Silica, or silicon dioxide (\(\text{SiO}_2\)), is one of the most abundant compounds on Earth, and its presence in water is nearly universal. As a naturally occurring mineral found in rocks and sand, trace amounts of silica are dissolved into almost every water source worldwide. While many compounds found in tap water are subject to strict regulation, silica is generally not treated as a primary contaminant, leading to frequent public questions about its safety and effects. Understanding the forms silica takes in water and its interaction with the human body and infrastructure is important for interpreting water quality information.
Defining Silica and Its Chemical Forms in Water
Silica exists in water in two primary chemical states, which determine its behavior and ease of removal. The most common form is dissolved silica, also known as reactive silica or monomeric silicic acid (\(\text{H}_4\text{SiO}_4\)). This dissolved form behaves as a weak acid and is completely mixed with the water molecules, making it chemically reactive and difficult to remove with standard filtration.
The second form is colloidal silica, often called unreactive silica. This is composed of ultra-fine, non-dissolved particles that are suspended in the water column. Colloidal silica results from the polymerization of silicic acid, a process that occurs when the concentration of dissolved silica becomes high or when the water’s pH is lowered. These microscopic particles act more like fine solids, presenting a distinct challenge for advanced water treatment processes.
Natural Sources and Typical Concentration Levels
Silica enters natural water supplies through the slow, continuous geological process of weathering. As groundwater and surface water flow over or through the Earth’s crust, they gradually dissolve silicate minerals, such as quartz, granite, and feldspar, which are the fundamental building blocks of most rocks. This dissolution releases the silicon dioxide into the water supply.
The concentration of silica in water varies significantly depending on the local geology and the water source. Fresh surface and groundwater sources typically contain dissolved silica concentrations ranging from 1 to 100 milligrams per liter (mg/L). Groundwater tends to have higher median concentrations, averaging around 17 mg/L, compared to stream water’s median of about 14 mg/L. Despite its ubiquity, the U.S. Environmental Protection Agency (EPA) has not established a maximum contaminant level for silica, meaning it is not regulated as a health-based primary drinking water contaminant.
Health Effects of Silica Consumption
The dissolved silica found in drinking water is generally considered safe for human consumption and is not associated with adverse health outcomes at typical concentrations. Dietary intake of silicon, including that from water, is a normal part of the human diet, with the average daily intake ranging between 43 and 107 mg of \(\text{SiO}_2\) per day. Silicon is considered beneficial for health, playing a role in the formation of collagen, a protein needed for connective tissue, skin, and bone health.
Dissolved silica, specifically the monomeric silicic acid, is readily absorbed from the digestive tract and excreted by the kidneys. A notable health effect involves its interaction with aluminum, a metal that can accumulate in the body. Soluble silicon can interact with dietary aluminum to form large, poorly absorbed complexes called hydroxyaluminosilicates in the gut.
This complex formation significantly reduces the absorption and bioavailability of aluminum, which is then promoted for urinary excretion. Studies suggest that the consumption of dissolved silicon can substantially reduce the amount of aluminum that enters the bloodstream. This protective effect is particularly pronounced with the oligomeric (polymeric) form of silica, which can reduce aluminum absorption by up to 67%. This dietary silicon must be clearly differentiated from crystalline silica dust, which is a known occupational lung hazard that causes severe respiratory diseases like silicosis when inhaled.
Practical Implications for Water Systems
While dissolved silica is generally safe to drink, high concentrations can create operational problems for water systems and industry. The primary issue is scaling, which is the formation of hard mineral deposits on surfaces. This scaling typically becomes noticeable in residential settings when silica concentrations exceed approximately 20 to 25 mg/L.
The aesthetic issues include etching on glassware and the formation of white, chalky deposits in toilets and on fixtures. In industrial applications, silica scaling is a major concern for equipment such as boilers, cooling towers, and heat exchangers, where it reduces efficiency and can cause damage. Silica is a foulant for reverse osmosis (RO) membranes, often limiting the practical recovery rate of water treatment systems. The polymerized or glassy form of silica is especially difficult to remove once it forms on membrane surfaces, often requiring specialized chemical cleaning agents.