A comprehensive adsorption study of 1-Hydroxy-2-Naphthoic acid using cost effective engineered materials
Summary (3 min read)
- The naphthoic acids are challenging and costly to remove from water and soil.
- Petroleum hydrocarbons (PH) are widely found pollutants in soil and contaminated water due to their ubiquity and frequency of use (Qadir et al., 2008).
- Wastewaters containing PH are also used for agricultural irrigation creating a pathway for contamination of soil (Feenstra et al., 2000) and accumulation within crops, soil and surface water.
- The authors hypothesis was that magnetite enrichment of biochar and clay minerals would enhance the adsorption properties (number and availability of active sites) of natural biochar adsorbents towards HNA for the remediation of contaminated waters.
- 1-Hydroxy-2-naphthoic acid, sodium hydroxide (NaOH) both with a purity ≥97 % and hydrochloric acid (HCl) were obtained from Sigma-Aldrich (Islamabad, Pakistan).
- Two types of clay minerals (natural zeolite and montmorillonite), three types of biochars (wheat straw, sugarcane and rice husk biochars) and one iron oxide were used for investigating the sorption of organic pollutants from water.
- Biochar produced by pyrolysis at 350 ⁰C were obtained from NARC (Islamabad, Pakistan).
- Magnetite and natural zeolite were obtained from Meiqi Industry and Trade Co., Ltd (Gongyi City, China) both having a purity of ≥99 percent and particle sizes of 53 and 150 µm respectively.
2.2. Enrichment of biochars and clays with magnetite
- The properties of the magnetite enriched product depend on the enrichment technique and the physical-chemical conditions at which enrichment was carried out.
- Subsequently, sodium hydroxide solution (0.1 M) was added to the magnetite to create a disperse solution of particles which formed a covering layer which was deposited on the surface of each adsorbent particles (visual inspection using microscopy) (Mohan et al., 2014).
- The functionalized mixture (henceforth enriched compound) was filtered and the residue was washed with buffered deionized water (pH 7.0).
- After drying in a laminar flow hood at room temperature, the product of this reaction was stored in air tight bottles.
- The three mixtures prepared were considered magnetite enriched biochars (WSB, SB and RHB).
2.3. Batch experiments
- Batch experiments were performed at ambient temperature (25±2 °C).
- Adsorption of HNA was investigated using both original and magnetite enriched biochars (WSB, RHB and SB), clay minerals, (zeolite and montmorillonite) and magnetite.
- The influence of a number of important process factors were investigated including: contact time of the adsorbent, pollutant concentration and ionization effects at different pH.
- The HNA concentration obtained from spectrophotometric method was validated against appropriate external quality standards for HNA.
- Values obtained from the spectrophotometer were corrected based on the absorbance of double deionized water blanks.
2.4. Adsorption kinetics
- Batch adsorption equilibrium kinetic experiments were carried out in 50 mL total organic carbon (TOC) free borosilicate glass beakers each containing 25 mL of solution at ambient temperature.
- Each experiment was undertaken in triplicate using a HNA solution with a concentration of 0.32 mmolL-1 and 0.05 g of each solid adsorbent material.
- The kinetic data for all the materials tested was evaluated fitting to the pseudo-first order model, the pseudo-second order model and intra-particle diffusion model.
- The linear form of the pseudo-first order model was obtained by integrating and applying the boundary conditions t=0 to t=t and qt=0 to qt=qt (Equation 2).
2.5. Adsorption equilibrium isotherms
- Adsorption equilibrium isotherms were investigated using five different HNA pollutant concentrations at equal intervals between 0.02 and 0.32 mmolL-1, obtained from a single stock solution of HNA.
- To investigate the potential for degradation of HNA, five chemical only blank samples (without solid sorbent material) were placed in covered beakers at room temperature for 3 days (in the dark).
- The adsorption equilibrium isotherm was established by adding 0.05 g each of the solid adsorbent into the HNA solutions having a pH range from 4.5 to 5.3, followed by shaking in the orbital shaker at a rate of 200 rpm at room temperature for 24 hours.
- After 24 hours, samples from each of the solutions were collected by filtering through a sterile polypropylene syringe filter (0.2 µm).
