INVESTIGATION OF THE PROCESSES OF ADDITIONAL TREATMENT OF WASTEWATER FROM HEAVY METAL IONS

ABSTRACT

Among the main pollutants that have a negative impact on the state of natural water bodies are heavy metal ions coming with wastewater from oil refining, metallurgical, galvanic, tanning and other enterprises. According to the classification of the World Health Organization, heavy metals are the most dangerous substances for living organisms with toxic, mutagenic and carcinogenic properties. At present, reagent methods are most often used to remove heavy metal ions from wastewater, in which metal ions are converted into an insoluble form with subsequent sedimentation of sludge. At the same time, these methods are uneconomical and have a number of disadvantages, such as incomplete removal of metals, high consumption of reagents and energy, as well as the formation of toxic sludge or other waste that requires disposal or processing. The purpose of this work was to study the process of purification of wastewater generated in the production of mineral fertilizers at "Maxam-Chirchik" enterprise with effective adsorbents based on local raw materials. Recent studies show that one of the effective adsorption materials that can be used instead of expensive synthetic adsorbents are bioadsorbents obtained from agricultural waste. In this work, studies were carried out on the use of bioadsorbents obtained on the basis of rice husks by modifying it with a 5% solution of alkali NaOH and caustic soda Na₂CO₃ for additional purification of wastewater from the production of mineral fertilizers from heavy metal ions. The obtained adsorbents were used for additional purification of wastewater from copper and nickel ions formed during the production of mineral fertilizers at the “Maxam-Chirchik” enterprise. Studies have shown that adsorbents obtained by modifying rice husks with alkali and soda have a high adsorption capacity in relation to heavy metal ions and can be used for additional treatment of industrial wastewater.

АННОТАЦИЯ

К основным загрязняющим веществам, негативно влияющим на состояние природных водоемов, относятся ионы тяжелых металлов, поступающие со сточными водами нефтеперерабатывающих, металлургических, гальванических, кожевенных и других предприятий. Согласно классификации Всемирной организации здравоохранения тяжелые металлы являются наиболее опасными веществами для живых организмов, обладающими токсическими, мутагенными и канцерогенными свойствами. В настоящее время для удаления ионов тяжелых металлов из сточных вод чаще всего используются реактивные методы, в которых ионы металлов переводятся в нерастворимую форму с последующим осаждением ила. В то же время эти методы неэкономичны и имеют ряд недостатков, таких как неполное удаление металлов, высокий расход реагентов и энергии, а также образование токсичного осадка или других отходов, требующих утилизации или переработки. Целью данной работы было изучение процесса очистки сточных вод, образующихся при производстве минеральных удобрений на предприятии «Максам-Чирчик», эффективными адсорбентами на основе местного сырья. Недавние исследования показывают, что одним из эффективных адсорбционных материалов, которые можно использовать вместо дорогостоящих синтетических адсорбентов, являются биоадсорбенты, полученные из сельскохозяйственных отходов. В данной работе были проведены исследования по использованию биоадсорбентов, полученных на основе рисовой шелухи путем модификации ее 5% -ным раствором щелочи NaOH и каустической соды Na₂CO₃ для дополнительной очистки сточных вод производства минеральных удобрений от ионов тяжелых металлов. Полученные адсорбенты были использованы для дополнительной очистки сточных вод от ионов меди и никеля, образующихся при производстве минеральных удобрений на предприятии «Максам-Чирчик». Исследования показали, что адсорбенты, полученные путем модификации рисовой шелухи щелочью и содой, обладают высокой адсорбционной способностью по отношению к ионам тяжелых металлов и могут использоваться для дополнительной очистки промышленных сточных вод.

Keywords: adsorption, bioadsorbents, heavy metal ions, wastewater treatment, fertilizers, modification.

Ключевые слова: адсорбция, биоадсорбенты, ионы тяжелых металлов, очистка сточных вод, удобрения, модификация.

INTRODUCTION

Among the main pollutants that have a negative impact on the state of natural water bodies are heavy metal ions coming with wastewater from oil refining, metallurgical, galvanic, tanning and other enterprises. 

The production of mineral fertilizers is one of the most intensively developing industries in the Republic of Uzbekistan. At the same time, an increase in the production of various types of fertilizers, an expansion of their range, an increase in the quality and competitiveness of products is associated with an increase in emissions of various types of pollutants into the environment in the form of gas and dust emissions, waste water and solid waste. The production of mineral fertilizers is associated with the formation of large volumes of wastewater containing nitrogen compounds, heavy metals, oil products, etc.

Despite the fact that at present at the enterprises for the production of mineral fertilizers there are treatment facilities that include mechanical, physicochemical and biological treatment methods, the degree of wastewater treatment does not always meet the requirements for the quality of treated water. This leads to the discharge of incompletely treated wastewater into natural reservoirs and disruption of the biological balance of the reservoir, the death of flora and fauna.

At present, reagent methods are most often used to remove heavy metal ions from wastewater, in which metal ions are converted into an insoluble form with subsequent sedimentation of sludge. At the same time, these methods are uneconomical and have a number of disadvantages, such as incomplete removal of metals, high consumption of reagents and energy, as well as the formation of toxic sludge or other waste that requires disposal or processing [1, p. 373].

The purpose of this work was to study the process of purification of wastewater generated in the production of mineral fertilizers at "Maxam-Chirchik" enterprise. The technological scheme of wastewater treatment from Cu²⁺ and Ni^{2 +} ions by the reagent method includes averaging the incoming effluents, treating them with reagents to convert them into an insoluble form, settling the formed particles in a sump and dewatering sludge.

