Study on synthesis of TiO2- Fe2O3/GNP materials from ilmenite and graphite ore to orient Cr(VI) transformation in defense industrial wastewater

Ensuring that the concentration of treated wastewater reaches the allowable concentration according to Vietnamese standards on industrial wastewater also plays an important role in eliminating the explosive properties of these explosive components.

Table 1.3. Characteristics and composition of hazardous waste in primary explosives production technology [12]

STT

Necklace

technology

Main ingredients

Hazardous waste

Production of mercury fuminate

Hg metal, HNO 3 , C 2 H 5 OH, HCl, copper, NH 4 NO 3 , NaN 3 , Pb(NO 3 ) 2 ,

ethanol

- Wastewater contaminated with Hg(ONC) 2 , Pb(NO 3 ) 2 ,

acid, ethanol

Production of lead azotide

NH 4 NO 3 , Na metal, NaN 3 , Pb(NO 3 ) 2 , ethanol

- Wastewater contaminated with Pb(N 3 ) 2 , Pb(NO 3 ) 2 ,

NaN3 , ethanol

Production of lead stypnate

Stypnic acid

(trinitrorezocxin TNR),

NaHCO 3 , Pb(NO 3 ) 2 , bitumen benzene, ethanol

- Wastewater contaminated with TNR. Sodium stypnate, lead stypnate.

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The third source is from the production of pyrotechnics and blasting agents. These two technologies are grouped together because their main raw materials are similar because the oxidizing and combustible components used in pyrotechnics are also used in the composition of pyrotechnics and blasting agents. The characteristics and composition of hazardous waste from the production lines of these products are shown in Table 1.5. The composition of these wastewaters also contains heavy metal ions and toxic, explosive organic substances.

Table 1.4. Heavy metal pollution in the production technology of pyrotechnics and fire-powder, explosive tubes [12].

STT

Necklace

technology

Main ingredients

Hazardous waste

Manufacture

pyrotechnics

- Combustible substances, oxidizing substances: W, Si, S, Sb 2 S 3 , Pb 3 O 4 , Fe 2 O 3 , CuO, Sb 2 S 5 , PbCrO 4 , BaCrO 4 , Ba(NO 3 ) 2 , KClO 3 ,

KClO 4 , Zr, potassium picrate.... rubber powder and adhesives: lac, phenol resin,

paraffin, NC glue…

- Wastewater contaminated with heavy metal ions Pb 2+ , Pb 4+ , Fe 2+ , Cu 2+ , Cr 6+ ,

Sb 3+ , Sb 5+ …, toxic organic substances such as picrate, NC.

Production of fire powder and explosives

- Combustible substances, oxidizing substances: W, Si, S, Sb 2 S 3 , Pb 3 O 4 , Fe 2 O 3 , CuO, Sb 2 S 5 , PbCrO 4 , BaCrO 4 , Ba(NO 3 ) 2 , KClO 3 ,

KClO 4 , Zr, potassium picrate.... rubber powder and adhesives: lac, phenol resin, paraffin, NC glue...

- Explosives such as lead stypnate, lead azotide, mercury sulfide, tetrazene.

- Secondary explosives

such as hexogen, pentrite.

- Wastewater contaminated with heavy metal ions Pb 2+ , Pb 4+ , Fe 2+ , Cu 2+ , Cr 6+ ,

Sb 3+ , Sb 5+ …, toxic organic substances such as picrate, NC, stypnate, azotide, hexogen, pentrite.

Due to the characteristics of the technology of producing pyrotechnics and the technology of producing fire-seeds and explosives, which are currently mainly manual, the composition of pyrotechnics, fire-seeds and explosives is diverse, so the characteristics of the wastewater generated from these two technologies are diverse, unstable, and always changing in composition and content of pollutants. On the other hand,

Wastewater mainly arises from the process of washing tools, equipment, and cleaning the factory, so the wastewater flow is not high but the pollutant content is high.

Table 1.5 is the data on the concentration of pollutants at the explosives manufacturing factory Z131/General Department of Defense Industry [13]. The explosives manufacturing factory Z131 has production lines for industrial explosives, general assembly, and filling of explosives into various types of ammunition.