- The remaining concentration of the HNA pollutant in each solution was measured using a UV-visible spectrophotometer as above.
3. Results and Discussion
- The removal of HNA from contaminated water through adsorption is an attractive and low cost method for wastewater treatment (Lu et al., 2010).
- Here, the adsorption of HNA was investigated using three types of biochar, natural materials, magnetite and magnetic companions conducted under a variety of experimental parameters to simulate the range of expected operating parameters (contact time, contaminant concentration and the ionization effects at different pH).
3.1. Adsorption kinetics
- The adsorption behaviour of adsorbents to HNA was assessed at different time intervals, initially the adsorption occurred rapidly up to 180 min of contact time and then increased more gradually until 400 min.
- This is pertinent as the HNA removal character by magnetic biochars was similar to magnetite suggesting this enrichment could be utilized to upgrade the adsorption capacity of local and available biochar.
- The adsorption capacity was in the following sequence and from greatest to least: magnetite > SB > WSB ≈ RHB and after enrichment the adsorption capacity increased but was in the same order of performance.
- The HNA adsorption capacity in order and from greatest to least was: enriched zeolite ≈ enriched montmorillonite >magnetite.
3.2. Kinetic modelling of HNA adsorption.
- The kinetic data was fitted to the pseudo-first order, pseudo-second order and intra-particle diffusion models (Table 1).
- This was evidenced by linear correlation coefficient (r2) values exceeding 0.99 and the closeness of modelled Qe values to the experimental derived Qe values.
- Here, the intra-particle diffusion model revealed that the fastest adsorption rates were observed for WSB and RHB evidenced by Kid values of 0.006 and 0.0067 mmol.[g.min0.5]-1 respectfully (Table 2).
- For example the WSB Kid decreased from 0.006 to 0.005 with similar reductions for other biochar.
- More rigorous stirring or agitation could be used to reduce apparent BLT and potentially increase the rate of diffusion.
3.3. Adsorption equilibrium isotherm
- All adsorbents had lower HNA amounts adsorbed at elevated contaminant concentrations which implies that full saturation of adsorbents’ active sites had occurred preventing further HNA uptake from solution.
- Therefore adsorption was monolayer and chemisorption could not proceed (Singh et al. 2018).
3.4. Impact of pH on adsorption to biochar
- The pH is an important factor on the HNA adsorption process, which influences surface complex reactions which occur at specific mineral adsorption sites (Hanna et al., 2010).
- Previous studies have shown that compounds with stronger electronegativity are bound more strongly to the adsorbent surface which influences the pollutant adsorption rates and regeneration efficacy of adsorbents (Chiou et al., 1979).
- This suggested that the electrostatic attraction between the HNA and the protonated functional groups of the adsorbent increased during each decrease in pH.
- Biochar showed a pronounced decrease in HNA adsorption before enrichment as compared to clay and magnetite adsorption but after enrichment the adsorption behaviour of magnetic biochar decreased more gradually, similar to clay and magnetite for pH, time and concentration of contaminant.
- Further work could investigate the influence of point of zero charge on HNA adsorption and undertake column tests to assess the adsorption using realistic volumes of real water with environmentally relevant concentrations of pollutants.
- The batch adsorption data demonstrated that HNA adsorption occurred rapidly before gradual removal occurred which stopped after all the active sites were saturated.
- The HNA adsorption character fitted the pseudo-second order model for all materials tested including enriched and conventional companions, suggesting chemisorption dominated the removal mechanism.
- The removal of HNA was better at lower pH with values of 5-6 being critical for enhancing the activity of key functional groups to reaction with HNA.
- These findings translate into equivalence between magnetite and magnetic biochars, suggesting cheaper alternative materials (to magnetite in isolation) could be synthesized in situ with the biochar acting as a carrier material.
- These findings are of interest to waste and water treatment practitioners interested in low-cost solutions for removal of organic pollutants and their breakdown products.
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...The surface layers of biochar studied were considered heterogeneous ( Zhao et al., 2013) this is in part due to bound carbonate and non-carbonaceous components on the surface layers of the biochar (Chen et al., 2008, Cao et al., 2009)....