**1. Introduction**

While nanosilica is becoming one of the most widely used nanomaterials for many industries, with an annual growth of 5.6%, silica is used as an important filler for rubber in a range of products, such as tires and other industrial materials, because it increases its mechanical durability, heat resistance, shrinkage, thermal expansion, and stress [1–4]. In addition, it improves the wear resistance of rubber based composites by replacing a soft matrix with a hard inorganic filler. Therefore, silica-based polymer nanocomposites have many exciting features for their many applications in the automotive, electronics, marine, and other industries. Many researchers have used silica as a reinforcement material for rubber-based composites [5–9].

Mechanical characteristics of the rubber can be enhanced using nanosilica only or combined with other fillers [10,11]. In particular, high-quality silica also has an absolute advantage in the manufacture of household products (fashion footwear soles, rubber mattresses, and others) or medical rubber (gloves, boots). In the pharmaceutical industry, silica is used as a carrier for some proprietary medicines [12–14]. In the organic chemical industry, silica acts as a catalyst for some organic reactions,

helps acceleration rates, and improves reaction yields [15–18]. To meet the increasing demand for silica in industrial applications, many studies focused on the fabrication of silica from different sources [19–28]. In addition to the many common sources, such as silane compounds of Na2SiO3, hexafluorosilicic acid (H2SiF6) becomes a potential economical candidate for the production of silica. Hexafluorosilicic acid is a by-product from the fertilizer industry, produced in huge quantities annually [29]. On the other hand, it is toxic and harmful to the environment, requiring either chemical treatment or conversion to a highly economical product [30]. For example, in Vietnam, Lam Thao Fertilizers and Chemicals, the largest production company of fertilizers with an estimated capacity of approximately 850,000 tons/year, produces approximately 30,000 tons of hexafluorosilicic acid annually. The production of hexafluorosilicic acid in the fertilizer production line can be explained as follows: First, fluorapatite (Ca5(PO4)3F (calcium fluorophosphate)) is reacted with either H2SO4 or HNO3 to generate HF according to the following reactions:

$$\text{Ca}\_5(\text{PO}\_4)\_3\text{F} + 5\text{H}\_2\text{SO}\_4 + \text{nH}\_2\text{O} \rightarrow 3\text{H}\_3\text{PO}\_4 + 5\text{CaSO}\_4\cdot\text{nH}\_2\text{O} + \text{HF}$$

Or

$$\text{Ca}\_5\text{(PO}\_4\text{)}\_3\text{F} + 10\text{HNO}\_3 \rightarrow 3\text{H}\_3\text{PO}\_4 + 5\text{Ca(NO}\_3\text{)}\_2 + \text{HF}$$

Obtained HF reacts with SiO2, which exists in the composition of the raw materials, to form SiF4 gas:

$$\text{4HF} + \text{SiO}\_2 \rightarrow \text{SiF}\_4 + 2\text{H}\_2\text{O}$$

The collection of H2SiF6 is usually performed by an absorption method of gaseous SiF4 in a water scrubber.

$$\text{\textbulletSiF}\_4 + \text{2H}\_2\text{O} = \text{2H}\_2\text{SiF}\_6 + \text{SiO}\_2$$

Many groups have reported its utilization, such as in silica recovery [30–37]. Dragicevic and Hraste [38] prepared silica from the neutralization of fluosilicic acid with ammonia while Sarawade et al. [25] used Na2CO3 to recover mesoporous silica with a large surface area from waste H2SiF6 from the fertilizer company. Hexafluorosilicic acid was further adopted as a silica source to fabricate SZM-5 as a trans-alkylation catalyst [29]. Cicala et al. [39] synthesized amorphous silicon alloys from fluorinated gases by plasma deposition, while Guzeev et al. [40] produced zircon and zirconium tetrafluoride with silicon tetrafluoride and zirconium dioxide as a raw material. Liu et al. [41] also synthesized titanium containing Mobil Composition of Matter No. 41 (MCM-41) with the industrial H2SiF6 and applied for cyclohexene epoxidation reaction.

We report a simple method of recovering amorphous silica nanoparticles from hexafluorosilicic acid waste and their application as a reinforcing filler in natural rubber (NR) with enhanced mechanical and thermal characteristics in this study. Both fabricated nanosilica and nanosilica-added NR are characterized. These efforts could not only reduce the environmental pollution of hexafluorosilicic acid wastes, but also enhance the value of waste from a fertilizer plant as an inorganic filler of NR. Note that of the total cost for the final product in factory including the cost for raw material, equipment, energies, and waste water treatment etc., the cost for waste water treatment grows higher as a result of the government policy and type of the waste water. This results in the higher price of final products, reducing their competitiveness. In case of Vietnamese fertilizer plants, the by-product of H2SiF6 with its emission rate is about 35,000 tons per year. With its highly toxic and corrosive characteristics, the H2SiF6 solution could threaten the environment by contaminating rivers and oceans.

Therefore, we strongly believe that the production and utilization of nanosilica in this study could become a reliable and sustainable solution for dealing with waste water from fertilizer plants environmentally as well as economically.