Table 1.5. Concentrations of pollutants in wastewater before treatment of Factory 2, Z131 factory

Parameter	Unit	Analysis results (TB)	QCVN40:2011/ BTNMT (B) Cmax: Kf = 1.2 ; Kq = 0.9
1	pH	-	11.8	5.5-9
2	Smell	-	-	-
3	Color, Co-Pt at pH=7	-	172	150
4	BOD5 at 20ºC	mg/L	78	54
5	COD	mg/L	203	162
6	Suspended solids	mg/L	164	108
7	Arsenic	mg/L	0.0172	0.108
8	Mercury	mg/L	0.00047	0.0108
9	Lead	mg/L	KPHĐ	0.54
10	Cadmium	mg/L	KPHĐ	0.108
11	Chromium(VI)	mg/L	0.68	0.108
12	Chromium(III)	mg/L	3.52	1.08
13	Copper	mg/L	0.0328	2.16
14	Zinc	mg/L	0.894	3.24
15	Nickel	mg/L	1.12	0.54
16	Manganese	mg/L	0.94	1.08
17	Iron	mg/L	1.7	5.4

Parameter	Unit	Analysis results (TB)	QCVN40:2011/ BTNMT (B) Cmax: Kf = 1.2 ; Kq = 0.9
18	Mineral oil	mg/L	14.2	10.8
19	Sulfide	mg/L	0.48	0.54
20	Fluoride	mg/L	KPHĐ	10.8
21	Chloride	mg/L	48	1,080
22	Ammonium (as N)	mg/L	3.34	10.8
23	Total N	mg/L	80	43.2
24	TNT	mg/L	20.1	0.5a
25	Coliform	MPN/100mL	330	5000

In Table 1.5, it can be seen that in addition to the concentration index of heavy metals such as Cr(III), Cr(VI) and Ni exceeding the permissible level, the wastewater also contains mineral oil and TNT.

Table 1.6 is the data on the concentration of pollutants in wastewater at facility 2 (Explosives Enterprise) of the Institute of Explosives and Propellants [14], where there is a production line for pyrotechnics, industrial explosives, mixed rocket fuel, production of various types of detonation stations, explosive charges, military explosives...

Table 1.6. Concentrations of pollutants in wastewater before treatment at the Explosives Factory/Explosives Propellant Institute

Parameter	Unit	Analysis results (TB)	QCVN40:2011/ BTNMT (B) Cmax: Kf = 1.2; Kq = 0.9
1	Temperature	o C	25.7	40
2	Color	Pt/Co	216	150
3	pH	-	3.5	5.5 - 9
4	TSS	mg/L	210	100

Parameter	Unit	Analysis results (TB)	QCVN40:2011/ BTNMT (B) Cmax: Kf = 1.2; Kq = 0.9
5	BOD5 ( 20 o C)	mg/L	98.5	50
6	COD	mg/L	319.3	150
7	Arsenic	mg/L	0.008	0.1
8	Mercury	mg/L	0.0003	0.01
9	Lead	mg/L	1,550	0.5
10	Cadmium	mg/L	0.315	0.1
11	Chromium (III)	mg/L	2,187	1
12	Chromium (VI)	mg/L	0.316	0.1
13	Copper	mg/L	0.915	2
14	Zinc	mg/L	1,356	3
15	Nickel	mg/L	0.215	0.5
16	Manganese	mg/L	0.545	1
17	Iron	mg/L	3,981	5
18	Total Cyanide	mg/L	0.035	0.1
19	Total Phenol	mg/L	0.136	0.5
20	Total mineral oil and grease	mg/L	15.3	10
21	Sulfide (S 2- )	mg/L	0.67	0.5
22	Fluoride	mg/L	1.45	10
23	Ammonium (N)	mg/L	19.34	10
24	Total N	mg/L	31.16	40
25	Total P	mg/L	3.98	6
26	Chloride	mg/L	91	1,000
27	Residual chlorine	mg/L	<0.026	2
19	TNT	mg/L	3.15	0.5a
29	Coliform	MPN/ 100mL	120	5,000

The measurement results in Table 1.6 show that the concentration of pollutants in untreated wastewater of the Explosives Enterprise/Explosives Propellant Institute exceeds the permissible limit.

The permissible levels include heavy metal ions such as lead, cadmium, chromium (III), chromium (VI), ammonium, mineral oil and TNT. It can be seen that most of the wastewater from explosives production mostly contains heavy metal ions and explosive, difficult-to-decompose organic substances.

In most defense factories in the mechanical, metallurgical and chemical sectors of the defense and technical industry, there are lines for treating waste contaminated with heavy metals.

