The contamination of aquatic environments in developing countries like Vietnam, driven by industrialization and population growth, poses significant challenges Pollutants from both point and nonpoint sources lead to the degradation of water bodies and present serious public health risks In Vietnam, particular attention is focused on pollutants that resist biodegradation, such as complex dyes, and those that are difficult to eliminate through conventional wastewater treatment, including heavy metals and emerging contaminants like antibiotics Addressing the effective removal of these pollutants from wastewater has become a critical issue globally.
To date, several methods have been developed to deal with hazardous substances in effluents (Cengiz et al 2012; Hequet et al 2001; Mohammadi et al 2011; Sud et al 2008; Wang
Precipitation and ion exchange are commonly employed techniques for the removal of heavy metal ions from wastewater Additionally, photocatalysis and advanced oxidation processes effectively decompose dyes Despite the efficacy of these methods, researchers are increasingly exploring simpler, cost-effective, and environmentally friendly alternatives Among these, adsorption stands out as a frequently used process, with materials that possess high adsorption capacities significantly reducing the costs associated with water and wastewater treatment.
In the past, activated carbon (AC) was used as the preferential sorbent in this regard, in
Activated carbon (AC) is particularly effective for adsorbing non-ionic organic pollutants due to its high specific surface area and pore volume Non-ionic organic compounds adhere to AC surfaces primarily through van der Waals forces and other mechanisms (Tran et al 2017b; Tran et al 2017c) However, traditional synthesis methods involving high pyrolysis and activation temperatures render AC a costly material (Zhang et al 2015) Additionally, the elevated carbonization temperatures during synthesis can diminish surface functional groups like OH and COOH (Tran et al 2018), which are crucial for the adsorption of cationic contaminants.
The increasing challenges posed by recalcitrant dyes, heavy metals, and emerging contaminants in water necessitate the development of carbonaceous adsorbents These adsorbents should be created through simple, eco-friendly, and efficient methods to effectively address these environmental issues.
Hydrochar, a promising environmentally-friendly adsorbent derived from the hydrothermal carbonization (HTC) of organic wastes such as agricultural residues and animal manures, has garnered recent attention (Fang et al 2015; Fang et al 2017; Wang et al 2018b) The HTC process involves carbonizing these organic materials at low temperatures (180°C – 350°C) in a closed reactor under autogenous pressure, leading to the formation of hydrochars through hydrolysis, condensation, decarboxylation, and dehydration (Fang et al 2017; Jain et al 2015; Libra et al 2011; Berge et al 2011; Sevilla and Fuertes 2009; Xiao et al 2012) While hydrochar typically has a lower specific surface area than biochar, another cost-effective carbonaceous sorbent, it may offer superior adsorption capacity for certain contaminants due to its higher abundance of oxygenated functional groups.
3 carboxylic, phenolic, hydroxyl) on the surface (Elaigwu and Greenway 2016; Jain et al 2016)
Hydrochar can be utilized to produce activated carbon (AC) through chemical activation, preserving the abundance of surface functional groups essential for enhancing adsorption capacity The presence of these functional groups is crucial for the effective removal of heavy metal ions, suggesting that AC synthesized from hydrothermal carbonization (HTC) followed by chemical activation will exhibit high adsorption capabilities for cationic contaminants.
Recent studies have explored the effectiveness of hydrochar in removing various contaminants from water (Islam et al 2017; Sun et al 2015; Xue et al 2012) Research by Sun et al showed that hydrochar has a greater uptake capacity for polar and nonpolar organic contaminants, such as bisphenol A and 17α-ethinylestradiol, compared to biochar under similar conditions (Sun et al 2011) Additionally, Tran et al found that the increased oxygenated groups on hydrochar surfaces led to electrostatic attraction as the main adsorption mechanism for cationic dyes like methylene green (Tran et al 2017a) The type of feedstock used to synthesize hydrochar can significantly influence the variety and abundance of its surface functional groups (Libra et al 2011) Furthermore, various chemical agents, including nitric acid and potassium hydroxide, have been utilized to enhance hydrochar's surface structure, improving its adsorption capabilities, although such modifications may also alter the adsorbent's surface area and pore size distribution (Güzel et al 2017; Xue et al 2012).
2017) However, the underlying adsorption mechanisms have not been investigated and
A significant portion of solid waste in developing countries originates from agricultural activities, highlighting the importance of exploring agro-based wastes as cost-effective materials for wastewater treatment Various agricultural byproducts, such as orange peels, have been identified as potential resources for this purpose (Fernandez et al 2015; Tran et al 2016).
Various agricultural by-products, including rice straw (Xu et al 2014), rice husk (Mohanty et al 2006), and sugar cane bagasse, along with fruits such as avocado, hami melon, and dragon fruit (Mallampati et al 2015; Yahya et al 2015), have been effectively utilized to create adsorbents.
