Why Sawdust Is The Most Promising Material For The Removal Of Heavy Metals from Water? (Material Science)

Many studies show how dangerous heavy metals can be. For instance, copper, it is not only very important element in life, but can also have toxic effects as it can contaminate water and foods such as chocolate, nuts and shellfish. If there is an increase in the inhalation of copper it can directly provoke lung cancer and large doses of copper can also cause irritation in the nervous system and in the liver and kidneys; moreover, copper is also highly linked with depression. Another example is cadmium, which is not even essential and is highly toxic due to its ability to combine with sulfur and displace essential elements such as zinc and calcium. Heavy metals not only pose a threat to human life but also contaminate the environment and are not biodegradable. This means that heavy metals will not transform into harmless elements with time and measures have to be taken to deal with their potential threat.

Its not like we doesnt have any methods to remove these heavy metals from water. But the fact is, most of the methods we have, cannot be implemented due to factors like high cost of operations, low selectivity of the metal ions, generation of sludge, the complicated nature of the treatment and their inability to achieve total water purification when concentrations of heavy metal ions are lower than 50 ppm. So, there is a need of another alternative. But, what if that alternative is nothing, but an adsorbent.

The elemental compositions of various types of sawdust. © Authors

Although there are many potential adsorbents, but as I already mentioned above, we need a low cost, biodegradable etc. alternative. And, sawdust present all these advantages. The important fact is, the behavior of sawdust depends on the chemical composition. Meaning, there are many types of sawdust like pine sawdust, Meranti sawdust, Chinese fir sawdust, Red wood sawdust, Rubber-wood etc. The question is, which one to choose? Well, for that you have to closely examine the shape of the surface of sawdust with the help of micrographs and when you will do this, you will realize that Pine and Meranti displays a rough surface showing tubular formations and pores compared to others. These features lead to an increase in the surface area of the Pine and Meranti sawdust, hence increasing the sorption capacity.

Scanning electron micrographs of Pinus halepensis. Reprinted with permission of Elsevier from Semerjian et al. 2018. © Meez et al.

But, though, sawdust effectively adsorb heavy metals, its capacity to adsorb heavy metals are affected by many parameters like:

  • pH: the adsorption percentage of sawdust increases with increasing pH until it reaches a maximum at a pH of 5 and a pH of 7 for lead and copper ions, respectively, and then the adsorption percentage starts to decrease with further increasing of pH.
  • Contact time: is the amount of time in which heavy metals are present in the same solution as sawdust. Adsorption increases rapidly in the early contact time within a few minutes, and starts to decrease gradually until equilibrium is achieved. Moreover, the sawdust concentration also affects time as it is observed that the lower the concentration of sawdust, the higher is the adsorption percentage.
  • Dosage of adsorbent: The removal rate of the heavy metals is directly proportional to the concentration or dosage of the adsorbent. As the dosage of the adsorbent increases, the adsorption percentage of heavy metals into sawdust increases. Interestingly, the number of metal ions present in the solution also plays a role in determining the adsorption process.
  • Temperature: Temperature also plays a crucial role in the adsorption process of heavy metals and should be considered when dealing with purification of wastewater. Usually, as the temperature increases, the number of heavy metals adsorbed increases until a certain temperature, which is recognized as the optimum temperature. For instance, the optimum temperature of copper by sawdust was found to be 40 °C, but the increase in temperature led to a direct decrease in adsorption percentage.

But, what if we can do some modifications in sawdust in order to enhance the selectivity/capacity? Well, as the adsorption process occurs on the surface of the sawdust, its modification can strongly affect the adsorptive capacity of this material. A number of chemical, physical, and other modification methods have been employed for this objective. In fact, to enhance the efficiency of the adsorbent, several pretreatment methods have been investigated employing various modifying agents, such as: (i) basic solutions (NaOH, Ca(OH)2, KOH, Na2CO3); (ii) acid solutions (HCl, H2SO4, H3PO4, CH3COOH, HNO3, citric acid); (iii) mineral salts (NaCl, KCl, Na2HPO4, NaHCO3); (iv) organic compounds (ethylene diamine, formaldehyde, epichloro-hydrin, methanol, dyes); (v) phosphorylation treatment (CO(NH2)2 (urea) + H3PO4).

Another important point is, which adsorption mechanism take place between sawdust and heavy metal ions? Well, most researchers have assumed, with no evidence, an ion exchange mechanism that occurs between hydrogen from cellulose molecules and adsorbing metal ions. However, recent study pointed out that predominantly calcium and to a lesser extent magnesium acts in the ion exchange process that occurs during the adsorption of heavy metal ions.

Finally, what adsoption processes have to follow? Well, lets have a look of them below:

  • Isotherms: Every adsorption process has to follow an isotherm, as isotherms determine the way in which heavy metal ions are deposited into a certain surface area. The adsorption of heavy metals into sawdust follow the Freundlich or Langmuir isotherms, meaning that the adsorption can happen in both a monolayer or multilayer manner.
  • Adsorption kinetics: Adsorption kinetics are used in order to determine the rate-determining step of adsorption of any material into the specified adsorbent. Kinetics are very important in order to design an efficient adsorption model to improve the whole process. The adsorption by sawdust usually follows the pseudo second-order kinetic model which means that, usually, the adsorption into sawdust is a chemisorption, but it is not always the only rate-determining step nonetheless.
  • Adsorption thermodynamics: There are three main thermodynamic parameters that are important for the adsorption process: enthalpy, entropy and free energy change. The thermodynamics studies in some special cases, such as in the removal of Cu(II) and Pb(II), showed that the adsorption into sawdust is an endothermic reaction and spontaneous with an increase in randomness between the solid and liquid surface of contact. So, from this one can conclude that sawdust is a very promising material for the purification of wastewater.

Reference: Meez, E.; Rahdar, A.; Kyzas, G.Z. Sawdust for the Removal of Heavy Metals from Water: Α Review. Molecules 2021, 26(14), 4318; https://doi.org/10.3390/molecules26144318

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