The advancement of water treatment technologies remains crucial in protecting the health and prosperity of both current and future generations, ensuring universal access to water. Among the various technologies, membrane filtration and adsorption are the most utilized advanced technologies, due to their effectiveness in achieving high-quality water standards. Yet, there is a pressing need for new research focused on enhancing adsorbent and membrane materials to improve their effectiveness in removing heavy metal ions and addressing newly emerging pollutants. It is also crucial to tackle key challenges like adsorbent recovery and membrane fouling issues. In this dissertation, atomic layer deposition (ALD) and vapor-phase grafting are explored to engineer adsorbent and membrane surfaces. We synthesize and characterize model adsorption sites with well-defined structure and chemistry for the advancement of adsorbent and membrane materials for efficient water purification. We begin by exploring the growth mechanisms of ALD metal oxides through both in situ and ex situ measurements, establishing ALD’s capability for uniform thin-film formation with precise control over thickness and composition. This foundational work enables us to synthesize and characterize 17 different metal oxides via ALD, assessing their water stability, surface wetting properties, and charge characteristics to determine their suitability for water treatment applications. Building on this foundational understanding, the research applies ALD techniques to fine-tune membrane pore sizes and surface charges using various metal oxides, optimizing their permeability and functionality for specific water treatment needs. Additionally, the dissertation explores the enhancement of porous carbon adsorbents for metal ion removal, focusing on ALD Al2O3 coatings. This section details the experimental process and evaluates the improved performance of coated adsorbents in removing metal ions from aqueous solutions. The next phase of the research delves into vapor-phase organic modification of ALD oxide surfaces using aminosilanes. Through comprehensive in situ and ex situ analyses, we demonstrate the formation of uniform aminosilane monolayers. The dissertation examines how reaction temperatures, the underlying oxide nature, and water vapor presence affect silanization efficiency and surface chemistry, offering new insights into the silane reaction mechanisms. Next, building on this, we explored vapor-phase grafting of various silanes on ALD Al2O3 surfaces. Our investigations revealed that the chemical nature and grafting conditions significantly affect surface properties. Finally, our approach further extends to developing a surface functionalization technique using ALD and vapor-phase silanization onto porous substrates for selective heavy metal ion adsorption. By modifying porous silica gel with ALD Al2O3 and a thiol-functional silane, we achieved high selectivity in adsorbing hazardous ions like Cd(II), As(V), Pb(II), Hg(II), and Cu(II) from mixed solutions. This work not only advances the understanding of selective adsorption mechanisms but also demonstrates the potential of surface functionalization techniques in creating more efficient water treatment solutions.