(159ag) Removal of Cadmium (II) from Aqueous Solution By Adsorption on an Hydrochar Prepared from Water Hyacinth

Heavy metal contamination in Mexico is a serious problem caused by human activities, specifically mining. The metal pollutants of significant concern are mercury, lead, cadmium and chromium. High concentrations of heavy metals in soils have been detected in sites located in the central region of Mexico. Cd(II) and Pb(II) are among the most hazardous metals to human beings since they can cause adverse effects to the enzymatic, renal, respiratory and digestive systems. Besides, they can bioaccumulate and are persistent in the environment.

The water hyacinth (Eichhornia crassipes) (WH) is an invasive weed that can cause a negative environmental impact in lakes and ponds if the WH is not kept under control. WH can cover large extensions of surface water resources preventing sunlight from reaching the native aquatic plants impeding photosynthesis, and depleting oxygen from the water. WH is very tolerant and can grow in heavily polluted water. WH has a high capacity for bioaccumulating heavy metals and can be used as a phytoremediation plant for cleaning industrial wastewater.

Hydrothermal carbonization (HTC) is a process in which organic matter decomposes under the influence of temperature in the presence of water. Hydrocarbonization has many applications, and the hydrochars produced can be applied as adsorbents for remediation of polluted water, soil amendment processes and producing catalysts and nanostructured materials.

One of the most effective processes for removing heavy metals from the aqueous solution is adsorption, which is a surface phenomenon that involves the accumulation of pollutants on the surface of a porous solid. Recently, hydrochar has been applied for the removal of heavy metals. Unlike activated carbons, hydrochars do not have microporosity but contain many oxygenated groups on the surface, favoring the adsorption of heavy metal cations.

This work's main objective was to hydrocarbonize WH by hydrothermal treatment and produce hydrochar from WH (HCWH). Moreover, the capacity of HCWH for adsorbing Cd(II) from water solutions was studied in detail. The HCWH was modified with citric acid to enhance its adsorption capacity towards Cd(II). The effect of temperature and pH were investigated thoroughly, and the adsorption mechanism was elucidated.

The water hyacinth (WH) was collected from a water damp at San Luis Potosí, Mexico, was sun-dried for 24 h and ground in an analytical mill. The WH hydrochar (WHHC) was synthesized by adding 8 g WH and 80 mL of the deionized water into an Autoclave Reactor with PTFE lined vessel, and the reactor was heated up in a muffle furnace at 220 ° C for 3 h. Afterward, the WHHC was washed with ethanol and then in deionized water. Finally, WHHC was dried in an oven at 110 °C and ground in a tungsten ball mill until passing through US mesh sizes 200 and 400 mesh. The WH and WHHC were modified by a hydrothermal method using a 1 M citric acid (CA) solution and continuously stirred and heated for 2 h at T = 60 °C. The modified materials were separated by decanting the solution and dried in an oven at 80 °C for 12 h. The WH and WHHC modified with CA were labeled as WH-CA and WHHC-CA.

The experimental adsorption equilibrium data of Cd(II) on WH, WH-CA, WHHC and WHHC-CA were obtained in batch adsorbers, conical vials of 50 mL. A particular mass of adsorbent and 45 mL of a Cd(II) solution having a known initial concentration were added into the adsorber, which was placed in a thermostatic bath. The adsorbers were shaken for 1 h daily, and the solution pH was kept constant at a given value by supplementing 0.01 N NaOH or HCl solutions. The Cd(II) solution and the adsorbent were contacted for 10 days to reach equilibrium. Afterward, a sample of the adsorber solution was analyzed by atomic absorption spectroscopic to determine the equilibrium concentration of Cd(II), and the mass of Cd(II) adsorbed was calculated by performing a mass balance.

All the materials were characterized by N2 physisorption, infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The textural properties of all the materials showed that they have low BET surface area and total pore volume and are not microporous. The FTIR technique confirmed the presence of the carboxylic groups where the Cd(II) cations can be adsorbed on all these materials. The TGA shows that the organic compounds of the materials (lignin, cellulose and hemicellulose), decompose thermally in the range 160-400 ° C. The Z potential was measured in a zetameter, and the results revealed that the surface of all materials was negatively charged in pH range 3-7 and the negative charge increased by raising the pH.

The adsorption isotherms of Cd(II) on the various adsorbents are depicted in Figure 1. The increasing order of the adsorption capacities of the materials was the following: WH < WHHC < WH-CA < WHHC-CA. At an equilibrium Cd(II) concentration of 600 mg/L, the uptakes of Cd(II) adsorbed on WH, WHHC, WH-CA and WHHC-CA were 60, 70, 122 and 158 mg/g, respectively. These results revealed that the adsorption capacity of WHHC-CA was 2.6, 2.3 and 1.3 times those of WH, WHHC and WH-CA, respectively.

The above results indicated that the hydrocarbonization of WH promotes its adsorption capacity slightly; however, the hydrocarbonization followed by the modification with AC enhanced the adsorption capacity of WHHC-AC considerably. The high capacity of WHHC-AC for adsorbing Cd²⁺ from aqueous solutions was much larger than those of various conventional adsorbents, namely activated carbons and natural clays and zeolites. The adsorption isotherm of WHHC-AC increased considerably at low equilibrium concentrations of Cd(II), showing that the WHHC-AC presented a very high affinity for Cd(II). The WHHC-AC exhibited an exceptionally high adsorption capacity, indicating that Cd(II) can be effectively eliminated from water solutions by adsorption on WHHC-AC.

The experimental adsorption equilibrium data were interpreted using the isotherm models of Langmuir, Freundlich and Radke-Prausnitz. The Radke-Prausnitz isotherm best fitted the Cd(II) adsorption data. The effect of pH on the adsorption capacity of WHHC-AC and WH-AC was studied by varying the solution pH, and it was also found that the adsorption capacity of WHHC-AC and WH-AC increased while the solution pH was raised from 3 to 6. This behavior was attributed to electrostatic interactions between Cd²⁺ cations and the negatively charged surface of WHHC-AC and WH-AC. The adsorption capacity of WHHC-AC was diminished notably by increasing the ionic strength of the solution, corroborating the importance of electrostatic attraction in the adsorption of Cd²⁺ on WHHC-AC. The adsorption equilibrium experiments at the temperatures of 15 and 25 °C demonstrated that the adsorption capacity of WHHC-AC towards Cd(II) increased with increasing temperature so that the adsorption process was endothermic.