Extraction of copper from wastewater is carried out by precipitation of copper with a CuS-solution of sodium sulphide with a mass fraction of 5-10%. The extraction of nickel is carried out by precipitation with a solution of lime milk with a mass fraction of 5% in the form of nickel hydroxide, as well as by galvanic coagulation in the form of nickel. Wastewater contains copper sulphide CuS; nickel hydroxide Ni(OH₂); iron hydroxide Fe(OH₂); chromium hydroxide Cr(OH₂); zinc hydroxide Zn(OH₂). During the sedimentation process, a large amount of sludge is formed, which has poor sedimentation properties and is difficult to separate from water. The sedimentation process takes place in a sump for 5 hours [2, p. 312 ].

After reagent treatment, wastewater is fed to biological treatment facilities to remove nitrogen compounds by nitro-denitrification and then to clean them from organic compounds into aerotanks, after which they are discharged into the Chirchik river. However, recently, due to the tightening of environmental legislation and the introduction of strict standards for maximum permissible discharges into water bodies, reagent methods do not provide the required quality of wastewater treatment. In addition, metal ions remaining in the water, even in small amounts, can have a detrimental effect on activated sludge microorganisms during biological treatment, which also reduces the degree of water purification.

  In this situation, from an economic point of view, many industrial enterprises do not have the opportunity to reconstruct the existing treatment facilities and switch to more efficient technologies for treating wastewater from heavy metal ions capable of providing complete purification. In this regard, to solve this problem, it seems to be effective to use additional wastewater treatment. Post-treatment means methods and methods that complement reagent cleaning methods. The most effective and cheapest method of post-treatment is the adsorption method of purification using natural and synthetic sorbents.

Recent studies show that one of the effective adsorption materials that can be used instead of expensive synthetic adsorbents are bio-adsorbents obtained from agricultural waste [3, p.2350].

In this work, studies were carried out on the use of bioadsorbents obtained on the basis of rice husks by modifying it with a 1% solution of alkali NaOH and caustic soda Na₂CO₃ for additional purification of wastewater from the production of mineral fertilizers from heavy metal ions.

To obtain adsorbents, rice husks were washed with water and dried at 105°C. Next, a 10-gram sample was poured with 1% NaOH alkali solution and kept for 24 hours with constant shaking. After that, the water was filtered off, washed with water until neutral, and dried to constant weight. Samples treated with soda Na₂CO₃ were prepared in the same way.

The obtained adsorbents were used for additional purification of waste water from copper and nickel ions formed at the “Maxam-Chirchik” enterprise. The studies were carried out with model solutions that completely imitate the effluents of this production.

The study of the adsorption properties of the obtained adsorbents was carried out on model solutions containing copper and nickel ions under static conditions. To determine the adsorption capacity of the obtained adsorbents, waste water solutions with a concentration of 5 to 200 mg/L were prepared by diluting the initial solution. In flasks with a volume of 250 ml, 100 ml of a solution of various concentrations were poured and 1 g of the modified adsorbent was added. The flasks were kept for 24 hours with constant shaking on the plug. After that, the solutions were poured off, filtered, and the content of metal ions was determined by the atomic absorption method.

The amount of adsorption was determined from the decrease in the concentration of metal ions in the solution by the formula:

V – volume of the test solution, ml;

C_initial и С_final – initial and final concentration of metals in water, mg / l;

m – mass of adsorbent, g.

The fig. 1 shows the curves of the dependence of the adsorption value of copper and nickel ions on the equilibrium concentration of the investigated solutions.

Figure. 1. Dependence of adsorption on the equilibrium concentration of solutions:

1- Cu²⁺, 2- Ni²⁺

The value of adsorption is significantly affected by the pH of the solution. As can be seen from Fig. 2, upon going from an acidic pH range to a neutral one, the degree of absorption of metals increases, which is associated with the formation of metal hydroxides.

Figure 2. Dependence of the degree of adsorption of Cu^{2 +} ions on the pH of the solution at concentrations: 1-5.2 mg/l; 2 - 7.6 mg/l; 3- 13.58 mg/l

  The study of the dependence of the degree of adsorption on time showed that within 1 hour the maximum extraction of ions from the solution is achieved (Fig. 3).

Figure 3. Dependence of the degree of adsorption on time

Thus, the studies carried out have shown that adsorbents obtained by modifying rice hulls with alkali and soda have a high adsorption capacity in relation to heavy metal ions and can be used for additional treatment of industrial wastewater, which will reduce their harmful effect on the environment.

References:

1. Richard O. Mines, Jr. "Environmental Engineering. Principlеs and practice". 2014 by JohnWiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK, ISBN 978-1-118-80145-1.
2. Rodionov A.I., Klushin V.N., Torocheshnikov N.S. Tehnika zashiti okrujayushey sredi [Environmental protection technology]./Uchebnik dlya vuzov. – 2-e izd., pererab. i dop. – M.: Khimiya, 1989. – 512 s: il. [in Russian].
3. Jyotikusum Acharya, Upendra Kumar and P. Mahammed Rafi. Removal of Heavy Metal Ions from Wastewater by Chemically Modified Agricultural Waste Material as Potential Adsorbent-A Review. International Journal of Accrent Engineering and Technology, 2347-5661 Vol. 8, №3 (May-June 2018).