However, the results of the survey and assessment of the environmental status of these facilities in the past time show that due to the lack of unified and complete technological models, the operational efficiency of many treatment systems is still low. Many treatment lines have degraded and are no longer capable of treating pollutants to meet environmental standards. The operation of these lines is manual, so the stability of the quality of wastewater after treatment is not guaranteed [8].

In general, wastewater from some production lines that generate heavy metals in defense facilities has only been treated to meet pH standards, while other indicators, especially those on heavy metal content, in many cases have not yet met the requirements of current environmental standards.

Therefore, with a large amount of daily discharge, wastewater containing heavy metals is one of the types of defense-specific waste that is at risk of having a major impact on the environment if it does not receive appropriate and timely attention. In recent years, defense facilities have received attention from the leaders of the State and the Ministry of Defense, and production lines and wastewater treatment systems have also been newly invested and modernized. Production wastewater is treated more thoroughly, and the process of treating and controlling the discharge into the environment is automated. However, the method of treating heavy metal wastewater at defense production facilities is mainly chemical. This treatment method has the advantage of fast treatment time, quite

thoroughly, but the treatment process uses a lot of chemicals, causing secondary waste sources. With wastewater sources with many organic and inorganic components such as wastewater from explosives production, chemical methods need to be combined with other treatment methods, making the treatment system cumbersome and complicated. Researching modern treatment methods that use less chemicals and are more suitable for waste sources with many components such as wastewater from explosives production is an urgent requirement and is of interest to scientists at home and abroad.

1.2.3. Methods of treating heavy metals in general and Cr (VI) in particular Over the years, researchers have used different methods to remove heavy metals in general and Cr (VI) in particular from different types of wastewater, including: electrochemical methods (electrocoagulation, electrocoagulation)

chemical and electrolytic methods), physicochemical methods (chemical precipitation, ion exchange), adsorption (activated carbon, carbon nanotubes and wood sawdust adsorbents), membrane filtration, photocatalytic and nanotechnology methods.

1.2.3.1. Electrochemical method

The three common electrochemical treatment methods today are electrochemical coagulation (EC)[52], electrochemical flocculation (EF)[37] and electrolysis. Although electrochemical treatment methods for wastewater containing heavy metals have many outstanding advantages such as high heavy metal recovery efficiency, compact system design and no large area requirement, due to high investment costs and power consumption, these methods are not widely used on an industrial scale.

1.2.3.2. Chemical precipitation method

Chemical precipitation is a simple and easy method of heavy metal treatment. This treatment method is widely used in removing heavy metals from wastewater. Chemical precipitation requires a lot of chemicals to reduce metal ions to acceptable levels before discharging into the environment [63], these chemicals will be a major source of pollution later.

In chemical precipitation, chemical precipitants are agents that react with heavy metal ions and change them into insoluble solid particles.

dissolved [116]. The solid phase will be separated from the solution by sedimentation or filtration. In this process, the combination of coagulants such as iron salts, aluminum salts and some polymers can enhance the separation of heavy metals from wastewater. The existence of free radicals affects the precipitation of hydroxide, some reagents are often used to improve the precipitation of hydroxide [116].

Sulfide precipitation is similar to hydroxide precipitation. Both soluble and insoluble substances can be used to precipitate metal ions. Sulfides are used to precipitate heavy metal ions such as metal sulfides and sludge for removal by sedimentation or filtration. The sulfide precipitation portion of the process requires pre- and post-treatment and precise control of chemical addition due to the toxicity of sulfide ions and H 2 S.

The most commonly used coagulants are lime and calcium hydroxide due to their cost and availability. Lime (CaO) is often preferred over other coagulants, but when used requires high dosages and it may not be successful in reducing heavy metal concentrations to standard limits due to incomplete precipitation in many cases.

Besides the above mentioned advantages, the chemical precipitation method has the disadvantages of being expensive, creating sludge, increasing the cost of sludge treatment, poor settling process and slow metal precipitation. The above disadvantages make this method limited in practical application [119].

1.2.3.3. Ion exchange method

Ion exchange treatment is based on the reversible exchange of ions between solid and liquid phases. The process starts with ion exchange reactions, then heavy metal ions are physically adsorbed, and a complex is formed between the counterions and functional groups. Finally, hydration occurs at the surface of the solution or the pores of the adsorbent [42]. Various factors affecting the ion exchange activity include pH, anions in the solution, temperature, initial concentration of the adsorbent, adsorbate, and contact time [65].

In ion exchange, the exchange of ions between ions occurs in two phases (solid and liquid). In this process, a resin removes ions from