Coagulation studies on photodegraded and photocatalytically degraded polystyrene microplastics using polyaluminium chloride.

Microplastics are ubiquitous persistent emerging contaminants, and its presence has been detected even in the most pristine and fragile ecosystems. Advanced oxidation processes are one of the novel degradation technologies used for the elimination of microplastics from the environment. In this study, the effect of ultraviolet C (UV-C, 253.7 nm) and ultraviolet A (UV-A, 365 nm) irradiations on polystyrene (PS) microplastic properties in the presence and absence of titanium dioxide were studied along with their coagulation performances using polyaluminium chloride (PAC). The effects of solar irradiation on the chemical properties of microplastics in aqueous and dry conditions were also investigated. PS microplastics (1.5 g) in three size ranges, 300-150 µm, 150-75 µm, and <75 µm were used during this experiment. After 45 days of irradiation, samples showed discolouration, brittleness, and loss of hydrophobicity. Images obtained from scanning electron microscope revealed smoothening and melting of PS surfaces upon UV exposure. Attenuated total reflectance- Fourier transform infrared spectroscopy and X-ray photon spectroscopy of photoaged samples revealed chemical alterations, bond cleavage and formation of oxygenated functional groups on microplastic surfaces. PAC coagulation of samples before and after UV irradiation showed drastic differences in removal efficiencies, with UV-C irradiated microplastics exhibiting maximum efficiency. Large sized and photocatalytically degraded microplastics showed better removal efficiencies than small sized particles. The 300-150 µm sized PS microplastic, degraded photo catalytically under UV-C irradiation showed approximately 99 % removal efficiency, while PS < 75 µm photodegraded under UV-A irradiation showed only 74.2 % removal efficiency.

Electrochemical degradation of malachite green: Multivariate optimization, pathway identification and toxicity analysis.

Application of a newly developed electrode material, PbO2 coated on mild steel plate (MS-PbO2), for the degradation of malachite green (MG) by photocatalytic oxidation (PCO), electrochemical oxidation (ECO) and photoelectrochemical oxidation (PEC) was explored. PEC performed marginally better at lower current density. However, the performances of PEC and ECO were equally good at higher current densities. One variable at a time optimization was carried out to identify the major parameters influencing ECO. Multivariate optimization was carried out with NaCl concentration, current density and pH as the variables and chemical oxygen demand (COD) removal efficiency and current efficiency (CE) as the responses. Increasing the current density aided the COD removal efficiency, but decreased the CE. Low NaCl concentration and acidic pH were beneficial for both. The optimum condition for maximizing the COD removal efficiency and CE of MG (50 mg L(-1)) was obtained as NaCl concentration of 1.56 g L(-1), a current density of 1.91 mA cm(-2) and pH 5. The maximum predicted and experimental COD removal efficiencies were 89.41% and 90.8%, and CEs were 21.52% and 21.1%, respectively. Degradation intermediates were identified and a possible pathway of degradation was proposed. Disc inhibition study showed that the degraded samples are non-toxic. The efficacy of the method was tested for treating wastewater collected from dyebath having a COD of about 2000 mg L(-1). COD removal efficiency of greater than 90% was achieved within 12 h at a current density of 7.2 mA cm(-2).

Anodic oxidation of coke oven wastewater: Multiparameter optimization for simultaneous removal of cyanide, COD and phenol.

Anodic oxidation of industrial wastewater from a coke oven plant having cyanide including thiocyanate (280 mg L(-1)), chemical oxygen demand (COD - 1520 mg L(-1)) and phenol (900 mg L(-1)) was carried out using a novel PbO2 anode. From univariate optimization study, low NaCl concentration, acidic pH, high current density and temperature were found beneficial for the oxidation. Multivariate optimization was performed with cyanide including thiocyanate, COD and phenol removal efficiencies as a function of changes in initial pH, NaCl concentration and current density using Box-Behnken experimental design. Optimization was performed for maximizing the removal efficiencies of these three parameters simultaneously. The optimum condition was obtained as initial pH 3.95, NaCl as 1 g L(-1) and current density of 6.7 mA cm(-2), for which the predicted removal efficiencies were 99.6%, 86.7% and 99.7% for cyanide including thiocyanate, COD and phenol respectively. It was in agreement with the values obtained experimentally as 99.1%, 85.2% and 99.7% respectively for these parameters. The optimum conditions with initial pH constrained to a range of 6-8 was initial pH 6, NaCl as 1.31 g L(-1) and current density as 6.7 mA cm(-2). The predicted removal efficiencies were 99%, 86.7% and 99.6% for the three parameters. The efficiencies obtained experimentally were in agreement at 99%, 87.8% and 99.6% respectively. The cost of operation for degradation at optimum conditions was calculated as 21.4 USD m(-3).

Photocatalytic degradation of methylene blue dye from aqueous solution using silver ion-doped TiO2 and its application to the degradation of real textile wastewater.

Methylene blue dye (MB) was degraded photocatalytically in aqueous solution using Ag(+) doped TiO(2) under UV irradiation. The degradations of the dye using untreated TiO(2) and Ag(+) doped TiO(2) were compared. Ag(+) doped TiO(2) was found to be more efficient. Using Ag(+) doped TiO(2) the filtration process was eliminated, as the particles became more settleable. The effect of various parameters such as catalyst loading, initial dye concentration, depth of solution, degree of adsorption, pH and O(2) on dye degradation was studied. The extent of mineralization was studied by observing the COD removal at different time intervals. The effects of various interfering ions such as Cl(-), NO(3) (-), CO(3) (2-), SO(4) (2-), Ca(2+) and Fe(3+) and electron acceptors such as H(2)O(2), KBrO(3) and (NH(4))(2)S(2)O(8) on the dye degradation was also studied. The degradation kinetics fitted well to Langmuir-Hinshelwood pseudo first order rate law. An aqueous solution of MB (20ppm) degraded by more than 99% after UV irradiation for 180 min with Ag(+) doped TiO(2) (2 g/L) and by more than 95% with untreated TiO(2) (2 g/L)(.) The COD removal was more than 91% with Ag(+)doped TiO(2) and more than 86% with untreated TiO(2) after 240 min. The degradation and COD removal of 5 times diluted textile wastewater was more than 98% and 79% respectively with 1 g/L Ag(+) doped TiO(2) after UV irradiation for 420 min.