Heavy metal pollution in wastewater from industrial processes is a major environmental concern.
Untreated wastewater with heavy metals discharged into the environment can contaminate aquatic life and crops.
Heavy metals can subsequently enter the food chain, be absorbed by humans and cause major health problems owing to their toxicity.
Wastewaters produced from these industrial activities affect the environment, human health, and the ecosystem.
Heavy metals, such as Hg, Cr, Pb, Zn, Cu, Ni, Cd, As, Co, Sn, etc. must be removed from the water to avoid harmful effects on the environment and human health.
Why is it worried?
On a global scale, wastewater treatment is a crucial issue that has received attention from the highest levels of government and major enterprises.
Organizations can use a variety of tactics to go beyond compliance and begin the process of positively improving global water quality to develop a sustainable solution.
According to a United Nations World Water Development study, more than 80% of all wastewater from industry, residences, towns, and agriculture is discharged into the environment untreated and flows back into the ecosystem via lakes, rivers, and other bodies of surface water.
This process occurs every day around the world, contaminating the environment while wasting valuable fertilizers and other recoverable materials.
Many methods have been applied for removing metal ions from aqueous solution generally depending on physical, chemical and biological technologies.
Chemical precipitation, oxidation or reduction, ion exchange, electrochemical treatment, membrane technology, reverse osmosis and filtration are examples of traditional physiochemical methods.
Most of these are ineffective or excessively expensive when the heavy metals concentrations are less than 100 mg/L.
For example, some of these treatment methods will be very costly, especially when treating large amounts of wastewater, so it is becoming necessary to find a cheap, effective, and eco-friendly method to remove heavy metals from water.
Research on biosorption focus on the biosorbents the biosorption mechanism, and large-scale experiments.
Although many biological materials can bind heavy metals, only those with sufficiently high metal-binding capacity and selectivity for heavy metals are suitable for use in a full-scale biosorption process.
Raw leaves as bio sorbents
Leaf adsorbents are among the most studied bio sorbents for the removal of metal ions, because leaves are considered adsorbents. After all, it is:
- Available,
- Cheap
- Eco-friendlily materials
- The high sorption capacity.
But it has been often ignored because it has Low mechanical strength. So that it must be modified to avoid this advantage.
What is the mechanism of heavy metal removal using biomass leaves?
The biomass leaves contain functional groups such as carboxyl, amine, amide, methyl groups and hydroxyl groups which are considered the major groups responsible for the biosorption process.
The pH of the aqueous solution has been considered the most important parameter controlling metal adsorption by adsorbents.
The pH can affect the form and the number of metal ions in water and the form and quantity of an adsorbent’s surface sites.
In general, the removal of metal cations is due to the well-known competition between H ions and metal ions in the solution.
Modified biomass leaves as heavy metal bio sorbents
Methods of surface modification
The main goal of surface modification is to improve the biosorption efficiency of biomass leaves.
The greatest value and widely studied surface modification of biomass leaves is chemical modification.
Advantages of surface modification
- Low cost
- The procedure is very easy.
- It is a one-step process in most cases.
The most used reagents/solutions used for surface modification
The use of each modification method aims to a specific effect like to improve the chemical surface heterogeneity, increase the number and spreading of the functional groups available for mandatory with the metal, and/or alter the surface morphology; thus, the useful pretreatment method should be chosen according to the targeted heavy metals ion.
Maximum adsorbent capacity increased with the increase in temperature this is due to the increase in the number of available active sites on the adsorbent.
The most important factor that affected the adsorption performance is the particle size of the biomass leaves powder.
Two different approaches during the development of the raw biomass were followed:
– To control the particle size of the biomass leaves in a specific range by sieving. The most common range was between 250 and 500 μm.
– The second one is to collect and use the powder of less than a specific maximum in size value. Different maximum particle sizes were reported, such as500,180, 100, or even80 μm.
Regeneration of bio sorbents
The reusability of bio sorbents offers an economic benefit and is preferred for their practical and profitable usefulness in wastewater treatment processes.
Numerous studies have been done for the regeneration and reuse of modified biomass leaves after heavy metal adsorption.
Desorption studies also help to control the biosorption mechanisms such as ion exchange, complexation and physisorption.
The most common eluents used are diluted HCl, NaOH, HNO, and EDTA solutions, usually in concentrations up to0.1 mol/L.
The contact of bio sorbents in acidic conditions due to strong desorption agents such as HCl can affect the biomass leaves rigidity due to biomass degradation and decrease of binding sites number.
References
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[8] Ngah, W. W., & Hanafiah, M. A. K. M. (2008). Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: a review. Bioresource Technology, 99(10), 3935-3948.
[9] Larous, S., Meniai, A. H., & Lehocine, M. B. (2005). Experimental study of the removal of copper from aqueous solutions by adsorption using sawdust. Desalination, 185(1-3), 483-490.
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