 o with a continous scan rate of 3 o /min was applied and the phases identified using the Joint Committee on Powder Diffraction Society (JCPDS) database.The Fourier transform infrared (FTIR) spectra of products were obtained using the Bruker Equinox 55 (in the range 4000 -400 cm -1 ) equipped with a DTGS detector from FT -IR (Institute of Material Science -Vietnam Academy of Science and Technology).The morphology and particle size of products were

Figure 3 .
Figure 3. XRD patterns of products with different aging timesThe FTIR spectra of all synthesized products with aging times up to 60 minutes resemble each other, with typical peaks of the Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O structure.e.g. the stretching vibrations of the O -H bonds resulted a sharp and a strong peak at around 3570 cm -1 and 3500 cm -1 , respectively.The presence of

Figure 7 .Figure 8 . 144 Figure 9 .
Figure 7.Comparison of FT-IR spectra of products at volume scaling up experiments