An ash pretreatment process was developed and evaluated for improving sorption of 2,4-dichlorophenoxyacetic acid (2,4-D) and sulfamethoxazole (SMX) for pine and biosolids based biochars. Pine and biosolids pellets were soaked in a dissolved ash solution and pyrolized at 800 °C. The sorption performance of untreated pine biochar, pine ash pretreated biochar, untreated biosolids biochar, biosolids ash pretreated biochar, base pretreated pine biochar, and commercial powdered activated carbon (PAC) was compared in deionized water, lake water, wastewater, and stormwater for 2,4-D and SMX removal. Dose response curves were developed at 3 hour and 7 day contact times in continuously mixed batch tests. Ash pretreatment of pine yielded over an order of magnitude increase in 2,4-D and SMX sorption compared to untreated pine biochar, making it competitive with PAC. Base pretreatment was the dominant improvement process of ash pretreatment, and the improvement from ash pretreatment was linked to the precursor feedstock inherent ash content. Adsorbent performance was strongly correlated with non-micropore (>2 nm) surface area. Background organic matter character significantly impacted sorbent performance. Ash pretreatment of biochar increased organic micropollutant sorption and is applicable in low-cost water treatment scenarios, such as stormwater and wastewater treatment, as well as in low- and middle-income countries.