Inability to Completely Remove Trace Contaminants from Drinking Water by Adsorption
Article Information
Wenfa Ng*
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
*Corresponding Author: Wenfa Ng, Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
Received: 27 February 2022; Accepted: 07 March 2022; Published: 09 March 2022
Citation: Wenfa Ng. Inability to Completely Remove Trace Contaminants from Drinking Water by Adsorption. Journal of Environmental Science and Public Health 6 (2022): 129-134.
Share at FacebookAbstract
Trace contaminant removal is of contemporary concern in drinking water and wastewater treatment, given increasing evidence of potential toxicity of various heavy metals and metalloids in water at parts per billion (ppb) concentration levels. Currently, only high-pressure reverse osmosis can reliably remove trace concentration contaminants from water but with high energy cost. Thus, the search is on for techniques and methods able to remove trace concentration contaminants with a lower energy footprint. Adsorption, a low energy technique for water and wastewater treatment is one possibility, which when coupled with the use of low-cost biomass as adsorbent, would further improve process efficiency. This synopsis describes work done on examining the use of local marine seaweed, Sargassum sp. for removing copper at low (4 to 20 ppm, parts per million) and trace concentration (< 1000 ppb), with and without ammonium ion interference at different solution pH. Specifically, to reduce organic leaching and improve mechanical stability, formaldehyde crosslinking was used and was effective in enhancing stability of seaweed between pH 3 and 9. But, residual organic leaching of ~ 4 ppm meant that modified seaweed was not suitable for drinking water treatment. Batch kinetic and equilibrium studies revealed that up to a threshold ammonium concentration of 50 ppm [NH4+-N], there was good sorption of copper on formaldehyde crosslinked seaweed (treated seaweed), with residual copper concentration of 0.5 ppm. Residual copper concentration increased with increase in ammonium concentration (up to 2500 ppm [NH4+-N]); thereby, highlighting a competitive binding effect for treated seaweed’s surface functional groups. Attempts to remove trace concentration copper highlighted feasibility of the approach. However, significant residual equilibrium concentration of copper remained in the solution. More importantly, thermodynamic considerations point to the existence of a finite resid
Keywords
Adsorption; Seaweed; Ammonium; Copper; Residual equilibrium concentration; Reverse osmosis; Membrane; Competitive binding; Kinetic; Biosorption
Adsorption articles; Seaweed articles; Ammonium articles; Copper articles; Residual equilibrium concentration articles; Reverse osmosis articles; Membrane articles; Competitive binding articles; Kinetic articles; Biosorption articles
Adsorption articles Adsorption Research articles Adsorption review articles Adsorption PubMed articles Adsorption PubMed Central articles Adsorption 2023 articles Adsorption 2024 articles Adsorption Scopus articles Adsorption impact factor journals Adsorption Scopus journals Adsorption PubMed journals Adsorption medical journals Adsorption free journals Adsorption best journals Adsorption top journals Adsorption free medical journals Adsorption famous journals Adsorption Google Scholar indexed journals Seaweed articles Seaweed Research articles Seaweed review articles Seaweed PubMed articles Seaweed PubMed Central articles Seaweed 2023 articles Seaweed 2024 articles Seaweed Scopus articles Seaweed impact factor journals Seaweed Scopus journals Seaweed PubMed journals Seaweed medical journals Seaweed free journals Seaweed best journals Seaweed top journals Seaweed free medical journals Seaweed famous journals Seaweed Google Scholar indexed journals Ammonium articles Ammonium Research articles Ammonium review articles Ammonium PubMed articles Ammonium PubMed Central articles Ammonium 2023 articles Ammonium 2024 articles Ammonium Scopus articles Ammonium impact factor journals Ammonium Scopus journals Ammonium PubMed journals Ammonium medical journals Ammonium free journals Ammonium best journals Ammonium top journals Ammonium free medical journals Ammonium famous journals Ammonium Google Scholar indexed journals Copper articles Copper Research articles Copper review articles Copper PubMed articles Copper PubMed Central articles Copper 2023 articles Copper 2024 articles Copper Scopus articles Copper impact factor journals Copper Scopus journals Copper PubMed journals Copper medical journals Copper free journals Copper best journals Copper top journals Copper free medical journals Copper famous journals Copper Google Scholar indexed journals Residual equilibrium concentration articles Residual equilibrium