A comprehensive review of algal biochar for soil improvement: bottlenecks and opportunities

In recent years, the rapid increase of CO₂ emission in the atmosphere and the resulting issues such as global warming and climate change have now become significant barriers to environmental sustainability. Although fossil CO₂ emissions have decreased in some of the world's largest emitters, including 11% in the EU, 12% in the US and 1.7% in China annually, the estimated global CO₂ emission amount still reached 40 G tonnes in 2020. The purpose of studying biochar produced by pyrolysis is essential to develop the knowledge of carbon cycles and nutrient components in soil. Among all types of feedstocks, algae grow incredibly rapidly compared to other biological materials, about 500-1500 times higher, which will boot the carbon sequestration rate. Therefore, the study of algal biochar production through pyrolysis has great significance for migrating climate change and developing carbon capture and storage.

This study focuses on a comprehensive review of previous literature on conventional and advanced macroalgae and microalgae pyrolysis for producing biochar and related valuable by-products like bio-oil and bio-syngas, aiming to establish a state-of-the-art of algal biochar for different soil-related applications and demonstrate the bottlenecks and opportunities. Specifically, a thorough comparison of algae species (20 microalgae and 20 macroalgae) is developed to benefit future researchers, involving chemical compositions, proximate analysis, solid-product fraction, physical properties and chemical properties. Redox conditions, surface functional groups and pH conditions are determined in lab-scale. Moreover, different algal biochar applications on soil and plant are analysed to optimise the commercial value of algal biochar, including soil conditioner, compositing additives, carrier for fertilisers, manure treatment and stable blending. Due to the abundant mineral contents (0.23-1.21% Na, 0.03-2.92% K, 0.75-7.17% Al, 0.19-1.24% Mg, 6.5-7% Ca and 0.04-0.69% Fe) of algal biochar, this study not only reviews the positive effects on soil improvement but also negative effects such as phytotoxic effect and heavy-metal pollution. A laboratory-based chemical oxidation approach (Edinburgh Stability Tool) is used to assess relatively long-term biochar stability and the influence of nutrient cycling. The optimal pyrolysis conditions (temperature, retention time and heating rate) and potential future commercial applications are obtained through the comprehensive review of algal biochar for soil improvement.