New multifunctional synthetic nanoparticles are adapted for water treatment with environmentally-functional capping layers and magnetic cores. With high surface-to-volume ratio these nanoparticles are very efficient in water treatment including but not restricted to water disinfection photo-catalytic degradation contaminant adsorption etc. in the context of drinking water or waste water treatment. Meanwhile their magnetic cores are highly magnetically responsive and can be separated by 99 percent within 10 min using simply a permanent magnet. Moreover once some non-degradable chemicals like perfluorinated compounds are absorbed to the particle surface these chemicals can be further degraded by introducing hyperthermia or eddy current. These particles can be redispersed after the external magnetic field is removed and are therefore regenerative substantially reducing the cost while eliminating contaminated byproducts.