concentration Research articles Residual equilibrium concentration review articles Residual equilibrium concentration PubMed articles Residual equilibrium concentration PubMed Central articles Residual equilibrium concentration 2023 articles Residual equilibrium concentration 2024 articles Residual equilibrium concentration Scopus articles Residual equilibrium concentration impact factor journals Residual equilibrium concentration Scopus journals Residual equilibrium concentration PubMed journals Residual equilibrium concentration medical journals Residual equilibrium concentration free journals Residual equilibrium concentration best journals Residual equilibrium concentration top journals Residual equilibrium concentration free medical journals Residual equilibrium concentration famous journals Residual equilibrium concentration Google Scholar indexed journals Reverse osmosis articles Reverse osmosis Research articles Reverse osmosis review articles Reverse osmosis PubMed articles Reverse osmosis PubMed Central articles Reverse osmosis 2023 articles Reverse osmosis 2024 articles Reverse osmosis Scopus articles Reverse osmosis impact factor journals Reverse osmosis Scopus journals Reverse osmosis PubMed journals Reverse osmosis medical journals Reverse osmosis free journals Reverse osmosis best journals Reverse osmosis top journals Reverse osmosis free medical journals Reverse osmosis famous journals Reverse osmosis Google Scholar indexed journals Membrane articles Membrane Research articles Membrane review articles Membrane PubMed articles Membrane PubMed Central articles Membrane 2023 articles Membrane 2024 articles Membrane Scopus articles Membrane impact factor journals Membrane Scopus journals Membrane PubMed journals Membrane medical journals Membrane free journals Membrane best journals Membrane top journals Membrane free medical journals Membrane famous journals Membrane Google Scholar indexed journals Competitive binding articles Competitive binding Research articles Competitive binding review articles Competitive binding PubMed articles Competitive binding PubMed Central articles Competitive binding 2023 articles Competitive binding 2024 articles Competitive binding Scopus articles Competitive binding impact factor journals Competitive binding Scopus journals Competitive binding PubMed journals Competitive binding medical journals Competitive binding free journals Competitive binding best journals Competitive binding top journals Competitive binding free medical journals Competitive binding famous journals Competitive binding Google Scholar indexed journals Kinetic articles Kinetic Research articles Kinetic review articles Kinetic PubMed articles Kinetic PubMed Central articles Kinetic 2023 articles Kinetic 2024 articles Kinetic Scopus articles Kinetic impact factor journals Kinetic Scopus journals Kinetic PubMed journals Kinetic medical journals Kinetic free journals Kinetic best journals Kinetic top journals Kinetic free medical journals Kinetic famous journals Kinetic Google Scholar indexed journals Biosorption articles Biosorption Research articles Biosorption review articles Biosorption PubMed articles Biosorption PubMed Central articles Biosorption 2023 articles Biosorption 2024 articles Biosorption Scopus articles Biosorption impact factor journals Biosorption Scopus journals Biosorption PubMed journals Biosorption medical journals Biosorption free journals Biosorption best journals Biosorption top journals Biosorption free medical journals Biosorption famous journals Biosorption Google Scholar indexed journals
Article Details
Synopsis
The goal in water treatment has always been the search for cost effective techniques and methodologies for removing various contaminants from water. To this end, adsorption of heavy metals and other ionic contaminants onto adsorbents derived from biomass is one attractive option for reducing the cost of treating water, enabling reuse of waste biomass materials, as well as tap on a low energy water treatment technique. Indeed, compared to high pressure reverse osmosis treatment of drinking water which effectively remove all ionic and non-ionic contaminants through a non-porous water permeable membrane, adsorption is lower in energy consumption given the relatively low pressure at which water flow through the adsorbent column. However, the technique’s most significant drawback comes from the inability to remove all contaminants given the dependence on thermodynamic equilibration between the water and adsorbent. Thus, adsorption is effective in removing significant fraction of the pollutant load from water, but ineffective in realizing zero pollutant in drinking water. Nevertheless, the technique remains highly useful in applications not requiring the removal of trace contaminants in the parts per billion (ppb) range.