This study investigates the removal of organic contaminants, specifically methylene blue (MB), antibiotics like tetracycline, and heavy metals such as cadmium (Cd) and copper (Cu) from aqueous solutions using adsorbents derived from agricultural wastes Chapter 2 provides a comprehensive background on wastewater treatment technologies and recent advancements in the field It details the production and chemical composition of hydrochar and hydrochar-derived activated carbon, highlighting their effectiveness in removing both organic and inorganic contaminants The chapter also reviews theories related to the adsorption process, including isotherms, kinetics, mechanisms, and key parameters influencing adsorption efficiency Additionally, it emphasizes the dual capability of hydrochar-based materials to remove both anions and cations, concluding with a cost comparison of adsorbent production.
Chapter 3 introduces an innovative and eco-friendly method for producing hydrochar and oxidized-hydrochar using orange peel and D-glucose, with and without HNO3 reflux This chapter is structured into three sections, focusing on the preparation of hydrochar and oxidized-hydrochar from agricultural waste, specifically orange peel and D-glucose, along with their characterization It details the structural and physicochemical properties of both hydrochar and oxidized-hydrochar.
This section provides a detailed examination of five types of hydrochar, highlighting key parameters that influence the adsorption process, including solution pH, pHPZC, temperature, and contact time Additionally, it explores the interactions between hydrochar and methylene blue (MB) adsorbates, specifically focusing on the adsorption mechanisms of MB onto hydrochar and modified hydrochar The findings of this investigation were published in the journal article: Nguyen, D H., Tran, H N., Chao, H P., & Lin, C C (2019) "Effect of nitric acid oxidation on the surface of hydrochars to sorb methylene blue: An adsorption mechanism comparison," in Adsorption Science & Technology, 37(7-8), 607-622.
Chapter 4 presents two eco-friendly methods for removing water contaminants, specifically metallic species like Cu (II), Cd (II), and the cation dye Methylene Blue (MB) The first method involves adsorption using activated carbon derived from teak sawdust hydrochar, produced solely at high temperatures The second method utilizes chemical activation with ZnCl2/K2CO3 Findings indicate that hydrothermal carbonization combined with ZnCl2 activation significantly enhances the adsorption capacity of the activated carbon ACZ1175 The chapter also discusses the adsorption isotherms and mechanisms, contributing valuable insights to the field This research is published in the journal under the title "Activated Carbons Derived from Teak Sawdust-Hydrochars for Efficient Removal of Methylene Blue, Copper, and Cadmium from Aqueous Solution."
“Water” (Duy Nguyen, H., Nguyen Tran, H., Chao, H P., & Lin, C C (2019) Water, 11(12), 2581)
Chapter 5 presents experimental findings on the absorption of tetracycline onto hydrochar and hydrochar-derived activated carbon, aimed at validating the adsorption mechanisms between ionic organic compounds and synthetic sorbents The study extensively discusses how the pH of the solution affects the charge of tetracycline, influencing its adsorption Additionally, a cost analysis is conducted to assess the feasibility of the entire production process for the hydrochar/activated carbon samples.
6 cost is estimated The outcomes of this study provide helpful information for future applications of MGH and ACZ1175 for water treatment
Finally, chapter 6 summarizes the general conclusions drawn from this thesis and presents some suggestions for future studies
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The study by Güzel et al (2017) investigates the optimal oxidation of biochar produced from the pyrolysis of weeds using nitric acid This process enhances the biochar's effectiveness in removing the hazardous dye methylene blue from water The findings, published in the Journal of Cleaner Production, demonstrate the potential of modified biochar as a sustainable solution for wastewater treatment.
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Huang Y, Huang Y, Wang W, Zheng K (2018) Characterization of hydrogen peroxide (H2O2) modified hydrochars from walnut shell for enhanced adsorption performance of methylene blue from aqueous solution DESALINATION AND WATER TREATMENT 109:221-230
Water and Sources of Water Pollution
Water is essential for all life on Earth, yet access to clean drinking water remains a significant global challenge in the 21st century Although over 71% of the Earth's surface is water, less than 1% meets international standards for potability due to various contaminants (Briggs 2003) Ensuring pure and uncontaminated water is crucial for the survival of all living organisms.
Water pollution refers to the harmful alteration of water's physical, chemical, or biological properties, negatively impacting human health and ecosystems There are two primary types of water pollutants: point source and non-point source Point source pollution occurs when contaminants are directly released into water bodies from identifiable sources such as factories and wastewater treatment plants In contrast, non-point source pollution results from diffuse sources, where pollutants, like fertilizers, are carried into water systems through runoff, particularly during rainfall This type of pollution is more challenging to manage and is responsible for the majority of contaminants found in lakes and streams.
2009) Sources that contribute to water pollution can be illustrated in Table 2.1
Table 2 1 Properties of point and nonpoint sources of water pollution
Point Sources Non-point Sources
- Wastewater effluent (municipal and industrial)
- Runoff and leachate from waste disposal sites
- Runoff and infiltration from animal feedlots
- Runoff from mines, oil fields, unsewered industrial sites
- Storm sewer outfalls from cities with a population >100,000
- Overflows of combined storm and sanitary sewers
- Runoff from construction sites >2 ha
- Runoff from agriculture (including return flow from irrigated agriculture)
- Runoff from pasture and range
- Urban runoff unsewered and sewered areas with a population