Cationic Manihot esculenta (ME) peel starch was synthesized through etherification method using 3-chloro-2-hydroxypropyl trimethylammonium chloride (CHPTAC) as cationizing monomer. The optimization of the main factors influencing the degree of substitution (DS) was conducted using central composite design (CCD) and response surface methodology (RSM). The factors assessed include CHPTAC concentration, catalyst sodium hydroxide (NaOH) concentration, and reaction time. The DS values of the cationic starches were obtained between 0.39 and 0.99. The maximum DS value was up to 0.99 at 0.615 mol/L of CHPTAC, 30 % (w/v) NaOH, and a reaction time of 5 h. The finding based on the optimization using RSM reflected that CHPTAC and NaOH concentrations are the key variables determining the DS value, while reaction time has a negligible impact on the etherification process. Furthermore, the chemical composition, morphology, and structure of the cationic ME peel starch were characterized by scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX), Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD) and nuclear magnetic resonance spectroscopy (1H NMR). It was confirmed that the modifying monomers penetrated the surface layer of the starch granules and attached to the starch backbone.
Wastewater treatment activities in the chemical industry have generated abundant gypsum waste, classified as scheduled waste (SW205) under the Environmental Quality Regulations 2005. The waste needs to be disposed into a secure landfill due to the high heavy metals content which is becoming a threat to the environment. Hence, an alternative disposal method was evaluated by recycling the waste into fired clay brick. The brick samples were incorporated with different percentages of gypsum waste (0% as control, 10, 20, 30, 40 and 50%) and were fired at 1050 °C using 1 °C per minute heating rate. Shrinkage, dry density, initial rate of suction (IRS) and compressive strength tests were conducted to determine the physical and mechanical properties of the brick, while the synthetic precipitation leaching procedure (SPLP) was performed to scrutinize the leachability of heavy metals from the crushed brick samples. The results showed that the properties would decrease through the incorporation of gypsum waste and indicated the best result at 10% of waste utilization with 47.5% of shrinkage, 1.37% of dry density, 22.87% of IRS and 28.3% of compressive strength. In addition, the leachability test highlighted that the concentrations of Fe and Al was significantly reduced up to 100% from 4884 to 3.13 ppm (Fe) and from 16,134 to 0.81 ppm (Al), respectively. The heavy metals content in the bricks were oxidized during the firing process, which signified the successful remediation of heavy metals in the samples. Based on the permissible incorporation of gypsum waste into fired clay brick, this study promised a more green disposing method for gypsum waste, and insight as a potential towards achieving a sustainable end product.