Realization of potential carcinogenic effects of many heavy metal contaminants and non-ionic pollutants has driven the search for more effective methods for removing trace contaminants from drinking water, with the goal of producing reverse osmosis grade water with lower cost and energy consumption. To this end, optimization of reverse osmosis processes has helped improved process efficiencies and cost of energy usage relative to contaminant removal level. But the inherent high energy consuming nature of reverse osmosis placed the technique at the upper end of energy consumption for water and wastewater treatment.
Adsorption has drawn attention due to its lower energy requirement. Coupled with the use of plant biomass found to be useful in high capacity adsorption of various heavy metals such as copper, cadmium and lead, the technique of biosorption was intensively researched for its possible use in treating various wastewaters [1]. To this end, a study [2] attempted to remove copper at low concentration level of 4 to 20 ppm from water under the competition of ammonium cations at high concentrations of up to 2500 ppm [NH4+-N], via formaldehyde treated local marine seaweed (treated seaweed).
Specifically, marine seaweed was harvested from the local coastline, washed with copious amounts of tap water, dried and ground into small size fraction suitable for adsorption: 500 to 850 m size range. Due to the phenomenon of leaching of organics from unmodified seaweed during adsorption experiments, 0.2 wt% formaldehyde solution was contacted with seaweed for crosslinking the seaweed surface, which helped improved mechanical stability of the seaweed, Sargassum sp. After formaldehyde crosslinking, organic leaching from the treated seaweed significantly decreased to ~ 4 ppm; thereby, making it suitable for use in treating wastewater. However, use of formaldehyde crosslinked seaweed in drinking water treatment would pose a secondary pollution problem due to the organics leached from the modified seaweed.
Experiment results revealed that, up to an ammonium concentration of 50 ppm [NH4+N], adsorption by treated seaweed was effective in removing copper at low concentration range of 4 to 20 ppm. Specifically, residual copper concentration after adsorption on treated seaweed was ~ 0.5 ppm for ammonium concentration less than 50 ppm [NH4+-N]. With increase in ammonium concentration from 50 ppm to 2500 ppm [NH4+-N], time taken for the system to reach equilibrium increased. Similarly, the concentration of residual copper in the solution also increased. Thus, ammonium, as a cation, likely interfered with the adsorption of Cu2+ by treated seaweed through a competitive binding mechanism, where ammonium competed with Cu2+ for the same functional groups.
Investigating the speed at which copper was adsorbed by treated seaweed revealed that sorption was fast and could be depicted as a two-phase process. Specifically, the initial phase of fast sorption most likely occurred due to adsorption of copper onto surface functional groups of treated seaweed. On the other hand, the slower second phase of sorption could be accounted for by the diffusion of copper ions into the pores of treated seaweed, where they are adsorbed by functional groups within the pores. Similar to the case with batch equilibrium studies, ammonium ion interfered with the sorption of copper most likely through a competitive mechanism, where the time taken for the process to reach equilibrium increased with increase in ammonium ion concentration.
Solution pH is a critical variable in wastewater and water chemistry and plays a significant role in influencing sorption speed and capacity. Specifically, adsorption of copper from low concentration solutions (4 – 20 ppm copper) on treated seaweed was substantially lower at pH 2 and 3 compared to pH 4.5. This highlighted that treated seaweed’s application potential at treating acidic wastewater is limited, even though it has good mechanical stability with respect to organic leaching between pH 3 and 9. Broadly speaking, significant competitive effect from H+ ions as well as changes in the polarity of surface functional groups at low pH reduced the biosorption capacity of treated seaweed for positively charged copper ions at acidic pH.
