Author: Izaz Ul Islam
Introduction
The rapid growth of industrialization and urbanization has intensified the generation of wastewater contaminated with dyes, heavy metals, and nutrients. Conventional treatment methods are often expensive, energy-intensive, or ineffective for complex pollutants. In this context, hydrochars, carbon-rich materials produced via hydrothermal carbonization (HTC), have emerged as promising, low-cost, and sustainable adsorbents for wastewater treatment.
This blog summarizes recent scientific advances in hydrochar production, modification, and application for pollutant removal, drawing upon the comprehensive review by Azzaz et al. (2020) published in Renewable and Sustainable Energy Reviews
a) Evolution of the number of papers dealing with the hydrothermal carbonization and hydrochars production topics from 2009 to 2018
b) Papers partition about hydrochar topic by respective research field (Source: Scopus 2018).
What Is Hydrothermal Carbonization?
Hydrothermal carbonization is a thermochemical process that converts wet biomass into a solid carbonaceous product—hydrochar—under moderate temperatures (140–350 °C) and autogenous pressure in an aqueous environment. Unlike pyrolysis, HTC does not require energy-intensive drying, making it particularly suitable for high-moisture wastes such as sewage sludge, food waste, and agricultural residues.
During HTC, biomass undergoes dehydration, decarboxylation, and polymerization reactions, resulting in a solid material with enhanced carbon content and surface functionality.
Feedstock Matters: Tailoring Hydrochar Properties
One of the major strengths of HTC lies in its feedstock flexibility. Hydrochars can be produced from:
- Animal wastes (e.g., manure, poultry litter)
- Agricultural residues (e.g., corn stover, rice husk, coconut shell)
- Municipal solid waste and sewage sludge
- Food and industrial wastes
The chemical composition of the original biomass strongly influences the hydrochar’s carbon yield, surface chemistry, porosity, and ash content. For example, lignocellulosic feedstocks tend to produce hydrochars with more aromatic structures, while manure-based hydrochars often contain higher mineral content.
Role of HTC Operating Conditions
Temperature
Increasing HTC temperature generally:
- Reduces hydrochar yield
- Decreases O/C and H/C ratios
- Enhances aromaticity and stability
- Improves energy density
Residence Time
Longer residence times promote:
- Greater dehydration and decarboxylation
- Increased fixed carbon content
- More stable and condensed hydrochar structures
pH of Reaction Medium
Acidic conditions accelerate biomass hydrolysis and dehydration, influencing:
- Elemental composition
- Functional group distribution
- Heavy metal mobility and stabilization
Enhancing Hydrochars via Activation and Modification
Raw hydrochars often possess limited surface area. To improve their adsorption performance, physical and chemical modifications are applied:
Physical Activation
- CO₂ and steam activation significantly increase surface area and microporosity
- Microwave and ultrasound treatments enhance heating efficiency and structural homogeneity
Chemical Activation
- Acid treatments (HCl, H₂O₂) introduce oxygen-containing functional groups
- Alkaline activation (KOH, NaOH) increases porosity and surface reactivity
- Metal/salt impregnation (e.g., FeCl₃) improves adsorption through complexation
- Organic functionalization (amines, polymers) enhances selectivity toward charged pollutants
Application in Wastewater Treatment
Hydrochars have been extensively studied as adsorbents for:
Organic Pollutants (Dyes)
Hydrochars can remove dyes such as:
- Methylene blue
- Congo red
- Rhodamine B
Adsorption mechanisms include:
- Electrostatic attraction
- π–π interactions
- Hydrogen bonding
While raw hydrochars show moderate adsorption capacity, activated hydrochars can reach performances comparable to commercial activated carbon.
Inorganic Pollutants (Heavy Metals and Nutrients)
Hydrochars effectively adsorb:
- Heavy metals (Pb²⁺, Cd²⁺, Cu²⁺, Cr⁶⁺)
- Nutrients (phosphate, ammonium)
Mechanisms involve:
- Surface complexation
- Ion exchange
- Precipitation and electrostatic interactions
Challenges and Future Perspectives
Despite their potential, several challenges remain:
- Variability due to feedstock heterogeneity
- Scale-up and process optimization
- Regeneration and long-term stability
- Environmental risk assessment of spent hydrochars
Future research should focus on engineered hydrochars, life-cycle analysis, and integration into circular economy frameworks for waste and water management.
Conclusion
Hydrochars represent a versatile and sustainable class of carbon materials derived from waste. Through controlled hydrothermal carbonization and targeted modification, they can be transformed into efficient adsorbents for wastewater treatment. Their dual role in waste valorization and pollution control positions hydrochars as key materials in next-generation environmental technologies.
Reference
Azzaz, A. A., Khiari, B., Jellali, S., Matei Ghimbeu, C., & Jeguirim, M. (2020). Hydrochars production, characterization and application for wastewater treatment: A review. Renewable and Sustainable Energy Reviews, 127, 109882. https://doi.org/10.1016/j.rser.2020.109882
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