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Chemical mechanical polishing (CMP) wastewater generated from semiconductor manufacturing industries is known to contain residual organic and inorganic contaminants, i.e. photoresists, acids, including silicon dioxide (SiO2), nanoparticles (NPs) and others. Nanoscale colloids in CMP wastewater have strong inclination to remain in the suspension, leading to high turbidity and chemical oxygen demand (COD). Although various types of pre-treatment have been implemented, these nanoparticles remain diffused in small clusters that pass through the treatment system. Therefore, it is crucial to select suitable pH and coagulant type in the coagulation treatment process. In this research zeta potential and dynamic light scattering measurements are applied as preliminary step aimed at determining optimum pH and coagulant dosage range based on the observation of inter particle-particle behavior in a CMP suspension. The first phase of the conducted study is to analyze nanoscale colloids in the CMP suspension in terms of zeta potential and z-average particle size as a function of pH within a range of 2 to 12. Two types of coagulants were investigated - polyaluminum chloride (PACl) and ferrous sulfate heptahydrate (FeSO4·7H2O). Similar pH analysis was conducted for the coagulants with the same pH range separately. The second phase of the study involved evaluating the interaction between nanoscale colloids and coagulants in the suspension. The dynamics of zeta potential and corresponding particle size were observed as a function of coagulant concentration. Results indicated that CMP wastewater is negatively charged, with average zeta potential of -59.8 mV and 149 d.nm at pH value of 8.7. The interaction between CMP wastewater and PACl showed that positively charged PACl rapidly adsorbed colloids in the wastewater, reducing the negative surface charge of nanoscale clusters. The interaction between CMP wastewater and FeSO4·7H2O showed that larger dosage is required to aggregate nanoscale clusters, due to its low positive value to counter negative charges of CMP wastewater.

In this study, the zeta potential and z-average hydrodynamic diameter of zinc oxide (20 nm, 99.5% purity) were investigated as a function of pH and time. The nanoparticles readily formed aggregates and could not be disintegrated to its primary size eventhough ultrasonication was applied. In zinc oxide suspensions (25 mgL and 100 mg/L) the nanoparticles were examined for electrophoretic mobility and aggregation behaviour within the pH range of 3 to 11 and time duration of 60 minutes. In this study, the particle size and zeta potential were measured by dynamic light scattering technique. The application of nanoparticle tracking analysis was demonstrated as well to observe the aggregation and disaggregation of zinc oxide nanoparticles at various pH values. The NTA results obtained in this work were compared and validated by the DLS. The z-average hydrodynamic diameter of the particles was 389 d.nm with an average zeta potential value of +23 mV using the DLS compared to mean particle size of 365 d.nm by NTA. DLS analysis also identified the pH of the point of zero charge (pHPZC) and the isoelectric point (IEP) at pH 9.4 and pH 6.4 respectively. It was found that the zeta potential remained unaffected by the concentration of the suspension. However, results show that pH does have a pronounced effect on the stability of zeta potential, pH and particle size as a function of time in pH regions where dissolution (< pH 6.4 and 8.3 < pH < 10.5) and aggregation (pH 6.4 and 9.4) occurs. It is apparent that the pH of the suspension strongly influences the zeta potential of ZnO nanoparticles without external factors which will consequently govern their behaviour, agglomeration state and their interactions with each other and surfaces in the environment.

The surface charge and average size of ZnO was studied with a function of pH and time. The interactions between ZnO and Suwannee River humic acid (SRHA) were further investigated under a range of environmentally relevant conditions with the pHPZC as a point of reference. The anionic charge carried by aquatic humic substances plays a major part in the aggregation and disaggregation of metal oxide nanoparticles. At low concentrations of SRHA (PZC. With similar SRHA concentrations, at pH=pHPZC, SRHA was able to control the suspension behavior of the ZnO and promoted partial disaggregation in small volumes. This was more distinguishable as SRHA form a surface coating on the nanoparticles and enhances stability via electrostatic stabilization mechanism which induces the disaggregation behavior in the ZnO nanoparticles and hence, decreases the aggregate size.

The efficiency of using tapioca starch as a natural coagulant on semiconductor wastewater treatment was investigated. The silica content in the wastewater was pH dependent and it was found that the reduction of COD and turbidity occurred at pH between 12 and 14 with measurements below 100 mg/L and 100 NTU respectively. A three-level factorial design experiment in the Response Surface Methodology (RSM) was applied to determine the influence of retention time and dosage of the tapioca starch. The parameters studied were chemical oxygen demand (COD), turbidity and total suspended solids (TSS). The R2 for turbidity, COD and TSS were 0.92, 0.92 and 0.77 respectively. Results indicate that the dosage of the tapioca starch has a higher influence on COD and turbidity reduction. Tapioca starch as a natural coagulant has shown that the optimum turbidity reduction was 99% at a settling time of 30 minutes with dosage of 0.1 g/L. The COD reduction was achieved at 87% after 60 min of retention time and 0.1 g/L of dosage while higher coagulant dosages reduced the TSS concentration to 10.9 mg/L at retention time between 50 to 60 minutes.

A study was conducted to treat stabilized leachate by applying polyaluminium chloride (PAC) and tobacco leaf extract as a coagulant and coagulant aid. Experimental results indicated that the tobacco leaves were positively charged. The removal rate of the chemical oxygen demand, using 1500 mg/L PAC as a sole coagulant, was approximately 63% and increased to 91% when 1000 mg/L PAC was mixed with 1000 mg/L tobacco leaf. Additionally, 1500 mg/L PAC with 250 - 1000 mg/L tobacco leaf and 54% ammoniacal nitrogen was removed, compared with only 46% reduction using 1500 mg/L with only 46% reduction.