To understand the feasibility of treated seaweed in removing copper at the trace concentration level (i.e., [Cu2+] < 1000 ppb, parts per billion), both batch kinetic and equilibrium studies were conducted. Specifically, kinetic experiments revealed that about 90% of copper was removed from the solution within 10 seconds; thereby, highlighting the possibility of using biosorption for removing trace copper contamination. Importantly, the residual copper concentration was about 50 ppb, which reaffirmed that adsorption could not remove a contaminant completely. Batch equilibrium experiments, on the other hand, were equivocal on the ability of treated seaweed in removing copper at trace concentration. Specifically, significant scatter in the data points revealed that more care must be taken in removing background copper adsorption to the walls of the polyethylene bottles used as adsorption and sample bottles.
Figure 1: Thermodynamic driving force accounts for the adsorption of solute to a surface, where the residual equilibrium concentration of a solute at the seaweed surface could be higher than that at the concentration boundary layer for a trace concentration contaminant.
Hence, while possibility exists for adsorption of copper by treated seaweed at the trace concentration range (i.e., [Cu2+] < 1000 ppb), adsorption as a technique could not completely remove the contaminant due to thermodynamic equilibrium considerations, where the driving force for adsorption comes from the concentration difference between the solution and the seaweed surface (Figure 1).
Specifically, given that adsorption is an affinity-based process aided by a concentration gradient between the solution and the adsorbent surface, the smaller the concentration difference, the smaller would be the amount of adsorption of the contaminant on the adsorbent surface.
Thus, it is theoretically impossible to completely remove a contaminant from water by adsorption as a finite concentration would exist as residual equilibrium concentration in solution. Hence, deployment of adsorption as a water and wastewater treatment technique is based on the removal of contaminant to a level where the residual equilibrium concentration is lower than the water quality standard for drinking or for discharge, respectively.
More importantly, in treating water with trace contaminants, the adsorbent surface may have a higher concentration of contaminants compared to the bulk water; thus, a reverse concentration gradient exists between the adsorbent surface and the solution, where thermodynamic driving force may drive the desorption of the contaminant from the adsorbent back into solution. Thus, desorption of previously sorbed contaminants provide a residual contaminant concentration in solution that preclude the use of adsorption as a method for completely removing trace contaminants from water.
Collectively, batch kinetic and equilibrium studies have affirmed treated seaweed’s ability at removing copper from low concentration solutions (4 to 20 ppm). However, presence of ammonium co-solute would severely reduce adsorption of copper beyond a threshold concentration of 50 ppm [NH4+-N], where residual copper concentration would be significantly higher than without ammonium interference. Possible competitive effect from ammonium ions at circumneutral pH of 5 meant that ammonium co-solute is an important factor affecting biosorption processes for both drinking water and wastewater treatment applications and should be factored in modelling studies.
Additionally, presence of significant organic leaching from treated seaweed (i.e., total organic carbon concentration of 4 ppm) during biosorption meant that the biosorbent was not suitable for use in drinking water treatment. More importantly, given that adsorption processes are guided by fundamental thermodynamic equilibrium considerations where a residual equilibrium concentration exists for the solute in water, it is thus impossible to completely remove a contaminant from water through adsorption processes.
Thus, unlike reverse osmosis process where a barrier (i.e., non-porous membrane) prevents the mixing of the influent and treated water, the lack of a barrier in adsorption processes meant that a residual equilibrium concentration of the contaminant exists; thereby, preventing removal of the trace contaminant.
Conflicts of Interest
The author declares no conflicts of interest.
Funding
No funding was used in this work.
References
- De Gisi S, Lofrano G, Grassi M, et al. Characteristics and adsorption capacities of low-cost sorbents for wastewater treatment: A review. Sustain. Mater. Technol 9 (2016): 10-40.
- Ng W. Ammonium interference reduced copper uptake by formaldehyde crosslinked Sargassum sp. seaweed. PeerJ Prepr 5 (2017): e1228v2.