12-HEPP CO2 capture CO2 gasification CiteSpace DEA-12-PD FTIR and XRD studies Harare Langmuir and Freundlich N2O PID control Schiff base South Africa Technology: general issues VLE Zimbabwe ZnO a visual analysis acidification activation energy distribution adsorption adsorption capacity algal biomass amine ammonium ammonium sulfate roasting process antibiotics antimony aqueous solution ash-free coal batch pyrolysis