<p>Selenium in the form of either “seleno-proteins” or “seleno-compounds” that is stably existed in the human body, acts a vital role in reproductive mechanism, DNA production, metabolism, and protection from infectious diseases. The current approach represents a sole designing technique to prepare monoclinic SeNPs in α-Se form using ascorbic acid and sodium alginate at room temperature to be ascribed as a green/energy saving synthetic method, in order to be dual applied in environmental purposes as nano-catalyst for degradation of para-nitro-aniline as one of the most dangerous organic water pollutants and antimicrobial laborer. Mean size for SeNPs samples in the range of 62.2–141.8 nm. X-Ray diffraction analysis showed that, the produced nanoparticles were corresponding to m-Se (monoclinic-form) and t-Se (trigonal form). SeNPs showed excellent antimicrobial action with mortality percent of 94.2%, 91.6% &amp; 93.5% against <i>Staph. aureus, E. coli</i> &amp; <i>C. albicans</i>, respectively. The rate of the reductive degradation of para-nitro-aniline was accelerated by SeNPs. Rate constant was raised from 1.3 × 10^–3 min⁻¹ without SeNPs to 14.8 × 10^–3 min⁻¹ under the catalytic performance of SeNPs. The duration required to complete the reduction reaction can be decreased by 8 times with increasing the SeNPs dose from 25 to 100 mg.</p> Graphical Abstract <p></p>